This is tar.info, produced by makeinfo version 4.7.90 from tar.texi. This manual is for GNU `tar' (version 1.14.90, 4 October 2004), which creates and extracts files from archives. Copyright (C) 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001, 2003, 2004 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License", with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom." INFO-DIR-SECTION Archiving START-INFO-DIR-ENTRY * Tar: (tar). Making tape (or disk) archives. END-INFO-DIR-ENTRY INFO-DIR-SECTION Individual utilities START-INFO-DIR-ENTRY * tar: (tar)tar invocation. Invoking GNU `tar'. END-INFO-DIR-ENTRY File: tar.info, Node: Top, Next: Introduction, Up: (dir) GNU tar: an archiver tool ************************* This manual is for GNU `tar' (version 1.14.90, 4 October 2004), which creates and extracts files from archives. Copyright (C) 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001, 2003, 2004 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License", with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom." The first part of this master menu lists the major nodes in this Info document. The rest of the menu lists all the lower level nodes. * Menu: * Introduction:: * Tutorial:: * tar invocation:: * operations:: * Backups:: * Choosing:: * Date input formats:: * Formats:: * Media:: * Free Software Needs Free Documentation:: * Copying This Manual:: * Index:: --- The Detailed Node Listing --- Introduction * Book Contents:: What this Book Contains * Definitions:: Some Definitions * What tar Does:: What `tar' Does * Naming tar Archives:: How `tar' Archives are Named * Current status:: Current development status of GNU `tar' * Authors:: GNU `tar' Authors * Reports:: Reporting bugs or suggestions Tutorial Introduction to `tar' * assumptions:: * stylistic conventions:: * basic tar options:: Basic `tar' Operations and Options * frequent operations:: * Two Frequent Options:: * create:: How to Create Archives * list:: How to List Archives * extract:: How to Extract Members from an Archive * going further:: Two Frequently Used Options * file tutorial:: * verbose tutorial:: * help tutorial:: How to Create Archives * prepare for examples:: * Creating the archive:: * create verbose:: * short create:: * create dir:: How to List Archives * list dir:: How to Extract Members from an Archive * extracting archives:: * extracting files:: * extract dir:: * failing commands:: Invoking GNU `tar' * Synopsis:: * using tar options:: * Styles:: * All Options:: * help:: * verbose:: * interactive:: The Three Option Styles * Mnemonic Options:: Mnemonic Option Style * Short Options:: Short Option Style * Old Options:: Old Option Style * Mixing:: Mixing Option Styles All `tar' Options * Operation Summary:: * Option Summary:: * Short Option Summary:: GNU `tar' Operations * Basic tar:: * Advanced tar:: * create options:: * extract options:: * backup:: * Applications:: * looking ahead:: Advanced GNU `tar' Operations * Operations:: * append:: * update:: * concatenate:: * delete:: * compare:: How to Add Files to Existing Archives: `--append' * appending files:: Appending Files to an Archive * multiple:: Updating an Archive * how to update:: Options Used by `--create' * Ignore Failed Read:: Options Used by `--extract' * Reading:: Options to Help Read Archives * Writing:: Changing How `tar' Writes Files * Scarce:: Coping with Scarce Resources Options to Help Read Archives * read full records:: * Ignore Zeros:: Changing How `tar' Writes Files * Dealing with Old Files:: * Overwrite Old Files:: * Keep Old Files:: * Keep Newer Files:: * Unlink First:: * Recursive Unlink:: * Modification Times:: * Setting Access Permissions:: * Writing to Standard Output:: * remove files:: Coping with Scarce Resources * Starting File:: * Same Order:: Performing Backups and Restoring Files * Full Dumps:: Using `tar' to Perform Full Dumps * Inc Dumps:: Using `tar' to Perform Incremental Dumps * incremental and listed-incremental:: The Incremental Options * Backup Levels:: Levels of Backups * Backup Parameters:: Setting Parameters for Backups and Restoration * Scripted Backups:: Using the Backup Scripts * Scripted Restoration:: Using the Restore Script Setting Parameters for Backups and Restoration * General-Purpose Variables:: * Magnetic Tape Control:: * User Hooks:: * backup-specs example:: An Example Text of `Backup-specs' Choosing Files and Names for `tar' * file:: Choosing the Archive's Name * Selecting Archive Members:: * files:: Reading Names from a File * exclude:: Excluding Some Files * Wildcards:: * after:: Operating Only on New Files * recurse:: Descending into Directories * one:: Crossing Filesystem Boundaries Reading Names from a File * nul:: Excluding Some Files * controlling pattern-patching with exclude:: * problems with exclude:: Crossing Filesystem Boundaries * directory:: Changing Directory * absolute:: Absolute File Names Date input formats * General date syntax:: Common rules. * Calendar date items:: 19 Dec 1994. * Time of day items:: 9:20pm. * Time zone items:: EST, PDT, GMT, ... * Day of week items:: Monday and others. * Relative items in date strings:: next tuesday, 2 years ago. * Pure numbers in date strings:: 19931219, 1440. * Seconds since the Epoch:: @1078100502. * Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al. Controlling the Archive Format * Portability:: Making `tar' Archives More Portable * Compression:: Using Less Space through Compression * Attributes:: Handling File Attributes * Standard:: The Standard Format * Extensions:: GNU Extensions to the Archive Format * cpio:: Comparison of `tar' and `cpio' Making `tar' Archives More Portable * Portable Names:: Portable Names * dereference:: Symbolic Links * old:: Old V7 Archives * posix:: POSIX archives * Checksumming:: Checksumming Problems * Large or Negative Values:: Large files, negative time stamps, etc. Using Less Space through Compression * gzip:: Creating and Reading Compressed Archives * sparse:: Archiving Sparse Files Tapes and Other Archive Media * Device:: Device selection and switching * Remote Tape Server:: * Common Problems and Solutions:: * Blocking:: Blocking * Many:: Many archives on one tape * Using Multiple Tapes:: Using Multiple Tapes * label:: Including a Label in the Archive * verify:: * Write Protection:: Blocking * Format Variations:: Format Variations * Blocking Factor:: The Blocking Factor of an Archive Many Archives on One Tape * Tape Positioning:: Tape Positions and Tape Marks * mt:: The `mt' Utility Using Multiple Tapes * Multi-Volume Archives:: Archives Longer than One Tape or Disk * Tape Files:: Tape Files Copying This Manual * GNU Free Documentation License:: License for copying this manual File: tar.info, Node: Introduction, Next: Tutorial, Prev: Top, Up: Top 1 Introduction ************** GNU `tar' creates and manipulates "archives" which are actually collections of many other files; the program provides users with an organized and systematic method for controlling a large amount of data. The name "tar" originally came from the phrase "Tape ARchive", but archives need not (and these days, typically do not) reside on tapes. * Menu: * Book Contents:: What this Book Contains * Definitions:: Some Definitions * What tar Does:: What `tar' Does * Naming tar Archives:: How `tar' Archives are Named * Current status:: Current development status of GNU `tar' * Authors:: GNU `tar' Authors * Reports:: Reporting bugs or suggestions File: tar.info, Node: Book Contents, Next: Definitions, Up: Introduction 1.1 What this Book Contains =========================== The first part of this chapter introduces you to various terms that will recur throughout the book. It also tells you who has worked on GNU `tar' and its documentation, and where you should send bug reports or comments. The second chapter is a tutorial (*note Tutorial::) which provides a gentle introduction for people who are new to using `tar'. It is meant to be self contained, not requiring any reading from subsequent chapters to make sense. It moves from topic to topic in a logical, progressive order, building on information already explained. Although the tutorial is paced and structured to allow beginners to learn how to use `tar', it is not intended solely for beginners. The tutorial explains how to use the three most frequently used operations (`create', `list', and `extract') as well as two frequently used options (`file' and `verbose'). The other chapters do not refer to the tutorial frequently; however, if a section discusses something which is a complex variant of a basic concept, there may be a cross reference to that basic concept. (The entire book, including the tutorial, assumes that the reader understands some basic concepts of using a Unix-type operating system; *note Tutorial::.) The third chapter presents the remaining five operations, and information about using `tar' options and option syntax. The other chapters are meant to be used as a reference. Each chapter presents everything that needs to be said about a specific topic. One of the chapters (*note Date input formats::) exists in its entirety in other GNU manuals, and is mostly self-contained. In addition, one section of this manual (*note Standard::) contains a big quote which is taken directly from `tar' sources. In general, we give both long and short (abbreviated) option names at least once in each section where the relevant option is covered, so that novice readers will become familiar with both styles. (A few options have no short versions, and the relevant sections will indicate this.) File: tar.info, Node: Definitions, Next: What tar Does, Prev: Book Contents, Up: Introduction 1.2 Some Definitions ==================== The `tar' program is used to create and manipulate `tar' archives. An "archive" is a single file which contains the contents of many files, while still identifying the names of the files, their owner(s), and so forth. (In addition, archives record access permissions, user and group, size in bytes, and last modification time. Some archives also record the file names in each archived directory, as well as other file and directory information.) You can use `tar' to "create" a new archive in a specified directory. The files inside an archive are called "members". Within this manual, we use the term "file" to refer only to files accessible in the normal ways (by `ls', `cat', and so forth), and the term "member" to refer only to the members of an archive. Similarly, a "file name" is the name of a file, as it resides in the filesystem, and a "member name" is the name of an archive member within the archive. The term "extraction" refers to the process of copying an archive member (or multiple members) into a file in the filesystem. Extracting all the members of an archive is often called "extracting the archive". The term "unpack" can also be used to refer to the extraction of many or all the members of an archive. Extracting an archive does not destroy the archive's structure, just as creating an archive does not destroy the copies of the files that exist outside of the archive. You may also "list" the members in a given archive (this is often thought of as "printing" them to the standard output, or the command line), or "append" members to a pre-existing archive. All of these operations can be performed using `tar'. File: tar.info, Node: What tar Does, Next: Naming tar Archives, Prev: Definitions, Up: Introduction 1.3 What `tar' Does =================== The `tar' program provides the ability to create `tar' archives, as well as various other kinds of manipulation. For example, you can use `tar' on previously created archives to extract files, to store additional files, or to update or list files which were already stored. Initially, `tar' archives were used to store files conveniently on magnetic tape. The name `tar' comes from this use; it stands for `t'ape `ar'chiver. Despite the utility's name, `tar' can direct its output to available devices, files, or other programs (using pipes). `tar' may even access remote devices or files (as archives). You can use `tar' archives in many ways. We want to stress a few of them: storage, backup, and transportation. Storage Often, `tar' archives are used to store related files for convenient file transfer over a network. For example, the GNU Project distributes its software bundled into `tar' archives, so that all the files relating to a particular program (or set of related programs) can be transferred as a single unit. A magnetic tape can store several files in sequence. However, the tape has no names for these files; it only knows their relative position on the tape. One way to store several files on one tape and retain their names is by creating a `tar' archive. Even when the basic transfer mechanism can keep track of names, as FTP can, the nuisance of handling multiple files, directories, and multiple links makes `tar' archives useful. Archive files are also used for long-term storage. You can think of this as transportation from the present into the future. (It is a science-fiction idiom that you can move through time as well as in space; the idea here is that `tar' can be used to move archives in all dimensions, even time!) Backup Because the archive created by `tar' is capable of preserving file information and directory structure, `tar' is commonly used for performing full and incremental backups of disks. A backup puts a collection of files (possibly pertaining to many users and projects) together on a disk or a tape. This guards against accidental destruction of the information in those files. GNU `tar' has special features that allow it to be used to make incremental and full dumps of all the files in a filesystem. Transportation You can create an archive on one system, transfer it to another system, and extract the contents there. This allows you to transport a group of files from one system to another. File: tar.info, Node: Naming tar Archives, Next: Current status, Prev: What tar Does, Up: Introduction 1.4 How `tar' Archives are Named ================================ Conventionally, `tar' archives are given names ending with `.tar'. This is not necessary for `tar' to operate properly, but this manual follows that convention in order to accustom readers to it and to make examples more clear. Often, people refer to `tar' archives as "`tar' files," and archive members as "files" or "entries". For people familiar with the operation of `tar', this causes no difficulty. However, in this manual, we consistently refer to "archives" and "archive members" to make learning to use `tar' easier for novice users. File: tar.info, Node: Current status, Next: Authors, Prev: Naming tar Archives, Up: Introduction 1.5 Current development status of GNU `tar' =========================================== GNU `tar' is currently in the process of active development, whose primary aims are: * Improve compatibility between GNU `tar' and other `tar' implementations. * Switch to using POSIX archives. * Revise sparse file handling and multiple volume processing. * Merge with the GNU `paxutils' project. Some of these aims are already attained, while others are still being worked upon. From the point of view of an end user, the following issues need special mentioning: Use of short option `-o'. Earlier versions of GNU `tar' understood `-o' command line option as a synonym for `--old-archive'. GNU `tar' starting from version 1.13.90 understands this option as a synonym for `--no-same-owner'. This is compatible with UNIX98 `tar' implementations. However, to facilitate transition, `-o' option retains its old semantics when it is used with one of archive-creation commands. Users are encouraged to use `--format=oldgnu' instead. It is especially important, since versions of GNU Automake up to and including 1.8.4 invoke tar with this option to produce distribution tarballs. *Note v7: Formats, for the detailed discussion of this issue and its implications. Future versions of GNU `tar' will understand `-o' only as a synonym for `--no-same-owner'. Use of short option `-l' Earlier versions of GNU `tar' understood `-l' option as a synonym for `--one-file-system'. Such usage is deprecated. For compatibility with other implementations future versions of GNU `tar' will understand this option as a synonym for `--check-links'. Use of options `--portability' and `--old-archive' These options are deprecated. Please use `--format=v7' instead. Use of option `--posix' This option is deprecated. Please use `--format=posix' instead. File: tar.info, Node: Authors, Next: Reports, Prev: Current status, Up: Introduction 1.6 GNU `tar' Authors ===================== GNU `tar' was originally written by John Gilmore, and modified by many people. The GNU enhancements were written by Jay Fenlason, then Joy Kendall, and the whole package has been further maintained by Thomas Bushnell, n/BSG, Franc,ois Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of numerous and kind users. We wish to stress that `tar' is a collective work, and owes much to all those people who reported problems, offered solutions and other insights, or shared their thoughts and suggestions. An impressive, yet partial list of those contributors can be found in the `THANKS' file from the GNU `tar' distribution. Jay Fenlason put together a draft of a GNU `tar' manual, borrowing notes from the original man page from John Gilmore. This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy Gorin worked on a tutorial and manual for GNU `tar'. Franc,ois Pinard put version 1.11.8 of the manual together by taking information from all these sources and merging them. Melissa Weisshaus finally edited and redesigned the book to create version 1.12. For version 1.12, Daniel Hagerty contributed a great deal of technical consulting. In particular, he is the primary author of *Note Backups::. In July, 2003 GNU `tar' was put on CVS at savannah.gnu.org (see `http://savannah.gnu.org/projects/tar'), and active development and maintenance work has started again. Currently GNU `tar' is being maintained by Paul Eggert, Sergey Poznyakoff and Jeff Bailey. Support for POSIX archives was added by Sergey Poznyakoff. File: tar.info, Node: Reports, Prev: Authors, Up: Introduction 1.7 Reporting bugs or suggestions ================================= If you find problems or have suggestions about this program or manual, please report them to `bug-tar@gnu.org'. When reporting a bug, please be sure to include as much detail as possible, in order to reproduce it. . File: tar.info, Node: Tutorial, Next: tar invocation, Prev: Introduction, Up: Top 2 Tutorial Introduction to `tar' ******************************** This chapter guides you through some basic examples of three `tar' operations: `--create', `--list', and `--extract'. If you already know how to use some other version of `tar', then you may not need to read this chapter. This chapter omits most complicated details about how `tar' works. * Menu: * assumptions:: * stylistic conventions:: * basic tar options:: Basic `tar' Operations and Options * frequent operations:: * Two Frequent Options:: * create:: How to Create Archives * list:: How to List Archives * extract:: How to Extract Members from an Archive * going further:: File: tar.info, Node: assumptions, Next: stylistic conventions, Up: Tutorial 2.1 Assumptions this Tutorial Makes =================================== This chapter is paced to allow beginners to learn about `tar' slowly. At the same time, we will try to cover all the basic aspects of these three operations. In order to accomplish both of these tasks, we have made certain assumptions about your knowledge before reading this manual, and the hardware you will be using: * Before you start to work through this tutorial, you should understand what the terms "archive" and "archive member" mean (*note Definitions::). In addition, you should understand something about how Unix-type operating systems work, and you should know how to use some basic utilities. For example, you should know how to create, list, copy, rename, edit, and delete files and directories; how to change between directories; and how to figure out where you are in the filesystem. You should have some basic understanding of directory structure and how files are named according to which directory they are in. You should understand concepts such as standard output and standard input, what various definitions of the term "argument" mean, and the differences between relative and absolute path names. * This manual assumes that you are working from your own home directory (unless we state otherwise). In this tutorial, you will create a directory to practice `tar' commands in. When we show path names, we will assume that those paths are relative to your home directory. For example, my home directory path is `/home/fsf/melissa'. All of my examples are in a subdirectory of the directory named by that path name; the subdirectory is called `practice'. * In general, we show examples of archives which exist on (or can be written to, or worked with from) a directory on a hard disk. In most cases, you could write those archives to, or work with them on any other device, such as a tape drive. However, some of the later examples in the tutorial and next chapter will not work on tape drives. Additionally, working with tapes is much more complicated than working with hard disks. For these reasons, the tutorial does not cover working with tape drives. *Note Media::, for complete information on using `tar' archives with tape drives. File: tar.info, Node: stylistic conventions, Next: basic tar options, Prev: assumptions, Up: Tutorial 2.2 Stylistic Conventions ========================= In the examples, `$' represents a typical shell prompt. It precedes lines you should type; to make this more clear, those lines are shown in `this font', as opposed to lines which represent the computer's response; those lines are shown in `this font', or sometimes `like this'. File: tar.info, Node: basic tar options, Next: frequent operations, Prev: stylistic conventions, Up: Tutorial 2.3 Basic `tar' Operations and Options ====================================== `tar' can take a wide variety of arguments which specify and define the actions it will have on the particular set of files or the archive. The main types of arguments to `tar' fall into one of two classes: operations, and options. Some arguments fall into a class called "operations"; exactly one of these is both allowed and required for any instance of using `tar'; you may _not_ specify more than one. People sometimes speak of "operating modes". You are in a particular operating mode when you have specified the operation which specifies it; there are eight operations in total, and thus there are eight operating modes. The other arguments fall into the class known as "options". You are not required to specify any options, and you are allowed to specify more than one at a time (depending on the way you are using `tar' at that time). Some options are used so frequently, and are so useful for helping you type commands more carefully that they are effectively "required". We will discuss them in this chapter. You can write most of the `tar' operations and options in any of three forms: long (mnemonic) form, short form, and old style. Some of the operations and options have no short or "old" forms; however, the operations and options which we will cover in this tutorial have corresponding abbreviations. We will indicate those abbreviations appropriately to get you used to seeing them. (Note that the "old style" option forms exist in GNU `tar' for compatibility with Unix `tar'. We present a full discussion of this way of writing options and operations appears in *Note Old Options::, and we discuss the other two styles of writing options in *Note Mnemonic Options::, and *Note Short Options::.) In the examples and in the text of this tutorial, we usually use the long forms of operations and options; but the "short" forms produce the same result and can make typing long `tar' commands easier. For example, instead of typing tar --create --verbose --file=afiles.tar apple angst aspic you can type tar -c -v -f afiles.tar apple angst aspic or even tar -cvf afiles.tar apple angst aspic For more information on option syntax, see *Note Advanced tar::. In discussions in the text, when we name an option by its long form, we also give the corresponding short option in parentheses. The term, "option", can be confusing at times, since "operations" are often lumped in with the actual, _optional_ "options" in certain general class statements. For example, we just talked about "short and long forms of options and operations". However, experienced `tar' users often refer to these by shorthand terms such as, "short and long options". This term assumes that the "operations" are included, also. Context will help you determine which definition of "options" to use. Similarly, the term "command" can be confusing, as it is often used in two different ways. People sometimes refer to `tar' "commands". A `tar' "command" is the entire command line of user input which tells `tar' what to do -- including the operation, options, and any arguments (file names, pipes, other commands, etc). However, you will also sometimes hear the term "the `tar' command". When the word "command" is used specifically like this, a person is usually referring to the `tar' _operation_, not the whole line. Again, use context to figure out which of the meanings the speaker intends. File: tar.info, Node: frequent operations, Next: Two Frequent Options, Prev: basic tar options, Up: Tutorial 2.4 The Three Most Frequently Used Operations ============================================= Here are the three most frequently used operations (both short and long forms), as well as a brief description of their meanings. The rest of this chapter will cover how to use these operations in detail. We will present the rest of the operations in the next chapter. `--create' `-c' Create a new `tar' archive. `--list' `-t' List the contents of an archive. `--extract' `-x' Extract one or more members from an archive. File: tar.info, Node: Two Frequent Options, Next: create, Prev: frequent operations, Up: Tutorial 2.5 Two Frequently Used Options =============================== To understand how to run `tar' in the three operating modes listed previously, you also need to understand how to use two of the options to `tar': `--file' (which takes an archive file as an argument) and `--verbose'. (You are usually not _required_ to specify either of these options when you run `tar', but they can be very useful in making things more clear and helping you avoid errors.) * Menu: * file tutorial:: * verbose tutorial:: * help tutorial:: File: tar.info, Node: file tutorial, Next: verbose tutorial, Up: Two Frequent Options The `--file' Option ------------------- `--file=ARCHIVE-NAME' `-f ARCHIVE-NAME' Specify the name of an archive file. You can specify an argument for the `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') option whenever you use `tar'; this option determines the name of the archive file that `tar' will work on. If you don't specify this argument, then `tar' will use a default, usually some physical tape drive attached to your machine. If there is no tape drive attached, or the default is not meaningful, then `tar' will print an error message. The error message might look roughly like one of the following: tar: can't open /dev/rmt8 : No such device or address tar: can't open /dev/rsmt0 : I/O error To avoid confusion, we recommend that you always specify an archive file name by using `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') when writing your `tar' commands. For more information on using the `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') option, see *Note file::. File: tar.info, Node: verbose tutorial, Next: help tutorial, Prev: file tutorial, Up: Two Frequent Options The `--verbose' Option ---------------------- `--verbose' `-v' Show the files being worked on as `tar' is running. `--verbose' (`-v') shows details about the results of running `tar'. This can be especially useful when the results might not be obvious. For example, if you want to see the progress of `tar' as it writes files into the archive, you can use the `--verbose' option. In the beginning, you may find it useful to use `--verbose' at all times; when you are more accustomed to `tar', you will likely want to use it at certain times but not at others. We will use `--verbose' at times to help make something clear, and we will give many examples both using and not using `--verbose' to show the differences. Sometimes, a single instance of `--verbose' on the command line will show a full, `ls' style listing of an archive or files, giving sizes, owners, and similar information. Other times, `--verbose' will only show files or members that the particular operation is operating on at the time. In the latter case, you can use `--verbose' twice in a command to get a listing such as that in the former case. For example, instead of saying tar -cvf afiles.tar apple angst aspic above, you might say tar -cvvf afiles.tar apple angst aspic This works equally well using short or long forms of options. Using long forms, you would simply write out the mnemonic form of the option twice, like this: $ tar --create --verbose --verbose ... Note that you must double the hyphens properly each time. Later in the tutorial, we will give examples using `--verbose --verbose'. File: tar.info, Node: help tutorial, Prev: verbose tutorial, Up: Two Frequent Options Getting Help: Using the `--help' Option --------------------------------------- `--help' The `--help' option to `tar' prints out a very brief list of all operations and option available for the current version of `tar' available on your system. File: tar.info, Node: create, Next: list, Prev: Two Frequent Options, Up: Tutorial 2.6 How to Create Archives ========================== _(This message will disappear, once this node revised.)_ One of the basic operations of `tar' is `--create' (`-c'), which you use to create a `tar' archive. We will explain `--create' first because, in order to learn about the other operations, you will find it useful to have an archive available to practice on. To make this easier, in this section you will first create a directory containing three files. Then, we will show you how to create an _archive_ (inside the new directory). Both the directory, and the archive are specifically for you to practice on. The rest of this chapter and the next chapter will show many examples using this directory and the files you will create: some of those files may be other directories and other archives. The three files you will archive in this example are called `blues', `folk', and `jazz'. The archive is called `collection.tar'. This section will proceed slowly, detailing how to use `--create' in `verbose' mode, and showing examples using both short and long forms. In the rest of the tutorial, and in the examples in the next chapter, we will proceed at a slightly quicker pace. This section moves more slowly to allow beginning users to understand how `tar' works. * Menu: * prepare for examples:: * Creating the archive:: * create verbose:: * short create:: * create dir:: File: tar.info, Node: prepare for examples, Next: Creating the archive, Up: create 2.6.1 Preparing a Practice Directory for Examples ------------------------------------------------- To follow along with this and future examples, create a new directory called `practice' containing files called `blues', `folk' and `jazz'. The files can contain any information you like: ideally, they should contain information which relates to their names, and be of different lengths. Our examples assume that `practice' is a subdirectory of your home directory. Now `cd' to the directory named `practice'; `practice' is now your "working directory". (_Please note_: Although the full path name of this directory is `/HOMEDIR/practice', in our examples we will refer to this directory as `practice'; the HOMEDIR is presumed. In general, you should check that the files to be archived exist where you think they do (in the working directory) by running `ls'. Because you just created the directory and the files and have changed to that directory, you probably don't need to do that this time. It is very important to make sure there isn't already a file in the working directory with the archive name you intend to use (in this case, `collection.tar'), or that you don't care about its contents. Whenever you use `create', `tar' will erase the current contents of the file named by `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') if it exists. `tar' will not tell you if you are about to overwrite an archive unless you specify an option which does this. To add files to an existing archive, you need to use a different option, such as `--append' (`-r'); see *Note append:: for information on how to do this. File: tar.info, Node: Creating the archive, Next: create verbose, Prev: prepare for examples, Up: create 2.6.2 Creating the Archive -------------------------- To place the files `blues', `folk', and `jazz' into an archive named `collection.tar', use the following command: $ tar --create --file=collection.tar blues folk jazz The order of the arguments is not very important, _when using long option forms_. You could also say: $ tar blues --create folk --file=collection.tar jazz However, you can see that this order is harder to understand; this is why we will list the arguments in the order that makes the commands easiest to understand (and we encourage you to do the same when you use `tar', to avoid errors). Note that the part of the command which says, `--file=collection.tar' is considered to be _one_ argument. If you substituted any other string of characters for `collection.tar', then that string would become the name of the archive file you create. The order of the options becomes more important when you begin to use short forms. With short forms, if you type commands in the wrong order (even if you type them correctly in all other ways), you may end up with results you don't expect. For this reason, it is a good idea to get into the habit of typing options in the order that makes inherent sense. *Note short create::, for more information on this. In this example, you type the command as shown above: `--create' is the operation which creates the new archive (`collection.tar'), and `--file' is the option which lets you give it the name you chose. The files, `blues', `folk', and `jazz', are now members of the archive, `collection.tar' (they are "file name arguments" to the `--create' operation). Now that they are in the archive, they are called _archive members_, not files. (*note members: Definitions.). When you create an archive, you _must_ specify which files you want placed in the archive. If you do not specify any archive members, GNU `tar' will complain. If you now list the contents of the working directory (`ls'), you will find the archive file listed as well as the files you saw previously: blues folk jazz collection.tar Creating the archive `collection.tar' did not destroy the copies of the files in the directory. Keep in mind that if you don't indicate an operation, `tar' will not run and will prompt you for one. If you don't name any files, `tar' will complain. You must have write access to the working directory, or else you will not be able to create an archive in that directory. _Caution_: Do not attempt to use `--create' (`-c') to add files to an existing archive; it will delete the archive and write a new one. Use `--append' (`-r') instead. *Note append::. File: tar.info, Node: create verbose, Next: short create, Prev: Creating the archive, Up: create 2.6.3 Running `--create' with `--verbose' ----------------------------------------- If you include the `--verbose' (`-v') option on the command line, `tar' will list the files it is acting on as it is working. In verbose mode, the `create' example above would appear as: $ tar --create --verbose --file=collection.tar blues folk jazz blues folk jazz This example is just like the example we showed which did not use `--verbose', except that `tar' generated the remaining lines . In the rest of the examples in this chapter, we will frequently use `verbose' mode so we can show actions or `tar' responses that you would otherwise not see, and which are important for you to understand. File: tar.info, Node: short create, Next: create dir, Prev: create verbose, Up: create 2.6.4 Short Forms with `create' ------------------------------- As we said before, the `--create' (`-c') operation is one of the most basic uses of `tar', and you will use it countless times. Eventually, you will probably want to use abbreviated (or "short") forms of options. A full discussion of the three different forms that options can take appears in *Note Styles::; for now, here is what the previous example (including the `--verbose' (`-v') option) looks like using short option forms: $ tar -cvf collection.tar blues folk jazz blues folk jazz As you can see, the system responds the same no matter whether you use long or short option forms. One difference between using short and long option forms is that, although the exact placement of arguments following options is no more specific when using short forms, it is easier to become confused and make a mistake when using short forms. For example, suppose you attempted the above example in the following way: $ tar -cfv collection.tar blues folk jazz In this case, `tar' will make an archive file called `v', containing the files `blues', `folk', and `jazz', because the `v' is the closest "file name" to the `-f' option, and is thus taken to be the chosen archive file name. `tar' will try to add a file called `collection.tar' to the `v' archive file; if the file `collection.tar' did not already exist, `tar' will report an error indicating that this file does not exist. If the file `collection.tar' does already exist (e.g., from a previous command you may have run), then `tar' will add this file to the archive. Because the `-v' option did not get registered, `tar' will not run under `verbose' mode, and will not report its progress. The end result is that you may be quite confused about what happened, and possibly overwrite a file. To illustrate this further, we will show you how an example we showed previously would look using short forms. This example, $ tar blues --create folk --file=collection.tar jazz is confusing as it is. When shown using short forms, however, it becomes much more so: $ tar blues -c folk -f collection.tar jazz It would be very easy to put the wrong string of characters immediately following the `-f', but doing that could sacrifice valuable data. For this reason, we recommend that you pay very careful attention to the order of options and placement of file and archive names, especially when using short option forms. Not having the option name written out mnemonically can affect how well you remember which option does what, and therefore where different names have to be placed. (Placing options in an unusual order can also cause `tar' to report an error if you have set the shell environment variable `POSIXLY_CORRECT'.) File: tar.info, Node: create dir, Prev: short create, Up: create 2.6.5 Archiving Directories --------------------------- You can archive a directory by specifying its directory name as a file name argument to `tar'. The files in the directory will be archived relative to the working directory, and the directory will be re-created along with its contents when the archive is extracted. To archive a directory, first move to its superior directory. If you have followed the previous instructions in this tutorial, you should type: $ cd .. $ This will put you into the directory which contains `practice', i.e. your home directory. Once in the superior directory, you can specify the subdirectory, `practice', as a file name argument. To store `practice' in the new archive file `music.tar', type: $ tar --create --verbose --file=music.tar practice `tar' should output: practice/ practice/blues practice/folk practice/jazz practice/collection.tar Note that the archive thus created is not in the subdirectory `practice', but rather in the current working directory--the directory from which `tar' was invoked. Before trying to archive a directory from its superior directory, you should make sure you have write access to the superior directory itself, not only the directory you are trying archive with `tar'. For example, you will probably not be able to store your home directory in an archive by invoking `tar' from the root directory; *Note absolute::. (Note also that `collection.tar', the original archive file, has itself been archived. `tar' will accept any file as a file to be archived, regardless of its content. When `music.tar' is extracted, the archive file `collection.tar' will be re-written into the file system). If you give `tar' a command such as $ tar --create --file=foo.tar . `tar' will report `tar: ./foo.tar is the archive; not dumped'. This happens because `tar' creates the archive `foo.tar' in the current directory before putting any files into it. Then, when `tar' attempts to add all the files in the directory `.' to the archive, it notices that the file `./foo.tar' is the same as the archive `foo.tar', and skips it. (It makes no sense to put an archive into itself.) GNU `tar' will continue in this case, and create the archive normally, except for the exclusion of that one file. (_Please note:_ Other versions of `tar' are not so clever; they will enter an infinite loop when this happens, so you should not depend on this behavior unless you are certain you are running GNU `tar'.) File: tar.info, Node: list, Next: extract, Prev: create, Up: Tutorial 2.7 How to List Archives ======================== Frequently, you will find yourself wanting to determine exactly what a particular archive contains. You can use the `--list' (`-t') operation to get the member names as they currently appear in the archive, as well as various attributes of the files at the time they were archived. For example, you can examine the archive `collection.tar' that you created in the last section with the command, $ tar --list --file=collection.tar The output of `tar' would then be: blues folk jazz The archive `bfiles.tar' would list as follows: ./birds baboon ./box Be sure to use a `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') option just as with `--create' (`-c') to specify the name of the archive. If you use the `--verbose' (`-v') option with `--list', then `tar' will print out a listing reminiscent of `ls -l', showing owner, file size, and so forth. If you had used `--verbose' (`-v') mode, the example above would look like: $ tar --list --verbose --file=collection.tar folk -rw-rw-rw- myself user 62 1990-05-23 10:55 folk You can specify one or more individual member names as arguments when using `list'. In this case, `tar' will only list the names of members you identify. For example, `tar --list --file=afiles.tar apple' would only print `apple'. Because `tar' preserves paths, file names must be specified as they appear in the archive (ie., relative to the directory from which the archive was created). Therefore, it is essential when specifying member names to `tar' that you give the exact member names. For example, `tar --list --file=bfiles birds' would produce an error message something like `tar: birds: Not found in archive', because there is no member named `birds', only one named `./birds'. While the names `birds' and `./birds' name the same file, _member_ names are compared using a simplistic name comparison, in which an exact match is necessary. *Note absolute::. However, `tar --list --file=collection.tar folk' would respond with `folk', because `folk' is in the archive file `collection.tar'. If you are not sure of the exact file name, try listing all the files in the archive and searching for the one you expect to find; remember that if you use `--list' with no file names as arguments, `tar' will print the names of all the members stored in the specified archive. * Menu: * list dir:: File: tar.info, Node: list dir, Up: list Listing the Contents of a Stored Directory ------------------------------------------ To get information about the contents of an archived directory, use the directory name as a file name argument in conjunction with `--list' (`-t'). To find out file attributes, include the `--verbose' (`-v') option. For example, to find out about files in the directory `practice', in the archive file `music.tar', type: $ tar --list --verbose --file=music.tar practice `tar' responds: drwxrwxrwx myself user 0 1990-05-31 21:49 practice/ -rw-rw-rw- myself user 42 1990-05-21 13:29 practice/blues -rw-rw-rw- myself user 62 1990-05-23 10:55 practice/folk -rw-rw-rw- myself user 40 1990-05-21 13:30 practice/jazz -rw-rw-rw- myself user 10240 1990-05-31 21:49 practice/collection.tar When you use a directory name as a file name argument, `tar' acts on all the files (including sub-directories) in that directory. File: tar.info, Node: extract, Next: going further, Prev: list, Up: Tutorial 2.8 How to Extract Members from an Archive ========================================== _(This message will disappear, once this node revised.)_ Creating an archive is only half the job--there is no point in storing files in an archive if you can't retrieve them. The act of retrieving members from an archive so they can be used and manipulated as unarchived files again is called "extraction". To extract files from an archive, use the `--extract' (`--get', `-x') operation. As with `--create' (`-c'), specify the name of the archive with `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME'). Extracting an archive does not modify the archive in any way; you can extract it multiple times if you want or need to. Using `--extract', you can extract an entire archive, or specific files. The files can be directories containing other files, or not. As with `--create' (`-c') and `--list' (`-t'), you may use the short or the long form of the operation without affecting the performance. * Menu: * extracting archives:: * extracting files:: * extract dir:: * extracting untrusted archives:: * failing commands:: File: tar.info, Node: extracting archives, Next: extracting files, Up: extract 2.8.1 Extracting an Entire Archive ---------------------------------- To extract an entire archive, specify the archive file name only, with no individual file names as arguments. For example, $ tar -xvf collection.tar produces this: -rw-rw-rw- me user 28 1996-10-18 16:31 jazz -rw-rw-rw- me user 21 1996-09-23 16:44 blues -rw-rw-rw- me user 20 1996-09-23 16:44 folk File: tar.info, Node: extracting files, Next: extract dir, Prev: extracting archives, Up: extract 2.8.2 Extracting Specific Files ------------------------------- To extract specific archive members, give their exact member names as arguments, as printed by `--list' (`-t'). If you had mistakenly deleted one of the files you had placed in the archive `collection.tar' earlier (say, `blues'), you can extract it from the archive without changing the archive's structure. It will be identical to the original file `blues' that you deleted. . First, make sure you are in the `practice' directory, and list the files in the directory. Now, delete the file, `blues', and list the files in the directory again. You can now extract the member `blues' from the archive file `collection.tar' like this: $ tar --extract --file=collection.tar blues If you list the files in the directory again, you will see that the file `blues' has been restored, with its original permissions, creation times, and owner. (These parameters will be identical to those which the file had when you originally placed it in the archive; any changes you may have made before deleting the file from the file system, however, will _not_ have been made to the archive member.) The archive file, `collection.tar', is the same as it was before you extracted `blues'. You can confirm this by running `tar' with `--list' (`-t'). Remember that as with other operations, specifying the exact member name is important. `tar --extract --file=bfiles.tar birds' will fail, because there is no member named `birds'. To extract the member named `./birds', you must specify `tar --extract --file=bfiles.tar ./birds'. To find the exact member names of the members of an archive, use `--list' (`-t') (*note list::). You can extract a file to standard output by combining the above options with the `--to-stdout' (`-O') option (*note Writing to Standard Output::). If you give the `--verbose' (`-v') option, then `--extract' (`--get', `-x') will print the names of the archive members as it extracts them. File: tar.info, Node: extract dir, Next: extracting untrusted archives, Prev: extracting files, Up: extract 2.8.3 Extracting Files that are Directories ------------------------------------------- Extracting directories which are members of an archive is similar to extracting other files. The main difference to be aware of is that if the extracted directory has the same name as any directory already in the working directory, then files in the extracted directory will be placed into the directory of the same name. Likewise, if there are files in the pre-existing directory with the same names as the members which you extract, the files from the extracted archive will replace the files already in the working directory (and possible subdirectories). This will happen regardless of whether or not the files in the working directory were more recent than those extracted (there exist, however, special options that alter this behavior *note Writing::). However, if a file was stored with a directory name as part of its file name, and that directory does not exist under the working directory when the file is extracted, `tar' will create the directory. We can demonstrate how to use `--extract' to extract a directory file with an example. Change to the `practice' directory if you weren't there, and remove the files `folk' and `jazz'. Then, go back to the parent directory and extract the archive `music.tar'. You may either extract the entire archive, or you may extract only the files you just deleted. To extract the entire archive, don't give any file names as arguments after the archive name `music.tar'. To extract only the files you deleted, use the following command: $ tar -xvf music.tar practice/folk practice/jazz practice/folk practice/jazz If you were to specify two `--verbose' (`-v') options, `tar' would have displayed more detail about the extracted files, as shown in the example below: $ tar -xvvf music.tar practice/folk practice/jazz -rw-rw-rw- me user 28 1996-10-18 16:31 practice/jazz -rw-rw-rw- me user 20 1996-09-23 16:44 practice/folk Because you created the directory with `practice' as part of the file names of each of the files by archiving the `practice' directory as `practice', you must give `practice' as part of the file names when you extract those files from the archive. File: tar.info, Node: extracting untrusted archives, Next: failing commands, Prev: extract dir, Up: extract 2.8.4 Extracting Archives from Untrusted Sources ------------------------------------------------ Extracting files from archives can overwrite files that already exist. If you receive an archive from an untrusted source, you should make a new directory and extract into that directory, so that you don't have to worry about the extraction overwriting one of your existing files. For example, if `untrusted.tar' came from somewhere else on the Internet, and you don't necessarily trust its contents, you can extract it as follows: $ mkdir newdir $ cd newdir $ tar -xvf ../untrusted.tar It is also a good practice to examine contents of the archive before extracting it, using `--list' (`-t') option, possibly combined with `--verbose' (`-v'). File: tar.info, Node: failing commands, Prev: extracting untrusted archives, Up: extract 2.8.5 Commands That Will Fail ----------------------------- Here are some sample commands you might try which will not work, and why they won't work. If you try to use this command, $ tar -xvf music.tar folk jazz you will get the following response: tar: folk: Not found in archive tar: jazz: Not found in archive $ This is because these files were not originally _in_ the parent directory `..', where the archive is located; they were in the `practice' directory, and their file names reflect this: $ tar -tvf music.tar practice/folk practice/jazz practice/rock Likewise, if you try to use this command, $ tar -tvf music.tar folk jazz you would get a similar response. Members with those names are not in the archive. You must use the correct member names in order to extract the files from the archive. If you have forgotten the correct names of the files in the archive, use `tar --list --verbose' to list them correctly. File: tar.info, Node: going further, Prev: extract, Up: Tutorial 2.9 Going Further Ahead in this Manual ====================================== File: tar.info, Node: tar invocation, Next: operations, Prev: Tutorial, Up: Top 3 Invoking GNU `tar' ******************** _(This message will disappear, once this node revised.)_ This chapter is about how one invokes the GNU `tar' command, from the command synopsis (*note Synopsis::). There are numerous options, and many styles for writing them. One mandatory option specifies the operation `tar' should perform (*note Operation Summary::), other options are meant to detail how this operation should be performed (*note Option Summary::). Non-option arguments are not always interpreted the same way, depending on what the operation is. You will find in this chapter everything about option styles and rules for writing them (*note Styles::). On the other hand, operations and options are fully described elsewhere, in other chapters. Here, you will find only synthetic descriptions for operations and options, together with pointers to other parts of the `tar' manual. Some options are so special they are fully described right in this chapter. They have the effect of inhibiting the normal operation of `tar' or else, they globally alter the amount of feedback the user receives about what is going on. These are the `--help' and `--version' (*note help::), `--verbose' (`-v') (*note verbose::) and `--interactive' (`-w') options (*note interactive::). * Menu: * Synopsis:: * using tar options:: * Styles:: * All Options:: * help:: * verbose:: * interactive:: File: tar.info, Node: Synopsis, Next: using tar options, Up: tar invocation 3.1 General Synopsis of `tar' ============================= The GNU `tar' program is invoked as either one of: tar OPTION... [NAME]... tar LETTER... [ARGUMENT]... [OPTION]... [NAME]... The second form is for when old options are being used. You can use `tar' to store files in an archive, to extract them from an archive, and to do other types of archive manipulation. The primary argument to `tar', which is called the "operation", specifies which action to take. The other arguments to `tar' are either "options", which change the way `tar' performs an operation, or file names or archive members, which specify the files or members `tar' is to act on. You can actually type in arguments in any order, even if in this manual the options always precede the other arguments, to make examples easier to understand. Further, the option stating the main operation mode (the `tar' main command) is usually given first. Each NAME in the synopsis above is interpreted as an archive member name when the main command is one of `--compare' (`--diff', `-d'), `--delete', `--extract' (`--get', `-x'), `--list' (`-t') or `--update' (`-u'). When naming archive members, you must give the exact name of the member in the archive, as it is printed by `--list' (`-t'). For `--append' (`-r') and `--create' (`-c'), these NAME arguments specify the names of either files or directory hierarchies to place in the archive. These files or hierarchies should already exist in the file system, prior to the execution of the `tar' command. `tar' interprets relative file names as being relative to the working directory. `tar' will make all file names relative (by removing leading slashes when archiving or restoring files), unless you specify otherwise (using the `--absolute-names' (`-P') option). *Note absolute::, for more information about `--absolute-names' (`-P'). If you give the name of a directory as either a file name or a member name, then `tar' acts recursively on all the files and directories beneath that directory. For example, the name `/' identifies all the files in the filesystem to `tar'. The distinction between file names and archive member names is especially important when shell globbing is used, and sometimes a source of confusion for newcomers. *Note Wildcards::, for more information about globbing. The problem is that shells may only glob using existing files in the file system. Only `tar' itself may glob on archive members, so when needed, you must ensure that wildcard characters reach `tar' without being interpreted by the shell first. Using a backslash before `*' or `?', or putting the whole argument between quotes, is usually sufficient for this. Even if NAMEs are often specified on the command line, they can also be read from a text file in the file system, using the `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') option. If you don't use any file name arguments, `--append' (`-r'), `--delete' and `--concatenate' (`--catenate', `-A') will do nothing, while `--create' (`-c') will usually yield a diagnostic and inhibit `tar' execution. The other operations of `tar' (`--list' (`-t'), `--extract' (`--get', `-x'), `--compare' (`--diff', `-d'), and `--update' (`-u')) will act on the entire contents of the archive. Besides successful exits, GNU `tar' may fail for many reasons. Some reasons correspond to bad usage, that is, when the `tar' command is improperly written. Errors may be encountered later, while encountering an error processing the archive or the files. Some errors are recoverable, in which case the failure is delayed until `tar' has completed all its work. Some errors are such that it would not meaningful, or at least risky, to continue processing: `tar' then aborts processing immediately. All abnormal exits, whether immediate or delayed, should always be clearly diagnosed on `stderr', after a line stating the nature of the error. GNU `tar' returns only a few exit statuses. I'm really aiming simplicity in that area, for now. If you are not using the `--compare' (`--diff', `-d') option, zero means that everything went well, besides maybe innocuous warnings. Nonzero means that something went wrong. Right now, as of today, "nonzero" is almost always 2, except for remote operations, where it may be 128. File: tar.info, Node: using tar options, Next: Styles, Prev: Synopsis, Up: tar invocation 3.2 Using `tar' Options ======================= GNU `tar' has a total of eight operating modes which allow you to perform a variety of tasks. You are required to choose one operating mode each time you employ the `tar' program by specifying one, and only one operation as an argument to the `tar' command (two lists of four operations each may be found at *Note frequent operations:: and *Note Operations::). Depending on circumstances, you may also wish to customize how the chosen operating mode behaves. For example, you may wish to change the way the output looks, or the format of the files that you wish to archive may require you to do something special in order to make the archive look right. You can customize and control `tar''s performance by running `tar' with one or more options (such as `--verbose' (`-v'), which we used in the tutorial). As we said in the tutorial, "options" are arguments to `tar' which are (as their name suggests) optional. Depending on the operating mode, you may specify one or more options. Different options will have different effects, but in general they all change details of the operation, such as archive format, archive name, or level of user interaction. Some options make sense with all operating modes, while others are meaningful only with particular modes. You will likely use some options frequently, while you will only use others infrequently, or not at all. (A full list of options is available in *note All Options::.) The `TAR_OPTIONS' environment variable specifies default options to be placed in front of any explicit options. For example, if `TAR_OPTIONS' is `-v --unlink-first', `tar' behaves as if the two options `-v' and `--unlink-first' had been specified before any explicit options. Option specifications are separated by whitespace. A backslash escapes the next character, so it can be used to specify an option containing whitespace or a backslash. Note that `tar' options are case sensitive. For example, the options `-T' and `-t' are different; the first requires an argument for stating the name of a file providing a list of NAMEs, while the second does not require an argument and is another way to write `--list' (`-t'). In addition to the eight operations, there are many options to `tar', and three different styles for writing both: long (mnemonic) form, short form, and old style. These styles are discussed below. Both the options and the operations can be written in any of these three styles. File: tar.info, Node: Styles, Next: All Options, Prev: using tar options, Up: tar invocation 3.3 The Three Option Styles =========================== There are three styles for writing operations and options to the command line invoking `tar'. The different styles were developed at different times during the history of `tar'. These styles will be presented below, from the most recent to the oldest. Some options must take an argument. (For example, `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') takes the name of an archive file as an argument. If you do not supply an archive file name, `tar' will use a default, but this can be confusing; thus, we recommend that you always supply a specific archive file name.) Where you _place_ the arguments generally depends on which style of options you choose. We will detail specific information relevant to each option style in the sections on the different option styles, below. The differences are subtle, yet can often be very important; incorrect option placement can cause you to overwrite a number of important files. We urge you to note these differences, and only use the option style(s) which makes the most sense to you until you feel comfortable with the others. Some options _may_ take an argument (currently, there are two such options: `--backup' and `--occurrence'). Such options may have at most long and short forms, they do not have old style equivalent. The rules for specifying an argument for such options are stricter than those for specifying mandatory arguments. Please, pay special attention to them. * Menu: * Mnemonic Options:: Mnemonic Option Style * Short Options:: Short Option Style * Old Options:: Old Option Style * Mixing:: Mixing Option Styles File: tar.info, Node: Mnemonic Options, Next: Short Options, Up: Styles 3.3.1 Mnemonic Option Style --------------------------- Each option has at least one long (or mnemonic) name starting with two dashes in a row, e.g. `--list'. The long names are more clear than their corresponding short or old names. It sometimes happens that a single mnemonic option has many different different names which are synonymous, such as `--compare' and `--diff'. In addition, long option names can be given unique abbreviations. For example, `--cre' can be used in place of `--create' because there is no other mnemonic option which begins with `cre'. (One way to find this out is by trying it and seeing what happens; if a particular abbreviation could represent more than one option, `tar' will tell you that that abbreviation is ambiguous and you'll know that that abbreviation won't work. You may also choose to run `tar --help' to see a list of options. Be aware that if you run `tar' with a unique abbreviation for the long name of an option you didn't want to use, you are stuck; `tar' will perform the command as ordered.) Mnemonic options are meant to be obvious and easy to remember, and their meanings are generally easier to discern than those of their corresponding short options (see below). For example: $ tar --create --verbose --blocking-factor=20 --file=/dev/rmt0 gives a fairly good set of hints about what the command does, even for those not fully acquainted with `tar'. Mnemonic options which require arguments take those arguments immediately following the option name. There are two ways of specifying a mandatory argument. It can be separated from the option name either by an equal sign, or by any amount of white space characters. For example, the `--file' option (which tells the name of the `tar' archive) is given a file such as `archive.tar' as argument by using any of the following notations: `--file=archive.tar' or `--file archive.tar'. In contrast, optional arguments must always be introduced using an equal sign. For example, the `--backup' option takes an optional argument specifying backup type. It must be used as `--backup=BACKUP-TYPE'. File: tar.info, Node: Short Options, Next: Old Options, Prev: Mnemonic Options, Up: Styles 3.3.2 Short Option Style ------------------------ Most options also have a short option name. Short options start with a single dash, and are followed by a single character, e.g. `-t' (which is equivalent to `--list'). The forms are absolutely identical in function; they are interchangeable. The short option names are faster to type than long option names. Short options which require arguments take their arguments immediately following the option, usually separated by white space. It is also possible to stick the argument right after the short option name, using no intervening space. For example, you might write `-f archive.tar' or `-farchive.tar' instead of using `--file=archive.tar'. Both `--file=ARCHIVE-NAME' and `-f ARCHIVE-NAME' denote the option which indicates a specific archive, here named `archive.tar'. Short options which take optional arguments take their arguments immediately following the option letter, _without any intervening white space characters_. Short options' letters may be clumped together, but you are not required to do this (as compared to old options; see below). When short options are clumped as a set, use one (single) dash for them all, e.g. ``tar' -cvf'. Only the last option in such a set is allowed to have an argument(1). When the options are separated, the argument for each option which requires an argument directly follows that option, as is usual for Unix programs. For example: $ tar -c -v -b 20 -f /dev/rmt0 If you reorder short options' locations, be sure to move any arguments that belong to them. If you do not move the arguments properly, you may end up overwriting files. ---------- Footnotes ---------- (1) Clustering many options, the last of which has an argument, is a rather opaque way to write options. Some wonder if GNU `getopt' should not even be made helpful enough for considering such usages as invalid. File: tar.info, Node: Old Options, Next: Mixing, Prev: Short Options, Up: Styles 3.3.3 Old Option Style ---------------------- _(This message will disappear, once this node revised.)_ Like short options, old options are single letters. However, old options must be written together as a single clumped set, without spaces separating them or dashes preceding them(1). This set of letters must be the first to appear on the command line, after the `tar' program name and some white space; old options cannot appear anywhere else. The letter of an old option is exactly the same letter as the corresponding short option. For example, the old option `t' is the same as the short option `-t', and consequently, the same as the mnemonic option `--list'. So for example, the command `tar cv' specifies the option `-v' in addition to the operation `-c'. When options that need arguments are given together with the command, all the associated arguments follow, in the same order as the options. Thus, the example given previously could also be written in the old style as follows: $ tar cvbf 20 /dev/rmt0 Here, `20' is the argument of `-b' and `/dev/rmt0' is the argument of `-f'. On the other hand, this old style syntax makes it difficult to match option letters with their corresponding arguments, and is often confusing. In the command `tar cvbf 20 /dev/rmt0', for example, `20' is the argument for `-b', `/dev/rmt0' is the argument for `-f', and `-v' does not have a corresponding argument. Even using short options like in `tar -c -v -b 20 -f /dev/rmt0' is clearer, putting all arguments next to the option they pertain to. If you want to reorder the letters in the old option argument, be sure to reorder any corresponding argument appropriately. This old way of writing `tar' options can surprise even experienced users. For example, the two commands: tar cfz archive.tar.gz file tar -cfz archive.tar.gz file are quite different. The first example uses `archive.tar.gz' as the value for option `f' and recognizes the option `z'. The second example, however, uses `z' as the value for option `f' -- probably not what was intended. Old options are kept for compatibility with old versions of `tar'. This second example could be corrected in many ways, among which the following are equivalent: tar -czf archive.tar.gz file tar -cf archive.tar.gz -z file tar cf archive.tar.gz -z file As far as we know, all `tar' programs, GNU and non-GNU, support old options. GNU `tar' supports them not only for historical reasons, but also because many people are used to them. For compatibility with Unix `tar', the first argument is always treated as containing command and option letters even if it doesn't start with `-'. Thus, `tar c' is equivalent to `tar -c': both of them specify the `--create' (`-c') command to create an archive. ---------- Footnotes ---------- (1) Beware that if you precede options with a dash, you are announcing the short option style instead of the old option style; short options are decoded differently. File: tar.info, Node: Mixing, Prev: Old Options, Up: Styles 3.3.4 Mixing Option Styles -------------------------- All three styles may be intermixed in a single `tar' command, so long as the rules for each style are fully respected(1). Old style options and either of the modern styles of options may be mixed within a single `tar' command. However, old style options must be introduced as the first arguments only, following the rule for old options (old options must appear directly after the `tar' command and some white space). Modern options may be given only after all arguments to the old options have been collected. If this rule is not respected, a modern option might be falsely interpreted as the value of the argument to one of the old style options. For example, all the following commands are wholly equivalent, and illustrate the many combinations and orderings of option styles. tar --create --file=archive.tar tar --create -f archive.tar tar --create -farchive.tar tar --file=archive.tar --create tar --file=archive.tar -c tar -c --file=archive.tar tar -c -f archive.tar tar -c -farchive.tar tar -cf archive.tar tar -cfarchive.tar tar -f archive.tar --create tar -f archive.tar -c tar -farchive.tar --create tar -farchive.tar -c tar c --file=archive.tar tar c -f archive.tar tar c -farchive.tar tar cf archive.tar tar f archive.tar --create tar f archive.tar -c tar fc archive.tar On the other hand, the following commands are _not_ equivalent to the previous set: tar -f -c archive.tar tar -fc archive.tar tar -fcarchive.tar tar -farchive.tarc tar cfarchive.tar These last examples mean something completely different from what the user intended (judging based on the example in the previous set which uses long options, whose intent is therefore very clear). The first four specify that the `tar' archive would be a file named `-c', `c', `carchive.tar' or `archive.tarc', respectively. The first two examples also specify a single non-option, NAME argument having the value `archive.tar'. The last example contains only old style option letters (repeating option `c' twice), not all of which are meaningful (eg., `.', `h', or `i'), with no argument value. ---------- Footnotes ---------- (1) Before GNU `tar' version 1.11.6, a bug prevented intermixing old style options with mnemonic options in some cases. File: tar.info, Node: All Options, Next: help, Prev: Styles, Up: tar invocation 3.4 All `tar' Options ===================== The coming manual sections contain an alphabetical listing of all `tar' operations and options, with brief descriptions and cross references to more in-depth explanations in the body of the manual. They also contain an alphabetically arranged table of the short option forms with their corresponding long option. You can use this table as a reference for deciphering `tar' commands in scripts. * Menu: * Operation Summary:: * Option Summary:: * Short Option Summary:: File: tar.info, Node: Operation Summary, Next: Option Summary, Up: All Options 3.4.1 Operations ---------------- `--append' `-r' Appends files to the end of the archive. *Note append::. `--catenate' `-A' Same as `--concatenate'. *Note concatenate::. `--compare' `-d' Compares archive members with their counterparts in the file system, and reports differences in file size, mode, owner, modification date and contents. *Note compare::. `--concatenate' `-A' Appends other `tar' archives to the end of the archive. *Note concatenate::. `--create' `-c' Creates a new `tar' archive. *Note create::. `--delete' Deletes members from the archive. Don't try this on a archive on a tape! *Note delete::. `--diff' `-d' Same `--compare'. *Note compare::. `--extract' `-x' Extracts members from the archive into the file system. *Note extract::. `--get' `-x' Same as `--extract'. *Note extract::. `--list' `-t' Lists the members in an archive. *Note list::. `--update' `-u' Adds files to the end of the archive, but only if they are newer than their counterparts already in the archive, or if they do not already exist in the archive. *Note update::. File: tar.info, Node: Option Summary, Next: Short Option Summary, Prev: Operation Summary, Up: All Options 3.4.2 `tar' Options ------------------- `--absolute-names' `-P' Normally when creating an archive, `tar' strips an initial `/' from member names. This option disables that behavior. `--after-date' (See `--newer'.) `--anchored' An exclude pattern must match an initial subsequence of the name's components. `--atime-preserve' Tells `tar' to preserve the access time field in a file's inode when reading it. Due to limitations in the `utimes' system call, the modification time field is also preserved, which may cause problems if the file is simultaneously being modified by another program. This option is incompatible with incremental backups, because preserving the access time involves updating the last-changed time. Also, this option does not work on files that you do not own, unless you're root. `--backup=BACKUP-TYPE' Rather than deleting files from the file system, `tar' will back them up using simple or numbered backups, depending upon BACKUP-TYPE. `--block-number' `-R' With this option present, `tar' prints error messages for read errors with the block number in the archive file. `--blocking-factor=BLOCKING' `-b BLOCKING' Sets the blocking factor `tar' uses to BLOCKING x 512 bytes per record. `--bzip2' `-j' This option tells `tar' to read or write archives through `bzip2'. `--checkpoint' This option directs `tar' to print periodic checkpoint messages as it reads through the archive. Its intended for when you want a visual indication that `tar' is still running, but don't want to see `--verbose' output. `--check-links' `-l' If this option was given, `tar' will check the number of links dumped for each processed file. If this number does not match the total number of hard links for the file, a warning message will be output. Future versions will take `-l' as a short version of `--check-links'. However, current release still retains the old semantics for `-l'. *Note Current status::, for more information. `--compress' `--uncompress' `-Z' `tar' will use the `compress' program when reading or writing the archive. This allows you to directly act on archives while saving space. `--confirmation' (See `--interactive'.) `--dereference' `-h' When creating a `tar' archive, `tar' will archive the file that a symbolic link points to, rather than archiving the symlink. `--directory=DIR' `-C DIR' When this option is specified, `tar' will change its current directory to DIR before performing any operations. When this option is used during archive creation, it is order sensitive. `--exclude=PATTERN' When performing operations, `tar' will skip files that match PATTERN. `--exclude-from=FILE' `-X FILE' Similar to `--exclude', except `tar' will use the list of patterns in the file FILE. `--exclude-caches' Automatically excludes all directories containing a cache directory tag. `--file=ARCHIVE' `-f ARCHIVE' `tar' will use the file ARCHIVE as the `tar' archive it performs operations on, rather than `tar''s compilation dependent default. `--files-from=FILE' `-T FILE' `tar' will use the contents of FILE as a list of archive members or files to operate on, in addition to those specified on the command-line. `--force-local' Forces `tar' to interpret the filename given to `--file' as a local file, even if it looks like a remote tape drive name. `--format=FORMAT' Selects output archive format. FORMAT may be one of the following: `v7' Creates an archive that is compatible with Unix V7 `tar'. `oldgnu' Creates an archive that is compatible with GNU `tar' version 1.12 or earlier. `gnu' Creates archive in GNU tar 1.13 format. Basically it is the same as `oldgnu' with the only difference in the way it handles long numeric fields. `ustar' Creates a POSIX.1-1988 compatible archive. `posix' Creates a POSIX.1-2001 archive. *Note Formats::, for a detailed discussion of these formats. `--group=GROUP' Files added to the `tar' archive will have a group id of GROUP, rather than the group from the source file. GROUP is first decoded as a group symbolic name, but if this interpretation fails, it has to be a decimal numeric group ID. Also see the comments for the `--owner=USER' option. `--gzip' `--gunzip' `--ungzip' `-z' This option tells `tar' to read or write archives through `gzip', allowing `tar' to directly operate on several kinds of compressed archives transparently. `--help' `tar' will print out a short message summarizing the operations and options to `tar' and exit. `--ignore-case' Ignore case when excluding files. `--ignore-failed-read' Do not exit unsuccessfully merely because an unreadable file was encountered. *Note Reading::. `--ignore-zeros' `-i' With this option, `tar' will ignore zeroed blocks in the archive, which normally signals EOF. *Note Reading::. `--incremental' `-G' Used to inform `tar' that it is working with an old GNU-format incremental backup archive. It is intended primarily for backwards compatibility only. `--index-file=FILE' Send verbose output to FILE instead of to standard output. `--info-script=SCRIPT-FILE' `--new-volume-script=SCRIPT-FILE' `-F SCRIPT-FILE' When `tar' is performing multi-tape backups, SCRIPT-FILE is run at the end of each tape. If SCRIPT-FILE exits with nonzero status, `tar' fails immediately. `--interactive' `--confirmation' `-w' Specifies that `tar' should ask the user for confirmation before performing potentially destructive options, such as overwriting files. `--keep-newer-files' Do not replace existing files that are newer than their archive copies when extracting files from an archive. `--keep-old-files' `-k' Do not overwrite existing files when extracting files from an archive. *Note Writing::. `--label=NAME' `-V NAME' When creating an archive, instructs `tar' to write NAME as a name record in the archive. When extracting or listing archives, `tar' will only operate on archives that have a label matching the pattern specified in NAME. `--listed-incremental=SNAPSHOT-FILE' `-g SNAPSHOT-FILE' During a `--create' operation, specifies that the archive that `tar' creates is a new GNU-format incremental backup, using SNAPSHOT-FILE to determine which files to backup. With other operations, informs `tar' that the archive is in incremental format. `--mode=PERMISSIONS' When adding files to an archive, `tar' will use PERMISSIONS for the archive members, rather than the permissions from the files. The program `chmod' and this `tar' option share the same syntax for what PERMISSIONS might be. *Note Permissions: (fileutils)File permissions. This reference also has useful information for those not being overly familiar with the Unix permission system. Of course, PERMISSIONS might be plainly specified as an octal number. However, by using generic symbolic modifications to mode bits, this allows more flexibility. For example, the value `a+rw' adds read and write permissions for everybody, while retaining executable bits on directories or on any other file already marked as executable. `--multi-volume' `-M' Informs `tar' that it should create or otherwise operate on a multi-volume `tar' archive. `--new-volume-script' (see -info-script) `-n' `--seek' Assume that the archive media supports seeks to arbitrary locations. Usually `tar' determines automatically whether the archive can be seeked or not. This option is intended for use in cases when such recognition fails. `--newer=DATE' `--after-date=DATE' `-N' When creating an archive, `tar' will only add files that have changed since DATE. If DATE begins with `/' or `.', it is taken to be the name of a file whose last-modified time specifies the date. `--newer-mtime=DATE' Like `--newer', but add only files whose contents have changed (as opposed to just `--newer', which will also back up files for which any status information has changed). `--no-anchored' An exclude pattern can match any subsequence of the name's components. `--no-ignore-case' Use case-sensitive matching when excluding files. `--no-recursion' With this option, `tar' will not recurse into directories. `--no-same-owner' `-o' When extracting an archive, do not attempt to preserve the owner specified in the `tar' archive. This the default behavior for ordinary users; this option has an effect only for the superuser. `--no-same-permissions' When extracting an archive, subtract the user's umask from files from the permissions specified in the archive. This is the default behavior for ordinary users; this option has an effect only for the superuser. `--no-wildcards' Do not use wildcards when excluding files. `--no-wildcards-match-slash' Wildcards do not match `/' when excluding files. `--null' When `tar' is using the `--files-from' option, this option instructs `tar' to expect filenames terminated with `NUL', so `tar' can correctly work with file names that contain newlines. `--numeric-owner' This option will notify `tar' that it should use numeric user and group IDs when creating a `tar' file, rather than names. `-o' When extracting files, this option is a synonym for `--no-same-owner', i.e. it prevents `tar' from restoring ownership of files being extracted. When creating an archive, `-o' is a synonym for `--old-archive'. This behavior is for compatibility with previous versions of GNU `tar', and will be removed in the future releases. *Note Current status::, for more information. `--occurrence[=NUMBER]' This option can be used in conjunction with one of the subcommands `--delete', `--diff', `--extract' or `--list' when a list of files is given either on the command line or via `-T' option. This option instructs `tar' to process only the NUMBERth occurrence of each named file. NUMBER defaults to 1, so tar -x -f archive.tar --occurrence filename will extract the first occurrence of `filename' from `archive.tar' and will terminate without scanning to the end of the archive. `--old-archive' Synonym for `--format=v7'. `--one-file-system' `-l' Used when creating an archive. Prevents `tar' from recursing into directories that are on different file systems from the current directory. Earlier versions of GNU `tar' understood `-l' as a synonym for `--one-file-system'. Although such usage is still allowed in the present version, it is _strongly discouraged_. The future versions of GNU `tar' will use `-l' as a synonym for `--check-links'. *Note Current status::, for more information. `--overwrite' Overwrite existing files and directory metadata when extracting files from an archive. *Note Overwrite Old Files::. `--overwrite-dir' Overwrite the metadata of existing directories when extracting files from an archive. *Note Overwrite Old Files::. `--owner=USER' Specifies that `tar' should use USER as the owner of members when creating archives, instead of the user associated with the source file. USER is first decoded as a user symbolic name, but if this interpretation fails, it has to be a decimal numeric user ID. There is no value indicating a missing number, and `0' usually means `root'. Some people like to force `0' as the value to offer in their distributions for the owner of files, because the `root' user is anonymous anyway, so that might as well be the owner of anonymous archives. This option does not affect extraction from archives. `--pax-option=KEYWORD-LIST' This option is meaningful only with POSIX.1-2001 archives (). It modifies the way `tar' handles the extended header keywords. KEYWORD-LIST is a comma-separated list of keyword options, each keyword option taking one of the following forms: delete=PATTERN When used with one of archive-creation command (), this option instructs `tar' to omit from extended header records that it produces any keywords matching the string PATTERN. When used in extract or list mode, this option instructs tar to ignore any keywords matching the given PATTERN in the extended header records. In both cases, matching is performed using the pattern matching notation described in POSIX 1003.2, 3.13 . For example: --pax-option delete=security.* would suppress security-related information. exthdr.name=STRING This keyword allows user control over the name that is written into the ustar header blocks for the extended headers. The name is obtained from STRING after substituting the following meta-characters: Meta-character Replaced By %d The directory name of the file, equivalent to the result of the `dirname' utility on the translated pathname. %f The filename of the file, equivalent to the result of the `basename' utility on the translated pathname. %p The process ID of the `tar' process. %% A `%' character. Any other `%' characters in STRING produce undefined results. If no option `exthdr.name=string' is specified, `tar' will use the following default value: %d/PaxHeaders.%p/%f globexthdr.name=STRING This keyword allows user control over the name that is written into the ustar header blocks for global extended header records. The name shall will be obtained from the contents of STRING, after the following character substitutions have been made: Meta-character Replaced By %n An integer that represents the sequence number of the global extended header record in the archive, starting at 1. %p The process ID of the `tar' process. %% A `%' character. Any other `%' characters in string produce undefined results. If no option `globexthdr.name=string' is specified, `tar' will use the following default value: $TMPDIR/GlobalHead.%p.%n where `$TMPDIR' represents the value of the TMPDIR environment variable. If TMPDIR is not set, `tar' uses `/tmp'. KEYWORD=VALUE When used with one of archive-creation commands, these keyword/value pairs will be included at the beginning of the archive in a global extended header record. When used with one of archive-reading commands, `tar' will behave as if it has encountered these keyword/value pairs at the beginning of the archive in a global extended header record. KEYWORD:=VALUE When used with one of archive-creation commands, these keyword/value pairs will be included as records at the beginning of an extended header for each file. This is effectively equivalent to KEYWORD=VALUE form except that it creates no global extended header records. When used with one of archive-reading commands, `tar' will behave as if these keyword/value pairs were included as records at the end of each extended header; thus, they will override any global or file-specific extended header record keywords of the same names. For example, in the command: tar --format=posix --create \ --file archive --pax-option gname:=user . the group name will be forced to a new value for all files stored in the archive. `--portability' `--old-archive' Synonym for `--format=v7'. `--posix' Same as `--format=posix'. `--preserve' Synonymous with specifying both `--preserve-permissions' and `--same-order'. `--preserve-order' (See `--same-order'; *note Reading::.) `--preserve-permissions' `--same-permissions' `-p' When `tar' is extracting an archive, it normally subtracts the users' umask from the permissions specified in the archive and uses that number as the permissions to create the destination file. Specifying this option instructs `tar' that it should use the permissions directly from the archive. *Note Writing::. `--read-full-records' `-B' Specifies that `tar' should reblock its input, for reading from pipes on systems with buggy implementations. *Note Reading::. `--record-size=SIZE' Instructs `tar' to use SIZE bytes per record when accessing the archive. `--recursion' With this option, `tar' recurses into directories. `--recursive-unlink' Remove existing directory hierarchies before extracting directories of the same name from the archive. *Note Writing::. `--remove-files' Directs `tar' to remove the source file from the file system after appending it to an archive. `--rmt-command=CMD' Notifies `tar' that it should use CMD instead of the default `/usr/libexec/rmt' (*note Remote Tape Server::). `--rsh-command=CMD' Notifies `tar' that is should use CMD to communicate with remote devices. `--same-order' `--preserve-order' `-s' This option is an optimization for `tar' when running on machines with small amounts of memory. It informs `tar' that the list of file arguments has already been sorted to match the order of files in the archive. *Note Reading::. `--same-owner' When extracting an archive, `tar' will attempt to preserve the owner specified in the `tar' archive with this option present. This is the default behavior for the superuser; this option has an effect only for ordinary users. `--same-permissions' (See `--preserve-permissions'; *note Writing::.) `--show-defaults' Displays the default options used by `tar' and exits successfully. This option is intended for use in shell scripts. Here is an example of what you can see using this option: $ tar --show-defaults --format=gnu -f- -b20 `--show-omitted-dirs' Instructs `tar' to mention directories its skipping over when operating on a `tar' archive. `--sparse' `-S' Invokes a GNU extension when adding files to an archive that handles sparse files efficiently. `--starting-file=NAME' `-K NAME' This option affects extraction only; `tar' will skip extracting files in the archive until it finds one that matches NAME. *Note Scarce::. `--strip-components=NUMBER' Strip given NUMBER of leading components from file names before extraction.(1) For example, if archive `archive.tar' contained `/some/file/name', then running tar --extract --file archive.tar --strip-components=2 would extracted this file to file `name'. `--suffix=SUFFIX' Alters the suffix `tar' uses when backing up files from the default `~'. `--tape-length=NUM' `-L NUM' Specifies the length of tapes that `tar' is writing as being NUM x 1024 bytes long. `--to-stdout' `-O' During extraction, `tar' will extract files to stdout rather than to the file system. *Note Writing::. `--totals' Displays the total number of bytes written after creating an archive. `--touch' `-m' Sets the modification time of extracted files to the extraction time, rather than the modification time stored in the archive. *Note Writing::. `--uncompress' (See `--compress'.) `--ungzip' (See `--gzip'.) `--unlink-first' `-U' Directs `tar' to remove the corresponding file from the file system before extracting it from the archive. *Note Writing::. `--use-compress-program=PROG' Instructs `tar' to access the archive through PROG, which is presumed to be a compression program of some sort. `--utc' Display file modification dates in UTC. This option implies `--verbose'. `--verbose' `-v' Specifies that `tar' should be more verbose about the operations its performing. This option can be specified multiple times for some operations to increase the amount of information displayed. `--verify' `-W' Verifies that the archive was correctly written when creating an archive. `--version' `tar' will print an informational message about what version it is and a copyright message, some credits, and then exit. `--volno-file=FILE' Used in conjunction with `--multi-volume'. `tar' will keep track of which volume of a multi-volume archive its working in FILE. `--wildcards' Use wildcards when excluding files. `--wildcards-match-slash' Wildcards match `/' when excluding files. ---------- Footnotes ---------- (1) This option was called `--strip-path' in version 1.14. File: tar.info, Node: Short Option Summary, Prev: Option Summary, Up: All Options 3.4.3 Short Options Cross Reference ----------------------------------- Here is an alphabetized list of all of the short option forms, matching them with the equivalent long option. `-A' `--concatenate' `-B' `--read-full-records' `-C' `--directory' `-F' `--info-script' `-G' `--incremental' `-K' `--starting-file' `-L' `--tape-length' `-M' `--multi-volume' `-N' `--newer' `-O' `--to-stdout' `-P' `--absolute-names' `-R' `--block-number' `-S' `--sparse' `-T' `--files-from' `-U' `--unlink-first' `-V' `--label' `-W' `--verify' `-X' `--exclude-from' `-Z' `--compress' `-b' `--blocking-factor' `-c' `--create' `-d' `--compare' `-f' `--file' `-g' `--listed-incremental' `-h' `--dereference' `-i' `--ignore-zeros' `-j' `--bzip2' `-k' `--keep-old-files' `-l' `--one-file-system'. Use of this short option is deprecated. It is retained for compatibility with the earlier versions of GNU `tar', and will be changed in future releases. *Note Current status::, for more information. `-m' `--touch' `-o' When creating -- `--no-same-owner', when extracting -- `--portability'. The later usage is deprecated. It is retained for compatibility with the earlier versions of GNU `tar'. In the future releases `-o' will be equivalent to `--no-same-owner' only. `-p' `--preserve-permissions' `-r' `--append' `-s' `--same-order' `-t' `--list' `-u' `--update' `-v' `--verbose' `-w' `--interactive' `-x' `--extract' `-z' `--gzip' File: tar.info, Node: help, Next: verbose, Prev: All Options, Up: tar invocation 3.5 GNU `tar' documentation =========================== Being careful, the first thing is really checking that you are using GNU `tar', indeed. The `--version' option will generate a message giving confirmation that you are using GNU `tar', with the precise version of GNU `tar' you are using. `tar' identifies itself and prints the version number to the standard output, then immediately exits successfully, without doing anything else, ignoring all other options. For example, `tar --version' might return: tar (GNU tar) 1.14.90 The first occurrence of `tar' in the result above is the program name in the package (for example, `rmt' is another program), while the second occurrence of `tar' is the name of the package itself, containing possibly many programs. The package is currently named `tar', after the name of the main program it contains(1). Another thing you might want to do is checking the spelling or meaning of some particular `tar' option, without resorting to this manual, for once you have carefully read it. GNU `tar' has a short help feature, triggerable through the `--help' option. By using this option, `tar' will print a usage message listing all available options on standard output, then exit successfully, without doing anything else and ignoring all other options. Even if this is only a brief summary, it may be several screens long. So, if you are not using some kind of scrollable window, you might prefer to use something like: $ tar --help | less presuming, here, that you like using `less' for a pager. Other popular pagers are `more' and `pg'. If you know about some KEYWORD which interests you and do not want to read all the `--help' output, another common idiom is doing: tar --help | grep KEYWORD for getting only the pertinent lines. The perceptive reader would have noticed some contradiction in the previous paragraphs. It is written that both `--version' and `--help' print something, and have all other options ignored. In fact, they cannot ignore each other, and one of them has to win. We do not specify which is stronger, here; experiment if you really wonder! The short help output is quite succinct, and you might have to get back to the full documentation for precise points. If you are reading this paragraph, you already have the `tar' manual in some form. This manual is available in printed form, as a kind of small book. It may printed out of the GNU `tar' distribution, provided you have TeX already installed somewhere, and a laser printer around. Just configure the distribution, execute the command `make dvi', then print `doc/tar.dvi' the usual way (contact your local guru to know how). If GNU `tar' has been conveniently installed at your place, this manual is also available in interactive, hypertextual form as an Info file. Just call `info tar' or, if you do not have the `info' program handy, use the Info reader provided within GNU Emacs, calling `tar' from the main Info menu. There is currently no `man' page for GNU `tar'. If you observe such a `man' page on the system you are running, either it does not long to GNU `tar', or it has not been produced by GNU. Currently, GNU `tar' documentation is provided in Texinfo format only, if we except, of course, the short result of `tar --help'. ---------- Footnotes ---------- (1) There are plans to merge the `cpio' and `tar' packages into a single one which would be called `paxutils'. So, who knows if, one of this days, the `--version' would not yield `tar (GNU paxutils) 3.2' File: tar.info, Node: verbose, Next: interactive, Prev: help, Up: tar invocation 3.6 Checking `tar' progress =========================== Typically, `tar' performs most operations without reporting any information to the user except error messages. When using `tar' with many options, particularly ones with complicated or difficult-to-predict behavior, it is possible to make serious mistakes. `tar' provides several options that make observing `tar' easier. These options cause `tar' to print information as it progresses in its job, and you might want to use them just for being more careful about what is going on, or merely for entertaining yourself. If you have encountered a problem when operating on an archive, however, you may need more information than just an error message in order to solve the problem. The following options can be helpful diagnostic tools. Normally, the `--list' (`-t') command to list an archive prints just the file names (one per line) and the other commands are silent. When used with most operations, the `--verbose' (`-v') option causes `tar' to print the name of each file or archive member as it is processed. This and the other options which make `tar' print status information can be useful in monitoring `tar'. With `--create' (`-c') or `--extract' (`--get', `-x'), `--verbose' (`-v') used once just prints the names of the files or members as they are processed. Using it twice causes `tar' to print a longer listing (reminiscent of `ls -l') for each member. Since `--list' (`-t') already prints the names of the members, `--verbose' (`-v') used once with `--list' (`-t') causes `tar' to print an `ls -l' type listing of the files in the archive. The following examples both extract members with long list output: $ tar --extract --file=archive.tar --verbose --verbose $ tar xvvf archive.tar Verbose output appears on the standard output except when an archive is being written to the standard output, as with `tar --create --file=- --verbose' (`tar cfv -', or even `tar cv'--if the installer let standard output be the default archive). In that case `tar' writes verbose output to the standard error stream. If `--index-file=FILE' is specified, `tar' sends verbose output to FILE rather than to standard output or standard error. The `--totals' option--which is only meaningful when used with `--create' (`-c')--causes `tar' to print the total amount written to the archive, after it has been fully created. The `--checkpoint' option prints an occasional message as `tar' reads or writes the archive. In fact, it prints a message each 10 records read or written. It is designed for those who don't need the more detailed (and voluminous) output of `--block-number' (`-R'), but do want visual confirmation that `tar' is actually making forward progress. The `--show-omitted-dirs' option, when reading an archive--with `--list' (`-t') or `--extract' (`--get', `-x'), for example--causes a message to be printed for each directory in the archive which is skipped. This happens regardless of the reason for skipping: the directory might not have been named on the command line (implicitly or explicitly), it might be excluded by the use of the `--exclude=PATTERN' option, or some other reason. If `--block-number' (`-R') is used, `tar' prints, along with every message it would normally produce, the block number within the archive where the message was triggered. Also, supplementary messages are triggered when reading blocks full of NULs, or when hitting end of file on the archive. As of now, if the archive if properly terminated with a NUL block, the reading of the file may stop before end of file is met, so the position of end of file will not usually show when `--block-number' (`-R') is used. Note that GNU `tar' drains the archive before exiting when reading the archive from a pipe. This option is especially useful when reading damaged archives, since it helps pinpoint the damaged sections. It can also be used with `--list' (`-t') when listing a file-system backup tape, allowing you to choose among several backup tapes when retrieving a file later, in favor of the tape where the file appears earliest (closest to the front of the tape). File: tar.info, Node: interactive, Prev: verbose, Up: tar invocation 3.7 Asking for Confirmation During Operations ============================================= Typically, `tar' carries out a command without stopping for further instructions. In some situations however, you may want to exclude some files and archive members from the operation (for instance if disk or storage space is tight). You can do this by excluding certain files automatically (*note Choosing::), or by performing an operation interactively, using the `--interactive' (`-w') option. `tar' also accepts `--confirmation' for this option. When the `--interactive' (`-w') option is specified, before reading, writing, or deleting files, `tar' first prints a message for each such file, telling what operation it intends to take, then asks for confirmation on the terminal. The actions which require confirmation include adding a file to the archive, extracting a file from the archive, deleting a file from the archive, and deleting a file from disk. To confirm the action, you must type a line of input beginning with `y'. If your input line begins with anything other than `y', `tar' skips that file. If `tar' is reading the archive from the standard input, `tar' opens the file `/dev/tty' to support the interactive communications. Verbose output is normally sent to standard output, separate from other error messages. However, if the archive is produced directly on standard output, then verbose output is mixed with errors on `stderr'. Producing the archive on standard output may be used as a way to avoid using disk space, when the archive is soon to be consumed by another process reading it, say. Some people felt the need of producing an archive on stdout, still willing to segregate between verbose output and error output. A possible approach would be using a named pipe to receive the archive, and having the consumer process to read from that named pipe. This has the advantage of letting standard output free to receive verbose output, all separate from errors. File: tar.info, Node: operations, Next: Backups, Prev: tar invocation, Up: Top 4 GNU `tar' Operations ********************** * Menu: * Basic tar:: * Advanced tar:: * create options:: * extract options:: * backup:: * Applications:: * looking ahead:: File: tar.info, Node: Basic tar, Next: Advanced tar, Up: operations 4.1 Basic GNU `tar' Operations ============================== The basic `tar' operations, `--create' (`-c'), `--list' (`-t') and `--extract' (`--get', `-x'), are currently presented and described in the tutorial chapter of this manual. This section provides some complementary notes for these operations. `--create' (`-c') Creating an empty archive would have some kind of elegance. One can initialize an empty archive and later use `--append' (`-r') for adding all members. Some applications would not welcome making an exception in the way of adding the first archive member. On the other hand, many people reported that it is dangerously too easy for `tar' to destroy a magnetic tape with an empty archive(1). The two most common errors are: 1. Mistakingly using `create' instead of `extract', when the intent was to extract the full contents of an archive. This error is likely: keys `c' and `x' are right next to each other on the QWERTY keyboard. Instead of being unpacked, the archive then gets wholly destroyed. When users speak about "exploding" an archive, they usually mean something else :-). 2. Forgetting the argument to `file', when the intent was to create an archive with a single file in it. This error is likely because a tired user can easily add the `f' key to the cluster of option letters, by the mere force of habit, without realizing the full consequence of doing so. The usual consequence is that the single file, which was meant to be saved, is rather destroyed. So, recognizing the likelihood and the catastrophical nature of these errors, GNU `tar' now takes some distance from elegance, and cowardly refuses to create an archive when `--create' (`-c') option is given, there are no arguments besides options, and `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') option is _not_ used. To get around the cautiousness of GNU `tar' and nevertheless create an archive with nothing in it, one may still use, as the value for the `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') option, a file with no names in it, as shown in the following commands: tar --create --file=empty-archive.tar --files-from=/dev/null tar cfT empty-archive.tar /dev/null `--extract' (`--get', `-x') A socket is stored, within a GNU `tar' archive, as a pipe. `--list' (`-t') GNU `tar' now shows dates as `1996-08-30', while it used to show them as `Aug 30 1996'. (One can revert to the old behavior by defining `USE_OLD_CTIME' in `src/list.c' before reinstalling.) But preferably, people should get used to ISO 8601 dates. Local American dates should be made available again with full date localization support, once ready. In the meantime, programs not being localizable for dates should prefer international dates, that's really the way to go. Look up `http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html' if you are curious, it contains a detailed explanation of the ISO 8601 standard. ---------- Footnotes ---------- (1) This is well described in `Unix-haters Handbook', by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG Books, ISBN 1-56884-203-1. File: tar.info, Node: Advanced tar, Next: create options, Prev: Basic tar, Up: operations 4.2 Advanced GNU `tar' Operations ================================= Now that you have learned the basics of using GNU `tar', you may want to learn about further ways in which `tar' can help you. This chapter presents five, more advanced operations which you probably won't use on a daily basis, but which serve more specialized functions. We also explain the different styles of options and why you might want to use one or another, or a combination of them in your `tar' commands. Additionally, this chapter includes options which allow you to define the output from `tar' more carefully, and provide help and error correction in special circumstances. * Menu: * Operations:: * append:: * update:: * concatenate:: * delete:: * compare:: File: tar.info, Node: Operations, Next: append, Up: Advanced tar 4.2.1 The Five Advanced `tar' Operations ---------------------------------------- _(This message will disappear, once this node revised.)_ In the last chapter, you learned about the first three operations to `tar'. This chapter presents the remaining five operations to `tar': `--append', `--update', `--concatenate', `--delete', and `--compare'. You are not likely to use these operations as frequently as those covered in the last chapter; however, since they perform specialized functions, they are quite useful when you do need to use them. We will give examples using the same directory and files that you created in the last chapter. As you may recall, the directory is called `practice', the files are `jazz', `blues', `folk', `rock', and the two archive files you created are `collection.tar' and `music.tar'. We will also use the archive files `afiles.tar' and `bfiles.tar'. `afiles.tar' contains the members `apple', `angst', and `aspic'. `bfiles.tar' contains the members `./birds', `baboon', and `./box'. Unless we state otherwise, all practicing you do and examples you follow in this chapter will take place in the `practice' directory that you created in the previous chapter; see *Note prepare for examples::. (Below in this section, we will remind you of the state of the examples where the last chapter left them.) The five operations that we will cover in this chapter are: `--append' `-r' Add new entries to an archive that already exists. `--update' `-r' Add more recent copies of archive members to the end of an archive, if they exist. `--concatenate' `--catenate' `-A' Add one or more pre-existing archives to the end of another archive. `--delete' Delete items from an archive (does not work on tapes). `--compare' `--diff' `-d' Compare archive members to their counterparts in the file system. File: tar.info, Node: append, Next: update, Prev: Operations, Up: Advanced tar 4.2.2 How to Add Files to Existing Archives: `--append' ------------------------------------------------------- _(This message will disappear, once this node revised.)_ If you want to add files to an existing archive, you don't need to create a new archive; you can use `--append' (`-r'). The archive must already exist in order to use `--append'. (A related operation is the `--update' operation; you can use this to add newer versions of archive members to an existing archive. To learn how to do this with `--update', *note update::.) If you use `--append' (`-r') to add a file that has the same name as an archive member to an archive containing that archive member, then the old member is not deleted. What does happen, however, is somewhat complex. `tar' _allows_ you to have infinite number of files with the same name. Some operations treat these same-named members no differently than any other set of archive members: for example, if you view an archive with `--list' (`-t'), you will see all of those members listed, with their modification times, owners, etc. Other operations don't deal with these members as perfectly as you might prefer; if you were to use `--extract' (`--get', `-x') to extract the archive, only the most recently added copy of a member with the same name as four other members would end up in the working directory. This is because `--extract' extracts an archive in the order the members appeared in the archive; the most recently archived members will be extracted last. Additionally, an extracted member will _replace_ a file of the same name which existed in the directory already, and `tar' will not prompt you about this(1). Thus, only the most recently archived member will end up being extracted, as it will replace the one extracted before it, and so on. There exists a special option that allows you to get around this behavior and extract (or list) only a particular copy of the file. This is `--occurrence' option. If you run `tar' with this option, it will extract only the first copy of the file. You may also give this option an argument specifying the number of copy to be extracted. Thus, for example if the archive `archive.tar' contained three copies of file `myfile', then the command tar --extract --file archive.tar --occurrence=2 myfile would extract only the second copy. *Note --occurrence: Option Summary, for the description of `--occurrence' option. If you want to replace an archive member, use `--delete' to delete the member you want to remove from the archive, , and then use `--append' to add the member you want to be in the archive. Note that you can not change the order of the archive; the most recently added member will still appear last. In this sense, you cannot truly "replace" one member with another. (Replacing one member with another will not work on certain types of media, such as tapes; see *Note delete:: and *Note Media::, for more information.) * Menu: * appending files:: Appending Files to an Archive * multiple:: ---------- Footnotes ---------- (1) Unless you give it `--keep-old-files' option, or the disk copy is newer than the the one in the archive and you invoke `tar' with `--keep-newer-files' option File: tar.info, Node: appending files, Next: multiple, Up: append 4.2.2.1 Appending Files to an Archive ..................................... _(This message will disappear, once this node revised.)_ The simplest way to add a file to an already existing archive is the `--append' (`-r') operation, which writes specified files into the archive whether or not they are already among the archived files. When you use `--append', you _must_ specify file name arguments, as there is no default. If you specify a file that already exists in the archive, another copy of the file will be added to the end of the archive. As with other operations, the member names of the newly added files will be exactly the same as their names given on the command line. The `--verbose' (`-v') option will print out the names of the files as they are written into the archive. `--append' cannot be performed on some tape drives, unfortunately, due to deficiencies in the formats those tape drives use. The archive must be a valid `tar' archive, or else the results of using this operation will be unpredictable. *Note Media::. To demonstrate using `--append' to add a file to an archive, create a file called `rock' in the `practice' directory. Make sure you are in the `practice' directory. Then, run the following `tar' command to add `rock' to `collection.tar': $ tar --append --file=collection.tar rock If you now use the `--list' (`-t') operation, you will see that `rock' has been added to the archive: $ tar --list --file=collection.tar -rw-rw-rw- me user 28 1996-10-18 16:31 jazz -rw-rw-rw- me user 21 1996-09-23 16:44 blues -rw-rw-rw- me user 20 1996-09-23 16:44 folk -rw-rw-rw- me user 20 1996-09-23 16:44 rock File: tar.info, Node: multiple, Prev: appending files, Up: append 4.2.2.2 Multiple Files with the Same Name ......................................... You can use `--append' (`-r') to add copies of files which have been updated since the archive was created. (However, we do not recommend doing this since there is another `tar' option called `--update'; *note update:: for more information. We describe this use of `--append' here for the sake of completeness.) When you extract the archive, the older version will be effectively lost. This works because files are extracted from an archive in the order in which they were archived. Thus, when the archive is extracted, a file archived later in time will replace a file of the same name which was archived earlier, even though the older version of the file will remain in the archive unless you delete all versions of the file. Supposing you change the file `blues' and then append the changed version to `collection.tar'. As you saw above, the original `blues' is in the archive `collection.tar'. If you change the file and append the new version of the file to the archive, there will be two copies in the archive. When you extract the archive, the older version of the file will be extracted first, and then replaced by the newer version when it is extracted. You can append the new, changed copy of the file `blues' to the archive in this way: $ tar --append --verbose --file=collection.tar blues blues Because you specified the `--verbose' option, `tar' has printed the name of the file being appended as it was acted on. Now list the contents of the archive: $ tar --list --verbose --file=collection.tar -rw-rw-rw- me user 28 1996-10-18 16:31 jazz -rw-rw-rw- me user 21 1996-09-23 16:44 blues -rw-rw-rw- me user 20 1996-09-23 16:44 folk -rw-rw-rw- me user 20 1996-09-23 16:44 rock -rw-rw-rw- me user 58 1996-10-24 18:30 blues The newest version of `blues' is now at the end of the archive (note the different creation dates and file sizes). If you extract the archive, the older version of the file `blues' will be replaced by the newer version. You can confirm this by extracting the archive and running `ls' on the directory. If you wish to extract the first occurrence of the file `blues' from the archive, use `--occurrence' option, as shown in the following example: $ tar --extract -vv --occurrence --file=collection.tar blues -rw-rw-rw- me user 21 1996-09-23 16:44 blues *Note Writing::, for more information on `--extract' (`--get', `-x') and *Note -occurrence: Option Summary, for the description of `--occurrence' option. File: tar.info, Node: update, Next: concatenate, Prev: append, Up: Advanced tar 4.2.3 Updating an Archive ------------------------- _(This message will disappear, once this node revised.)_ In the previous section, you learned how to use `--append' (`-r') to add a file to an existing archive. A related operation is `--update' (`-u'). The `--update' operation updates a `tar' archive by comparing the date of the specified archive members against the date of the file with the same name. If the file has been modified more recently than the archive member, then the newer version of the file is added to the archive (as with `--append' (`-r')). Unfortunately, you cannot use `--update' with magnetic tape drives. The operation will fail. Both `--update' and `--append' work by adding to the end of the archive. When you extract a file from the archive, only the version stored last will wind up in the file system, unless you use the `--backup' option. * Menu: * how to update:: File: tar.info, Node: how to update, Up: update 4.2.3.1 How to Update an Archive Using `--update' ................................................. You must use file name arguments with the `--update' (`-u') operation. If you don't specify any files, `tar' won't act on any files and won't tell you that it didn't do anything (which may end up confusing you). To see the `--update' option at work, create a new file, `classical', in your practice directory, and some extra text to the file `blues', using any text editor. Then invoke `tar' with the `update' operation and the `--verbose' (`-v') option specified, using the names of all the files in the practice directory as file name arguments: $ tar --update -v -f collection.tar blues folk rock classical blues classical $ Because we have specified verbose mode, `tar' prints out the names of the files it is working on, which in this case are the names of the files that needed to be updated. If you run `tar --list' and look at the archive, you will see `blues' and `classical' at its end. There will be a total of two versions of the member `blues'; the one at the end will be newer and larger, since you added text before updating it. (The reason `tar' does not overwrite the older file when updating it is because writing to the middle of a section of tape is a difficult process. Tapes are not designed to go backward. *Note Media::, for more information about tapes. `--update' (`-u') is not suitable for performing backups for two reasons: it does not change directory content entries, and it lengthens the archive every time it is used. The GNU `tar' options intended specifically for backups are more efficient. If you need to run backups, please consult *Note Backups::. File: tar.info, Node: concatenate, Next: delete, Prev: update, Up: Advanced tar 4.2.4 Combining Archives with `--concatenate' --------------------------------------------- Sometimes it may be convenient to add a second archive onto the end of an archive rather than adding individual files to the archive. To add one or more archives to the end of another archive, you should use the `--concatenate' (`--catenate', `-A') operation. To use `--concatenate', name the archives to be concatenated on the command line. (Nothing happens if you don't list any.) The members, and their member names, will be copied verbatim from those archives. If this causes multiple members to have the same name, it does not delete any members; all the members with the same name coexist. To demonstrate how `--concatenate' works, create two small archives called `bluesrock.tar' and `folkjazz.tar', using the relevant files from `practice': $ tar -cvf bluesrock.tar blues rock blues classical $ tar -cvf folkjazz.tar folk jazz folk jazz If you like, You can run `tar --list' to make sure the archives contain what they are supposed to: $ tar -tvf bluesrock.tar -rw-rw-rw- melissa user 105 1997-01-21 19:42 blues -rw-rw-rw- melissa user 33 1997-01-20 15:34 rock $ tar -tvf folkjazz.tar -rw-rw-rw- melissa user 20 1996-09-23 16:44 folk -rw-rw-rw- melissa user 65 1997-01-30 14:15 jazz We can concatenate these two archives with `tar': $ cd .. $ tar --concatenate --file=bluesrock.tar jazzfolk.tar If you now list the contents of the `bluesclass.tar', you will see that now it also contains the archive members of `jazzfolk.tar': $ tar --list --file=bluesrock.tar blues rock jazz folk When you use `--concatenate', the source and target archives must already exist and must have been created using compatible format parameters. The new, concatenated archive will be called by the same name as the first archive listed on the command line. Like `--append' (`-r'), this operation cannot be performed on some tape drives, due to deficiencies in the formats those tape drives use. It may seem more intuitive to you to want or try to use `cat' to concatenate two archives instead of using the `--concatenate' operation; after all, `cat' is the utility for combining files. However, `tar' archives incorporate an end-of-file marker which must be removed if the concatenated archives are to be read properly as one archive. `--concatenate' removes the end-of-archive marker from the target archive before each new archive is appended. If you use `cat' to combine the archives, the result will not be a valid `tar' format archive. If you need to retrieve files from an archive that was added to using the `cat' utility, use the `--ignore-zeros' (`-i') option. *Note Ignore Zeros::, for further information on dealing with archives improperly combined using the `cat' shell utility. You must specify the source archives using `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') (*note file::). If you do not specify the target archive, `tar' uses the value of the environment variable `TAPE', or, if this has not been set, the default archive name. File: tar.info, Node: delete, Next: compare, Prev: concatenate, Up: Advanced tar 4.2.5 Removing Archive Members Using `--delete' ----------------------------------------------- _(This message will disappear, once this node revised.)_ You can remove members from an archive by using the `--delete' option. Specify the name of the archive with `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') and then specify the names of the members to be deleted; if you list no member names, nothing will be deleted. The `--verbose' (`-v') option will cause `tar' to print the names of the members as they are deleted. As with `--extract' (`--get', `-x'), you must give the exact member names when using `tar --delete'. `--delete' will remove all versions of the named file from the archive. The `--delete' operation can run very slowly. Unlike other operations, `--delete' has no short form. This operation will rewrite the archive. You can only use `--delete' on an archive if the archive device allows you to write to any point on the media, such as a disk; because of this, it does not work on magnetic tapes. Do not try to delete an archive member from a magnetic tape; the action will not succeed, and you will be likely to scramble the archive and damage your tape. There is no safe way (except by completely re-writing the archive) to delete files from most kinds of magnetic tape. *Note Media::. To delete all versions of the file `blues' from the archive `collection.tar' in the `practice' directory, make sure you are in that directory, and then, $ tar --list --file=collection.tar blues folk jazz rock practice/blues practice/folk practice/jazz practice/rock practice/blues $ tar --delete --file=collection.tar blues $ tar --list --file=collection.tar folk jazz rock $ The `--delete' option has been reported to work properly when `tar' acts as a filter from `stdin' to `stdout'. File: tar.info, Node: compare, Prev: delete, Up: Advanced tar 4.2.6 Comparing Archive Members with the File System ---------------------------------------------------- _(This message will disappear, once this node revised.)_ The `--compare' (`-d'), or `--diff' operation compares specified archive members against files with the same names, and then reports differences in file size, mode, owner, modification date and contents. You should _only_ specify archive member names, not file names. If you do not name any members, then `tar' will compare the entire archive. If a file is represented in the archive but does not exist in the file system, `tar' reports a difference. You have to specify the record size of the archive when modifying an archive with a non-default record size. `tar' ignores files in the file system that do not have corresponding members in the archive. The following example compares the archive members `rock', `blues' and `funk' in the archive `bluesrock.tar' with files of the same name in the file system. (Note that there is no file, `funk'; `tar' will report an error message.) $ tar --compare --file=bluesrock.tar rock blues funk rock blues tar: funk not found in archive Depending on the system where you are running `tar' and the version you are running, `tar' may have a different error message, such as: funk: does not exist The spirit behind the `--compare' (`--diff', `-d') option is to check whether the archive represents the current state of files on disk, more than validating the integrity of the archive media. For this later goal, *Note verify::. File: tar.info, Node: create options, Next: extract options, Prev: Advanced tar, Up: operations 4.3 Options Used by `--create' ============================== The previous chapter described the basics of how to use `--create' (`-c') to create an archive from a set of files. *Note create::. This section described advanced options to be used with `--create'. * Menu: * Ignore Failed Read:: File: tar.info, Node: Ignore Failed Read, Up: create options 4.3.1 Ignore Fail Read ---------------------- `--ignore-failed-read' Do not exit with nonzero on unreadable files or directories. File: tar.info, Node: extract options, Next: backup, Prev: create options, Up: operations 4.4 Options Used by `--extract' =============================== _(This message will disappear, once this node revised.)_ The previous chapter showed how to use `--extract' (`--get', `-x') to extract an archive into the filesystem. Various options cause `tar' to extract more information than just file contents, such as the owner, the permissions, the modification date, and so forth. This section presents options to be used with `--extract' when certain special considerations arise. You may review the information presented in *Note extract:: for more basic information about the `--extract' operation. * Menu: * Reading:: Options to Help Read Archives * Writing:: Changing How `tar' Writes Files * Scarce:: Coping with Scarce Resources File: tar.info, Node: Reading, Next: Writing, Up: extract options 4.4.1 Options to Help Read Archives ----------------------------------- _(This message will disappear, once this node revised.)_ Normally, `tar' will request data in full record increments from an archive storage device. If the device cannot return a full record, `tar' will report an error. However, some devices do not always return full records, or do not require the last record of an archive to be padded out to the next record boundary. To keep reading until you obtain a full record, or to accept an incomplete record if it contains an end-of-archive marker, specify the `--read-full-records' (`-B') option in conjunction with the `--extract' (`--get', `-x') or `--list' (`-t') operations. *Note Blocking::. The `--read-full-records' (`-B') option is turned on by default when `tar' reads an archive from standard input, or from a remote machine. This is because on BSD Unix systems, attempting to read a pipe returns however much happens to be in the pipe, even if it is less than was requested. If this option were not enabled, `tar' would fail as soon as it read an incomplete record from the pipe. If you're not sure of the blocking factor of an archive, you can read the archive by specifying `--read-full-records' (`-B') and `--blocking-factor=512-SIZE' (`-b 512-SIZE'), using a blocking factor larger than what the archive uses. This lets you avoid having to determine the blocking factor of an archive. *Note Blocking Factor::. * Menu: * read full records:: * Ignore Zeros:: File: tar.info, Node: read full records, Next: Ignore Zeros, Up: Reading Reading Full Records .................... `--read-full-records' `-B' Use in conjunction with `--extract' (`--get', `-x') to read an archive which contains incomplete records, or one which has a blocking factor less than the one specified. File: tar.info, Node: Ignore Zeros, Prev: read full records, Up: Reading Ignoring Blocks of Zeros ........................ Normally, `tar' stops reading when it encounters a block of zeros between file entries (which usually indicates the end of the archive). `--ignore-zeros' (`-i') allows `tar' to completely read an archive which contains a block of zeros before the end (i.e. a damaged archive, or one which was created by concatenating several archives together). The `--ignore-zeros' (`-i') option is turned off by default because many versions of `tar' write garbage after the end-of-archive entry, since that part of the media is never supposed to be read. GNU `tar' does not write after the end of an archive, but seeks to maintain compatiblity among archiving utilities. `--ignore-zeros' `-i' To ignore blocks of zeros (ie. end-of-archive entries) which may be encountered while reading an archive. Use in conjunction with `--extract' (`--get', `-x') or `--list' (`-t'). File: tar.info, Node: Writing, Next: Scarce, Prev: Reading, Up: extract options 4.4.2 Changing How `tar' Writes Files ------------------------------------- _(This message will disappear, once this node revised.)_ * Menu: * Dealing with Old Files:: * Overwrite Old Files:: * Keep Old Files:: * Keep Newer Files:: * Unlink First:: * Recursive Unlink:: * Modification Times:: * Setting Access Permissions:: * Writing to Standard Output:: * remove files:: File: tar.info, Node: Dealing with Old Files, Next: Overwrite Old Files, Up: Writing Options Controlling the Overwriting of Existing Files ..................................................... When extracting files, if `tar' discovers that the extracted file already exists, it normally replaces the file by removing it before extracting it, to prevent confusion in the presence of hard or symbolic links. (If the existing file is a symbolic link, it is removed, not followed.) However, if a directory cannot be removed because it is nonempty, `tar' normally overwrites its metadata (ownership, permission, etc.). The `--overwrite-dir' option enables this default behavior. To be more cautious and preserve the metadata of such a directory, use the `--no-overwrite-dir' option. To be even more cautious and prevent existing files from being replaced, use the `--keep-old-files' (`-k') option. It causes `tar' to refuse to replace or update a file that already exists, i.e., a file with the same name as an archive member prevents extraction of that archive member. Instead, it reports an error. To be more aggressive about altering existing files, use the `--overwrite' option. It causes `tar' to overwrite existing files and to follow existing symbolic links when extracting. Some people argue that GNU `tar' should not hesitate to overwrite files with other files when extracting. When extracting a `tar' archive, they expect to see a faithful copy of the state of the filesystem when the archive was created. It is debatable that this would always be a proper behavior. For example, suppose one has an archive in which `usr/local' is a link to `usr/local2'. Since then, maybe the site removed the link and renamed the whole hierarchy from `/usr/local2' to `/usr/local'. Such things happen all the time. I guess it would not be welcome at all that GNU `tar' removes the whole hierarchy just to make room for the link to be reinstated (unless it _also_ simultaneously restores the full `/usr/local2', of course!) GNU `tar' is indeed able to remove a whole hierarchy to reestablish a symbolic link, for example, but _only if_ `--recursive-unlink' is specified to allow this behavior. In any case, single files are silently removed. Finally, the `--unlink-first' (`-U') option can improve performance in some cases by causing `tar' to remove files unconditionally before extracting them. File: tar.info, Node: Overwrite Old Files, Next: Keep Old Files, Prev: Dealing with Old Files, Up: Writing Overwrite Old Files ................... `--overwrite' Overwrite existing files and directory metadata when extracting files from an archive. This causes `tar' to write extracted files into the file system without regard to the files already on the system; i.e., files with the same names as archive members are overwritten when the archive is extracted. It also causes `tar' to extract the ownership, permissions, and time stamps onto any preexisting files or directories. If the name of a corresponding file name is a symbolic link, the file pointed to by the symbolic link will be overwritten instead of the symbolic link itself (if this is possible). Moreover, special devices, empty directories and even symbolic links are automatically removed if they are in the way of extraction. Be careful when using the `--overwrite' option, particularly when combined with the `--absolute-names' (`-P') option, as this combination can change the contents, ownership or permissions of any file on your system. Also, many systems do not take kindly to overwriting files that are currently being executed. `--overwrite-dir' Overwrite the metadata of directories when extracting files from an archive, but remove other files before extracting. File: tar.info, Node: Keep Old Files, Next: Keep Newer Files, Prev: Overwrite Old Files, Up: Writing Keep Old Files .............. `--keep-old-files' `-k' Do not replace existing files from archive. The `--keep-old-files' (`-k') option prevents `tar' from replacing existing files with files with the same name from the archive. The `--keep-old-files' (`-k') option is meaningless with `--list' (`-t'). Prevents `tar' from replacing files in the file system during extraction. File: tar.info, Node: Keep Newer Files, Next: Unlink First, Prev: Keep Old Files, Up: Writing Keep Newer Files ................ `--keep-newer-files' Do not replace existing files that are newer than their archive copies. This option is meaningless with `--list' (`-t'). File: tar.info, Node: Unlink First, Next: Recursive Unlink, Prev: Keep Newer Files, Up: Writing Unlink First ............ `--unlink-first' `-U' Remove files before extracting over them. This can make `tar' run a bit faster if you know in advance that the extracted files all need to be removed. Normally this option slows `tar' down slightly, so it is disabled by default. File: tar.info, Node: Recursive Unlink, Next: Modification Times, Prev: Unlink First, Up: Writing Recursive Unlink ................ `--recursive-unlink' When this option is specified, try removing files and directory hierarchies before extracting over them. _This is a dangerous option!_ If you specify the `--recursive-unlink' option, `tar' removes _anything_ that keeps you from extracting a file as far as current permissions will allow it. This could include removal of the contents of a full directory hierarchy. File: tar.info, Node: Modification Times, Next: Setting Access Permissions, Prev: Recursive Unlink, Up: Writing Setting Modification Times .......................... Normally, `tar' sets the modification times of extracted files to the modification times recorded for the files in the archive, but limits the permissions of extracted files by the current `umask' setting. To set the modification times of extracted files to the time when the files were extracted, use the `--touch' (`-m') option in conjunction with `--extract' (`--get', `-x'). `--touch' `-m' Sets the modification time of extracted archive members to the time they were extracted, not the time recorded for them in the archive. Use in conjunction with `--extract' (`--get', `-x'). File: tar.info, Node: Setting Access Permissions, Next: Writing to Standard Output, Prev: Modification Times, Up: Writing Setting Access Permissions .......................... To set the modes (access permissions) of extracted files to those recorded for those files in the archive, use `--same-permissions' in conjunction with the `--extract' (`--get', `-x') operation. `--preserve-permission' `--same-permission' `--ignore-umask' `-p' Set modes of extracted archive members to those recorded in the archive, instead of current umask settings. Use in conjunction with `--extract' (`--get', `-x'). File: tar.info, Node: Writing to Standard Output, Next: remove files, Prev: Setting Access Permissions, Up: Writing Writing to Standard Output .......................... To write the extracted files to the standard output, instead of creating the files on the file system, use `--to-stdout' (`-O') in conjunction with `--extract' (`--get', `-x'). This option is useful if you are extracting files to send them through a pipe, and do not need to preserve them in the file system. If you extract multiple members, they appear on standard output concatenated, in the order they are found in the archive. `--to-stdout' `-O' Writes files to the standard output. Used in conjunction with `--extract' (`--get', `-x'). Extract files to standard output. When this option is used, instead of creating the files specified, `tar' writes the contents of the files extracted to its standard output. This may be useful if you are only extracting the files in order to send them through a pipe. This option is meaningless with `--list' (`-t'). This can be useful, for example, if you have a tar archive containing a big file and don't want to store the file on disk before processing it. You can use a command like this: tar -xOzf foo.tgz bigfile | process or even like this if you want to process the concatenation of the files: tar -xOzf foo.tgz bigfile1 bigfile2 | process File: tar.info, Node: remove files, Prev: Writing to Standard Output, Up: Writing Removing Files .............. `--remove-files' Remove files after adding them to the archive. File: tar.info, Node: Scarce, Prev: Writing, Up: extract options 4.4.3 Coping with Scarce Resources ---------------------------------- _(This message will disappear, once this node revised.)_ * Menu: * Starting File:: * Same Order:: File: tar.info, Node: Starting File, Next: Same Order, Up: Scarce Starting File ............. `--starting-file=NAME' `-K NAME' Starts an operation in the middle of an archive. Use in conjunction with `--extract' (`--get', `-x') or `--list' (`-t'). If a previous attempt to extract files failed due to lack of disk space, you can use `--starting-file=NAME' (`-K NAME') to start extracting only after member NAME of the archive. This assumes, of course, that there is now free space, or that you are now extracting into a different file system. (You could also choose to suspend `tar', remove unnecessary files from the file system, and then restart the same `tar' operation. In this case, `--starting-file=NAME' (`-K NAME') is not necessary. *Note Inc Dumps::, *Note interactive::, and *Note exclude::.) File: tar.info, Node: Same Order, Prev: Starting File, Up: Scarce Same Order .......... `--same-order' `--preserve-order' `-s' To process large lists of file names on machines with small amounts of memory. Use in conjunction with `--compare' (`--diff', `-d'), `--list' (`-t') or `--extract' (`--get', `-x'). The `--same-order' (`--preserve-order', `-s') option tells `tar' that the list of file names to be listed or extracted is sorted in the same order as the files in the archive. This allows a large list of names to be used, even on a small machine that would not otherwise be able to hold all the names in memory at the same time. Such a sorted list can easily be created by running `tar -t' on the archive and editing its output. This option is probably never needed on modern computer systems. File: tar.info, Node: backup, Next: Applications, Prev: extract options, Up: operations 4.5 Backup options ================== GNU `tar' offers options for making backups of files before writing new versions. These options control the details of these backups. They may apply to the archive itself before it is created or rewritten, as well as individual extracted members. Other GNU programs (`cp', `install', `ln', and `mv', for example) offer similar options. Backup options may prove unexpectedly useful when extracting archives containing many members having identical name, or when extracting archives on systems having file name limitations, making different members appear has having similar names through the side-effect of name truncation. (This is true only if we have a good scheme for truncated backup names, which I'm not sure at all: I suspect work is needed in this area.) When any existing file is backed up before being overwritten by extraction, then clashing files are automatically be renamed to be unique, and the true name is kept for only the last file of a series of clashing files. By using verbose mode, users may track exactly what happens. At the detail level, some decisions are still experimental, and may change in the future, we are waiting comments from our users. So, please do not learn to depend blindly on the details of the backup features. For example, currently, directories themselves are never renamed through using these options, so, extracting a file over a directory still has good chances to fail. Also, backup options apply to created archives, not only to extracted members. For created archives, backups will not be attempted when the archive is a block or character device, or when it refers to a remote file. For the sake of simplicity and efficiency, backups are made by renaming old files prior to creation or extraction, and not by copying. The original name is restored if the file creation fails. If a failure occurs after a partial extraction of a file, both the backup and the partially extracted file are kept. `--backup[=METHOD]' Back up files that are about to be overwritten or removed. Without this option, the original versions are destroyed. Use METHOD to determine the type of backups made. If METHOD is not specified, use the value of the `VERSION_CONTROL' environment variable. And if `VERSION_CONTROL' is not set, use the `existing' method. This option corresponds to the Emacs variable `version-control'; the same values for METHOD are accepted as in Emacs. This option also allows more descriptive names. The valid METHODs are: `t' `numbered' Always make numbered backups. `nil' `existing' Make numbered backups of files that already have them, simple backups of the others. `never' `simple' Always make simple backups. `--suffix=SUFFIX' Append SUFFIX to each backup file made with `--backup'. If this option is not specified, the value of the `SIMPLE_BACKUP_SUFFIX' environment variable is used. And if `SIMPLE_BACKUP_SUFFIX' is not set, the default is `~', just as in Emacs. Some people express the desire to _always_ use the `--backup' option, by defining some kind of alias or script. This is not as easy as one may think, due to the fact that old style options should appear first and consume arguments a bit unpredictably for an alias or script. But, if you are ready to give up using old style options, you may resort to using something like (a Bourne shell function here): tar () { /usr/local/bin/tar --backup $*; } File: tar.info, Node: Applications, Next: looking ahead, Prev: backup, Up: operations 4.6 Notable `tar' Usages ======================== _(This message will disappear, once this node revised.)_ You can easily use archive files to transport a group of files from one system to another: put all relevant files into an archive on one computer system, transfer the archive to another system, and extract the contents there. The basic transfer medium might be magnetic tape, Internet FTP, or even electronic mail (though you must encode the archive with `uuencode' in order to transport it properly by mail). Both machines do not have to use the same operating system, as long as they both support the `tar' program. For example, here is how you might copy a directory's contents from one disk to another, while preserving the dates, modes, owners and link-structure of all the files therein. In this case, the transfer medium is a "pipe", which is one a Unix redirection mechanism: $ cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -) The command also works using short option forms: $ cd sourcedir; tar --create --file=- . | (cd targetdir; tar --extract --file=-) This is one of the easiest methods to transfer a `tar' archive. File: tar.info, Node: looking ahead, Prev: Applications, Up: operations 4.7 Looking Ahead: The Rest of this Manual ========================================== You have now seen how to use all eight of the operations available to `tar', and a number of the possible options. The next chapter explains how to choose and change file and archive names, how to use files to store names of other files which you can then call as arguments to `tar' (this can help you save time if you expect to archive the same list of files a number of times), and so forth. If there are too many files to conveniently list on the command line, you can list the names in a file, and `tar' will read that file. *Note files::. There are various ways of causing `tar' to skip over some files, and not archive them. *Note Choosing::. File: tar.info, Node: Backups, Next: Choosing, Prev: operations, Up: Top 5 Performing Backups and Restoring Files **************************************** _(This message will disappear, once this node revised.)_ GNU `tar' is distributed along with the scripts which the Free Software Foundation uses for performing backups. There is no corresponding scripts available yet for doing restoration of files. Even if there is a good chance those scripts may be satisfying to you, they are not the only scripts or methods available for doing backups and restore. You may well create your own, or use more sophisticated packages dedicated to that purpose. Some users are enthusiastic about `Amanda' (The Advanced Maryland Automatic Network Disk Archiver), a backup system developed by James da Silva `jds@cs.umd.edu' and available on many Unix systems. This is free software, and it is available at these places: http://www.cs.umd.edu/projects/amanda/amanda.html ftp://ftp.cs.umd.edu/pub/amanda Here is a possible plan for a future documentation about the backuping scripts which are provided within the GNU `tar' distribution. .* dumps . + what are dumps . + different levels of dumps . - full dump = dump everything . - level 1, level 2 dumps etc, - A level n dump dumps everything changed since the last level n-1 dump (?) . + how to use scripts for dumps (ie, the concept) . - scripts to run after editing backup specs (details) . + Backup Specs, what is it. . - how to customize . - actual text of script [/sp/dump/backup-specs] . + Problems . - rsh doesn't work . - rtape isn't installed . - (others?) . + the --incremental option of tar . + tapes . - write protection . - types of media . : different sizes and types, useful for different things . - files and tape marks one tape mark between files, two at end. . - positioning the tape MT writes two at end of write, backspaces over one when writing again. This chapter documents both the provided shell scripts and `tar' options which are more specific to usage as a backup tool. To "back up" a file system means to create archives that contain all the files in that file system. Those archives can then be used to restore any or all of those files (for instance if a disk crashes or a file is accidentally deleted). File system "backups" are also called "dumps". * Menu: * Full Dumps:: Using `tar' to Perform Full Dumps * Inc Dumps:: Using `tar' to Perform Incremental Dumps * incremental and listed-incremental:: The Incremental Options * Backup Levels:: Levels of Backups * Backup Parameters:: Setting Parameters for Backups and Restoration * Scripted Backups:: Using the Backup Scripts * Scripted Restoration:: Using the Restore Script File: tar.info, Node: Full Dumps, Next: Inc Dumps, Up: Backups 5.1 Using `tar' to Perform Full Dumps ===================================== _(This message will disappear, once this node revised.)_ Full dumps should only be made when no other people or programs are modifying files in the filesystem. If files are modified while `tar' is making the backup, they may not be stored properly in the archive, in which case you won't be able to restore them if you have to. (Files not being modified are written with no trouble, and do not corrupt the entire archive.) You will want to use the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option to give the archive a volume label, so you can tell what this archive is even if the label falls off the tape, or anything like that. Unless the filesystem you are dumping is guaranteed to fit on one volume, you will need to use the `--multi-volume' (`-M') option. Make sure you have enough tapes on hand to complete the backup. If you want to dump each filesystem separately you will need to use the `--one-file-system' (`-l') option to prevent `tar' from crossing filesystem boundaries when storing (sub)directories. The `--incremental' (`-G') option is not needed, since this is a complete copy of everything in the filesystem, and a full restore from this backup would only be done onto a completely empty disk. Unless you are in a hurry, and trust the `tar' program (and your tapes), it is a good idea to use the `--verify' (`-W') option, to make sure your files really made it onto the dump properly. This will also detect cases where the file was modified while (or just after) it was being archived. Not all media (notably cartridge tapes) are capable of being verified, unfortunately. `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') take a file name argument always. If the file doesn't exist, run a level zero dump, creating the file. If the file exists, uses that file to see what has changed. `--incremental' (`-G') `--incremental' (`-G') handle old GNU-format incremental backup. This option should only be used when creating an incremental backup of a filesystem. When the `--incremental' (`-G') option is used, `tar' writes, at the beginning of the archive, an entry for each of the directories that will be operated on. The entry for a directory includes a list of all the files in the directory at the time the dump was done, and a flag for each file indicating whether the file is going to be put in the archive. This information is used when doing a complete incremental restore. Note that this option causes `tar' to create a non-standard archive that may not be readable by non-GNU versions of the `tar' program. The `--incremental' (`-G') option means the archive is an incremental backup. Its meaning depends on the command that it modifies. If the `--incremental' (`-G') option is used with `--list' (`-t'), `tar' will list, for each directory in the archive, the list of files in that directory at the time the archive was created. This information is put out in a format that is not easy for humans to read, but which is unambiguous for a program: each file name is preceded by either a `Y' if the file is present in the archive, an `N' if the file is not included in the archive, or a `D' if the file is a directory (and is included in the archive). Each file name is terminated by a null character. The last file is followed by an additional null and a newline to indicate the end of the data. If the `--incremental' (`-G') option is used with `--extract' (`--get', `-x'), then when the entry for a directory is found, all files that currently exist in that directory but are not listed in the archive _are deleted from the directory_. This behavior is convenient when you are restoring a damaged file system from a succession of incremental backups: it restores the entire state of the file system to that which obtained when the backup was made. If you don't use `--incremental' (`-G'), the file system will probably fill up with files that shouldn't exist any more. `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') handle new GNU-format incremental backup. This option handles new GNU-format incremental backup. It has much the same effect as `--incremental' (`-G'), but also the time when the dump is done and the list of directories dumped is written to the given FILE. When restoring, only files newer than the saved time are restored, and the directory list is used to speed up operations. `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') acts like `--incremental' (`-G'), but when used in conjunction with `--create' (`-c') will also cause `tar' to use the file FILE, which contains information about the state of the filesystem at the time of the last backup, to decide which files to include in the archive being created. That file will then be updated by `tar'. If the file FILE does not exist when this option is specified, `tar' will create it, and include all appropriate files in the archive. The file, which is archive independent, contains the date it was last modified and a list of devices, inode numbers and directory names. `tar' will archive files with newer mod dates or inode change times, and directories with an unchanged inode number and device but a changed directory name. The file is updated after the files to be archived are determined, but before the new archive is actually created. File: tar.info, Node: Inc Dumps, Next: incremental and listed-incremental, Prev: Full Dumps, Up: Backups 5.2 Using `tar' to Perform Incremental Dumps ============================================ _(This message will disappear, once this node revised.)_ Performing incremental dumps is similar to performing full dumps, although a few more options will usually be needed. A standard scheme is to do a _monthly_ (full) dump once a month, a _weekly_ dump once a week of everything since the last monthly and a _daily_ every day of everything since the last (weekly or monthly) dump. Here is a sample script to dump the directory hierarchies `/usr' and `/var'. #! /bin/sh tar --create \ --blocking-factor=126 \ --file=/dev/rmt/0 \ --label="`hostname` /usr /var `date +%Y-%m-%d`" \ --listed-incremental=/var/log/usr-var.snar \ --verbose \ /usr /var This script uses the file `/var/log/usr-var.snar' as a snapshot to store information about the previous tar dump. The blocking factor 126 is an attempt to make the tape drive stream. Some tape devices cannot handle 64 kB blocks or larger, and require the block size to be a multiple of 1 kB; for these devices, 126 is the largest blocking factor that can be used. File: tar.info, Node: incremental and listed-incremental, Next: Backup Levels, Prev: Inc Dumps, Up: Backups 5.3 The Incremental Options =========================== _(This message will disappear, once this node revised.)_ `--incremental' (`-G') is used in conjunction with `--create' (`-c'), `--extract' (`--get', `-x') or `--list' (`-t') when backing up and restoring file systems. An archive cannot be extracted or listed with the `--incremental' (`-G') option specified unless it was created with the option specified. This option should only be used by a script, not by the user, and is usually disregarded in favor of `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE'), which is described below. `--incremental' (`-G') in conjunction with `--create' (`-c') causes `tar' to write, at the beginning of the archive, an entry for each of the directories that will be archived. The entry for a directory includes a list of all the files in the directory at the time the archive was created and a flag for each file indicating whether or not the file is going to be put in the archive. Note that this option causes `tar' to create a non-standard archive that may not be readable by non-GNU versions of the `tar' program. `--incremental' (`-G') in conjunction with `--extract' (`--get', `-x') causes `tar' to read the lists of directory contents previously stored in the archive, _delete_ files in the file system that did not exist in their directories when the archive was created, and then extract the files in the archive. This behavior is convenient when restoring a damaged file system from a succession of incremental backups: it restores the entire state of the file system to that which obtained when the backup was made. If `--incremental' (`-G') isn't specified, the file system will probably fill up with files that shouldn't exist any more. `--incremental' (`-G') in conjunction with `--list' (`-t') causes `tar' to print, for each directory in the archive, the list of files in that directory at the time the archive was created. This information is put out in a format that is not easy for humans to read, but which is unambiguous for a program: each file name is preceded by either a `Y' if the file is present in the archive, an `N' if the file is not included in the archive, or a `D' if the file is a directory (and is included in the archive). Each file name is terminated by a null character. The last file is followed by an additional null and a newline to indicate the end of the data. `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') acts like `--incremental' (`-G'), but when used in conjunction with `--create' (`-c') will also cause `tar' to use the file SNAPSHOT-FILE, which contains information about the state of the file system at the time of the last backup, to decide which files to include in the archive being created. That file will then be updated by `tar'. If the file FILE does not exist when this option is specified, `tar' will create it, and include all appropriate files in the archive. The file FILE, which is archive independent, contains the date it was last modified and a list of devices, inode numbers and directory names. `tar' will archive files with newer mod dates or inode change times, and directories with an unchanged inode number and device but a changed directory name. The file is updated after the files to be archived are determined, but before the new archive is actually created. Incremental dumps depend crucially on time stamps, so the results are unreliable if you modify a file's time stamps during dumping (e.g. with the `--atime-preserve' option), or if you set the clock backwards. Despite it should be obvious that a device has a non-volatile value, NFS devices have non-dependable values when an automounter gets in the picture. This led to a great deal of spurious redumping in incremental dumps, so it is somewhat useless to compare two NFS devices numbers over time. So `tar' now considers all NFS devices as being equal when it comes to comparing directories; this is fairly gross, but there does not seem to be a better way to go. `tar' doesn't access SNAPSHOT-FILE when `--extract' (`--get', `-x') or `--list' (`-t') are specified, but the `--listed-incremental=SNAPSHOT-FILE' (`-g SNAPSHOT-FILE') option must still be given. A placeholder SNAPSHOT-FILE can be specified, e.g., `/dev/null'. File: tar.info, Node: Backup Levels, Next: Backup Parameters, Prev: incremental and listed-incremental, Up: Backups 5.4 Levels of Backups ===================== An archive containing all the files in the file system is called a "full backup" or "full dump". You could insure your data by creating a full dump every day. This strategy, however, would waste a substantial amount of archive media and user time, as unchanged files are daily re-archived. It is more efficient to do a full dump only occasionally. To back up files between full dumps, you can use "incremental dumps". A "level one" dump archives all the files that have changed since the last full dump. A typical dump strategy would be to perform a full dump once a week, and a level one dump once a day. This means some versions of files will in fact be archived more than once, but this dump strategy makes it possible to restore a file system to within one day of accuracy by only extracting two archives--the last weekly (full) dump and the last daily (level one) dump. The only information lost would be in files changed or created since the last daily backup. (Doing dumps more than once a day is usually not worth the trouble). GNU `tar' comes with scripts you can use to do full and level-one (actually, even level-two and so on) dumps. Using scripts (shell programs) to perform backups and restoration is a convenient and reliable alternative to typing out file name lists and `tar' commands by hand. Before you use these scripts, you need to edit the file `backup-specs', which specifies parameters used by the backup scripts and by the restore script. This file is usually located in `/etc/backup' directory. Once the backup parameters are set, you can perform backups or restoration by running the appropriate script. The name of the backup script is `backup'. The name of the restore script is `restore'. The following sections describe their use in detail. _Please Note:_ The backup and restoration scripts are designed to be used together. While it is possible to restore files by hand from an archive which was created using a backup script, and to create an archive by hand which could then be extracted using the restore script, it is easier to use the scripts. *Note Inc Dumps::, and *Note Inc Dumps::, before making such an attempt. File: tar.info, Node: Backup Parameters, Next: Scripted Backups, Prev: Backup Levels, Up: Backups 5.5 Setting Parameters for Backups and Restoration ================================================== The file `backup-specs' specifies backup parameters for the backup and restoration scripts provided with `tar'. You must edit `backup-specs' to fit your system configuration and schedule before using these scripts. Syntactically, `backup-specs' is a shell script, containing mainly variable assignments. However, any valid shell construct is allowed in this file. Particularly, you may wish to define functions within that script (e.g. see `RESTORE_BEGIN' below). For more information about shell script syntax, please refer to the definition of the Shell Command Language (http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta g_02). See also *Note Bash Features: (bashref)Top. The shell variables controlling behavior of `backup' and `restore' are described in the following subsections. * Menu: * General-Purpose Variables:: * Magnetic Tape Control:: * User Hooks:: * backup-specs example:: An Example Text of `Backup-specs' File: tar.info, Node: General-Purpose Variables, Next: Magnetic Tape Control, Up: Backup Parameters 5.5.1 General-Purpose Variables ------------------------------- -- Backup variable: ADMINISTRATOR The user name of the backup administrator. `Backup' scripts sends a backup report to this address. -- Backup variable: BACKUP_HOUR The hour at which the backups are done. This can be a number from 0 to 23, or the time specification in form HOURS:MINUTES, or the string `now'. This variable is used by `backup'. Its value may be overridden using `--time' option (*note Scripted Backups::). -- Backup variable: TAPE_FILE The device `tar' writes the archive to. This device should be attached to the host on which the dump scripts are run. -- Backup variable: BLOCKING The blocking factor `tar' will use when writing the dump archive. *Note Blocking Factor::. -- Backup variable: BACKUP_DIRS A list of file systems to be dumped (for `backup'), or restored (for `restore'). You can include any directory name in the list -- subdirectories on that file system will be included, regardless of how they may look to other networked machines. Subdirectories on other file systems will be ignored. The host name specifies which host to run `tar' on, and should normally be the host that actually contains the file system. However, the host machine must have GNU `tar' installed, and must be able to access the directory containing the backup scripts and their support files using the same file name that is used on the machine where the scripts are run (ie. what `pwd' will print when in that directory on that machine). If the host that contains the file system does not have this capability, you can specify another host as long as it can access the file system through NFS. If the list of file systems is very long you may wish to put it in a separate file. This file is usually named `/etc/backup/dirs', but this name may be overridden in `backup-specs' using `DIRLIST' variable. -- Backup variable: DIRLIST A path to the file containing the list of the filesystems to backup or restore. By default it is `/etc/backup/dirs'. -- Backup variable: BACKUP_FILES A list of individual files to be dumped (for `backup'), or restored (for `restore'). These should be accessible from the machine on which the backup script is run. If the list of file systems is very long you may wish to store it in a separate file. This file is usually named `/etc/backup/files', but this name may be overridden in `backup-specs' using `FILELIST' variable. -- Backup variable: FILELIST A path to the file containing the list of the individual files to backup or restore. By default it is `/etc/backup/files'. -- Backup variable: RSH Path to `rsh' binary or its equivalent. You may wish to set it to `ssh', to improve security. In this case you will have to use public key authentication. -- Backup variable: RSH_COMMAND Path to rsh binary on remote mashines. This will be passed via `--rsh-command' option to the remote invocation of GNU `tar'. -- Backup variable: VOLNO_FILE Name of temporary file to hold volume numbers. This needs to be accessible by all the machines which have filesystems to be dumped. -- Backup variable: XLIST Name of "exclude file list". An "exclude file list" is a file located on the remote machine and containing the list of files to be excluded from the backup. Exclude file lists are searched in /etc/tar-backup directory. A common use for exclude file lists is to exclude files containing security-sensitive information (e.g. `/etc/shadow' from backups). This variable affects only `backup'. -- Backup variable: SLEEP_TIME Time to sleep between dumps of any two successive filesystems This variable affects only `backup'. -- Backup variable: DUMP_REMIND_SCRIPT Script to be run when it's time to insert a new tape in for the next volume. Administrators may want to tailor this script for their site. If this variable isn't set, GNU `tar' will display its built-in prompt , and will expect confirmation from the console. -- Backup variable: SLEEP_MESSAGE Message to display on the terminal while waiting for dump time. Usually this will just be some literal text. -- Backup variable: TAR Pathname of the GNU `tar' executable. If this is not set, backup scripts will search `tar' in the current shell path. File: tar.info, Node: Magnetic Tape Control, Next: User Hooks, Prev: General-Purpose Variables, Up: Backup Parameters 5.5.2 Magnetic Tape Control --------------------------- Backup scripts access tape device using special "hook functions". These functions take a single argument - the name of the tape device. Their names are kept in the following variables: -- Backup variable: MT_BEGIN The name of "begin" function. This function is called before accessing the drive. By default it retensions the tape: MT_BEGIN=mt_begin mt_begin() { mt -f "$1" retension } -- Backup variable: MT_REWIND The name of "rewind" function. The default definition is as follows: MT_REWIND=mt_rewind mt_rewind() { mt -f "$1" rewind } -- Backup variable: MT_OFFLINE The name of the function switching the tape off line. By default it is defined as follows: MT_OFFLINE=mt_offline mt_offline() { mt -f "$1" offl } -- Backup variable: MT_STATUS The name of the function used to obtain the status of the archive device, including error count. Default definition: MT_STATUS=mt_status mt_status() { mt -f "$1" status } File: tar.info, Node: User Hooks, Next: backup-specs example, Prev: Magnetic Tape Control, Up: Backup Parameters 5.5.3 User Hooks ---------------- "User hooks" are shell functions executed before and after each `tar' invocation. Thus, there are "backup hooks", which are executed before and after dumping each file system, and "restore hooks", executed before and after restoring a file system. Each user hook is a shell function taking four arguments: -- User Hook Function: hook LEVEL HOST FS FSNAME Its arguments are: LEVEL Current backup or restore level. HOST Name or IP address of the host machine being dumped or restored. FS Full path name to the filesystem being dumped or restored. FSNAME Filesystem name with directory separators replaced with colons. This is useful e.g. for creating unique files. Following variables keep the names of user hook functions -- Backup variable: DUMP_BEGIN Dump begin function. It is executed before dumping the filesystem. -- Backup variable: DUMP_END Executed after dumping the filesystem. -- Backup variable: RESTORE_BEGIN Executed before restoring the filesystem. -- Backup variable: RESTORE_END Executed after restoring the filesystem. File: tar.info, Node: backup-specs example, Prev: User Hooks, Up: Backup Parameters 5.5.4 An Example Text of `Backup-specs' --------------------------------------- The following is an example of `backup-specs': # site-specific parameters for file system backup. ADMINISTRATOR=friedman BACKUP_HOUR=1 TAPE_FILE=/dev/nrsmt0 # Use `ssh' instead of the less secure `rsh' RSH=/usr/bin/ssh RSH_COMMAND=/usr/bin/ssh # Override MT_STATUS function: my_status() { mts -t $TAPE_FILE } MT_STATUS=my_status # Disable MT_OFFLINE function MT_OFFLINE=: BLOCKING=124 BACKUP_DIRS=" albert:/fs/fsf apple-gunkies:/gd albert:/fs/gd2 albert:/fs/gp geech:/usr/jla churchy:/usr/roland albert:/ albert:/usr apple-gunkies:/ apple-gunkies:/usr gnu:/hack gnu:/u apple-gunkies:/com/mailer/gnu apple-gunkies:/com/archive/gnu" BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]" File: tar.info, Node: Scripted Backups, Next: Scripted Restoration, Prev: Backup Parameters, Up: Backups 5.6 Using the Backup Scripts ============================ The syntax for running a backup script is: backup --level=LEVEL --time=TIME The `level' option requests the dump level. Thus, to produce a full dump, specify `--level=0' (this is the default, so `--level' may be omitted if its value is `0'). (1) The `--time' option determines when should the backup be run. TIME may take three forms: HH:MM The dump must be run at HH hours MM minutes. HH The dump must be run at HH hours now The dump must be run immediately. You should start a script with a tape or disk mounted. Once you start a script, it prompts you for new tapes or disks as it needs them. Media volumes don't have to correspond to archive files -- a multi-volume archive can be started in the middle of a tape that already contains the end of another multi-volume archive. The `restore' script prompts for media by its archive volume, so to avoid an error message you should keep track of which tape (or disk) contains which volume of the archive (*note Scripted Restoration::). The backup scripts write two files on the file system. The first is a record file in `/etc/tar-backup/', which is used by the scripts to store and retrieve information about which files were dumped. This file is not meant to be read by humans, and should not be deleted by them. The second file is a log file containing the names of the file systems and files dumped, what time the backup was made, and any error messages that were generated, as well as how much space was left in the media volume after the last volume of the archive was written. You should check this log file after every backup. The file name is `log-MM-DD-YYYY-level-N', where MM-DD-YYYY represents current date, and N represents current dump level number. The script also prints the name of each system being dumped to the standard output. Following is the full list of options accepted by `backup' script: `-l LEVEL' `--level=LEVEL' Do backup level LEVEL (default 0). `-f' `--force' Force backup even if today's log file already exists. `-v[LEVEL]' `--verbose[=LEVEL]' Set verbosity level. The higher the level is, the more debugging information will be output during execution. Devault LEVEL is 100, which means the highest debugging level. `-t START-TIME' `--time=START-TIME' Wait till TIME, then do backup. `-h' `--help' Display short help message and exit. `-L' `--license' Display program license and exit. `-V' `--version' Display program version and exit. ---------- Footnotes ---------- (1) For backward compatibility, the `backup' will also try to deduce the requested dump level from the name of the script itself. If the name consists of a string `level-' followed by a single decimal digit, that digit is taken as the dump level number. Thus, you may create a link from `backup' to `level-1' and then run `level-1' whenever you need to create a level one dump. File: tar.info, Node: Scripted Restoration, Prev: Scripted Backups, Up: Backups 5.7 Using the Restore Script ============================ To restore files that were archived using a scripted backup, use the `restore' script. Its usage is quite straightforward. In the simplest form, invoke `restore --all', it will then restore all the filesystems and files specified in `backup-specs' (*note BACKUP_DIRS: General-Purpose Variables.). You may select the filesystems (and/or files) to restore by giving `restore' list of "patterns" in its command line. For example, running restore 'albert:*' will restore all filesystems on the machine `albert'. A more complicated example: restore 'albert:*' '*:/var' This command will restore all filesystems on the machine `albert' as well as `/var' filesystem on all machines. By default `restore' will start restoring files from the lowest available dump level (usually zero) and will continue through all available dump levels. There may be situations where such a thorough restore is not necessary. For example, you may wish to restore only files from the recent level one backup. To do so, use `--level' option, as shown in the example below: restore --level=1 The full list of options accepted by `restore' follows: `-a' `--all' Restore all filesystems and files specified in `backup-specs' `-l LEVEL' `--level=LEVEL' Start restoring from the given backup level, instead of the default 0. `-v[LEVEL]' `--verbose[=LEVEL]' Set verbosity level. The higher the level is, the more debugging information will be output during execution. Devault LEVEL is 100, which means the highest debugging level. `-h' `--help' Display short help message and exit. `-L' `--license' Display program license and exit. `-V' `--version' Display program version and exit. You should start the restore script with the media containing the first volume of the archive mounted. The script will prompt for other volumes as they are needed. If the archive is on tape, you don't need to rewind the tape to to its beginning--if the tape head is positioned past the beginning of the archive, the script will rewind the tape as needed. *Warning:* The script will delete files from the active file system if they were not in the file system when the archive was made. *Note Inc Dumps::, and *Note Inc Dumps::, for an explanation of how the script makes that determination. File: tar.info, Node: Choosing, Next: Date input formats, Prev: Backups, Up: Top 6 Choosing Files and Names for `tar' ************************************ _(This message will disappear, once this node revised.)_ Certain options to `tar' enable you to specify a name for your archive. Other options let you decide which files to include or exclude from the archive, based on when or whether files were modified, whether the file names do or don't match specified patterns, or whether files are in specified directories. * Menu: * file:: Choosing the Archive's Name * Selecting Archive Members:: * files:: Reading Names from a File * exclude:: Excluding Some Files * Wildcards:: * after:: Operating Only on New Files * recurse:: Descending into Directories * one:: Crossing Filesystem Boundaries File: tar.info, Node: file, Next: Selecting Archive Members, Up: Choosing 6.1 Choosing and Naming Archive Files ===================================== _(This message will disappear, once this node revised.)_ By default, `tar' uses an archive file name that was compiled when it was built on the system; usually this name refers to some physical tape drive on the machine. However, the person who installed `tar' on the system may not set the default to a meaningful value as far as most users are concerned. As a result, you will usually want to tell `tar' where to find (or create) the archive. The `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') option allows you to either specify or name a file to use as the archive instead of the default archive file location. `--file=ARCHIVE-NAME' `-f ARCHIVE-NAME' Name the archive to create or operate on. Use in conjunction with any operation. For example, in this `tar' command, $ tar -cvf collection.tar blues folk jazz `collection.tar' is the name of the archive. It must directly follow the `-f' option, since whatever directly follows `-f' _will_ end up naming the archive. If you neglect to specify an archive name, you may end up overwriting a file in the working directory with the archive you create since `tar' will use this file's name for the archive name. An archive can be saved as a file in the file system, sent through a pipe or over a network, or written to an I/O device such as a tape, floppy disk, or CD write drive. If you do not name the archive, `tar' uses the value of the environment variable `TAPE' as the file name for the archive. If that is not available, `tar' uses a default, compiled-in archive name, usually that for tape unit zero (ie. `/dev/tu00'). `tar' always needs an archive name. If you use `-' as an ARCHIVE-NAME, `tar' reads the archive from standard input (when listing or extracting files), or writes it to standard output (when creating an archive). If you use `-' as an ARCHIVE-NAME when modifying an archive, `tar' reads the original archive from its standard input and writes the entire new archive to its standard output. $ cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -) To specify an archive file on a device attached to a remote machine, use the following: --file=HOSTNAME:/DEV/FILE NAME `tar' will complete the remote connection, if possible, and prompt you for a username and password. If you use `--file=@HOSTNAME:/DEV/FILE NAME', `tar' will complete the remote connection, if possible, using your username as the username on the remote machine. If the archive file name includes a colon (`:'), then it is assumed to be a file on another machine. If the archive file is `USER@HOST:FILE', then FILE is used on the host HOST. The remote host is accessed using the `rsh' program, with a username of USER. If the username is omitted (along with the `@' sign), then your user name will be used. (This is the normal `rsh' behavior.) It is necessary for the remote machine, in addition to permitting your `rsh' access, to have the `rmt' program installed (This command is included in the GNU `tar' distribution and by default is installed under `PREFIX/libexec/rmt', were PREFIX means your installation prefix). If you need to use a file whose name includes a colon, then the remote tape drive behavior can be inhibited by using the `--force-local' option. When the archive is being created to `/dev/null', GNU `tar' tries to minimize input and output operations. The Amanda backup system, when used with GNU `tar', has an initial sizing pass which uses this feature. File: tar.info, Node: Selecting Archive Members, Next: files, Prev: file, Up: Choosing 6.2 Selecting Archive Members ============================= "File Name arguments" specify which files in the file system `tar' operates on, when creating or adding to an archive, or which archive members `tar' operates on, when reading or deleting from an archive. *Note Operations::. To specify file names, you can include them as the last arguments on the command line, as follows: tar OPERATION [OPTION1 OPTION2 ...] [FILE NAME-1 FILE NAME-2 ...] If you specify a directory name as a file name argument, all the files in that directory are operated on by `tar'. If you do not specify files when `tar' is invoked with `--create' (`-c'), `tar' operates on all the non-directory files in the working directory. If you specify either `--list' (`-t') or `--extract' (`--get', `-x'), `tar' operates on all the archive members in the archive. If you specify any operation other than one of these three, `tar' does nothing. By default, `tar' takes file names from the command line. However, there are other ways to specify file or member names, or to modify the manner in which `tar' selects the files or members upon which to operate. In general, these methods work both for specifying the names of files and archive members. File: tar.info, Node: files, Next: exclude, Prev: Selecting Archive Members, Up: Choosing 6.3 Reading Names from a File ============================= _(This message will disappear, once this node revised.)_ Instead of giving the names of files or archive members on the command line, you can put the names into a file, and then use the `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') option to `tar'. Give the name of the file which contains the list of files to include as the argument to `--files-from'. In the list, the file names should be separated by newlines. You will frequently use this option when you have generated the list of files to archive with the `find' utility. `--files-from=FILE NAME' `-T FILE NAME' Get names to extract or create from file FILE NAME. If you give a single dash as a file name for `--files-from', (i.e., you specify either `--files-from=-' or `-T -'), then the file names are read from standard input. Unless you are running `tar' with `--create', you can not use both `--files-from=-' and `--file=-' (`-f -') in the same command. The following example shows how to use `find' to generate a list of files smaller than 400K in length and put that list into a file called `small-files'. You can then use the `-T' option to `tar' to specify the files from that file, `small-files', to create the archive `little.tgz'. (The `-z' option to `tar' compresses the archive with `gzip'; *note gzip:: for more information.) $ find . -size -400 -print > small-files $ tar -c -v -z -T small-files -f little.tgz * Menu: * nul:: File: tar.info, Node: nul, Up: files 6.3.1 `NUL' Terminated File Names --------------------------------- The `--null' option causes `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') to read file names terminated by a `NUL' instead of a newline, so files whose names contain newlines can be archived using `--files-from'. `--null' Only consider `NUL' terminated file names, instead of files that terminate in a newline. The `--null' option is just like the one in GNU `xargs' and `cpio', and is useful with the `-print0' predicate of GNU `find'. In `tar', `--null' also causes `--directory=DIRECTORY' (`-C DIRECTORY') options to be treated as file names to archive, in case there are any files out there called `-C'. This example shows how to use `find' to generate a list of files larger than 800K in length and put that list into a file called `long-files'. The `-print0' option to `find' just just like `-print', except that it separates files with a `NUL' rather than with a newline. You can then run `tar' with both the `--null' and `-T' options to specify that `tar' get the files from that file, `long-files', to create the archive `big.tgz'. The `--null' option to `tar' will cause `tar' to recognize the `NUL' separator between files. $ find . -size +800 -print0 > long-files $ tar -c -v --null --files-from=long-files --file=big.tar File: tar.info, Node: exclude, Next: Wildcards, Prev: files, Up: Choosing 6.4 Excluding Some Files ======================== _(This message will disappear, once this node revised.)_ To avoid operating on files whose names match a particular pattern, use the `--exclude=PATTERN' or `--exclude-from=FILE-OF-PATTERNS' (`-X FILE-OF-PATTERNS') options. `--exclude=PATTERN' Causes `tar' to ignore files that match the PATTERN. The `--exclude=PATTERN' option prevents any file or member whose name matches the shell wildcard (PATTERN) from being operated on. For example, to create an archive with all the contents of the directory `src' except for files whose names end in `.o', use the command `tar -cf src.tar --exclude='*.o' src'. You may give multiple `--exclude' options. `--exclude-from=FILE' `-X FILE' Causes `tar' to ignore files that match the patterns listed in FILE. Use the `--exclude-from=FILE-OF-PATTERNS' option to read a list of patterns, one per line, from FILE; `tar' will ignore files matching those patterns. Thus if `tar' is called as `tar -c -X foo .' and the file `foo' contains a single line `*.o', no files whose names end in `.o' will be added to the archive. `--exclude-caches' Causes `tar' to ignore directories containing a cache directory tag. When creating an archive, the `--exclude-caches' option causes `tar' to exclude all directories that contain a "cache directory tag". A cache directory tag is a short file with the well-known name `CACHEDIR.TAG' and having a standard header specified in `http://www.brynosaurus.com/cachedir/spec.html'. Various applications write cache directory tags into directories they use to hold regenerable, non-precious data, so that such data can be more easily excluded from backups. * Menu: * controlling pattern-patching with exclude:: * problems with exclude:: File: tar.info, Node: controlling pattern-patching with exclude, Next: problems with exclude, Up: exclude Controlling Pattern-Matching with the `exclude' Options ------------------------------------------------------- Normally, a pattern matches a name if an initial subsequence of the name's components matches the pattern, where `*', `?', and `[...]' are the usual shell wildcards, `\' escapes wildcards, and wildcards can match `/'. Other than optionally stripping leading `/' from names (*note absolute::), patterns and names are used as-is. For example, trailing `/' is not trimmed from a user-specified name before deciding whether to exclude it. However, this matching procedure can be altered by the options listed below. These options accumulate. For example: --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme' ignores case when excluding `makefile', but not when excluding `readme'. `--anchored' `--no-anchored' If anchored, a pattern must match an initial subsequence of the name's components. Otherwise, the pattern can match any subsequence. Default is `--no-anchored'. `--ignore-case' `--no-ignore-case' When ignoring case, upper-case patterns match lower-case names and vice versa. When not ignoring case (the default), matching is case-sensitive. `--wildcards' `--no-wildcards' When using wildcards (the default), `*', `?', and `[...]' are the usual shell wildcards, and `\' escapes wildcards. Otherwise, none of these characters are special, and patterns must match names literally. `--wildcards-match-slash' `--no-wildcards-match-slash' When wildcards match slash (the default), a wildcard like `*' in the pattern can match a `/' in the name. Otherwise, `/' is matched only by `/'. The `--recursion' and `--no-recursion' options (*note recurse::) also affect how exclude patterns are interpreted. If recursion is in effect, a pattern excludes a name if it matches any of the name's parent directories. File: tar.info, Node: problems with exclude, Prev: controlling pattern-patching with exclude, Up: exclude Problems with Using the `exclude' Options ----------------------------------------- Some users find `exclude' options confusing. Here are some common pitfalls: * The main operating mode of `tar' does not act on a path name explicitly listed on the command line if one of its file name components is excluded. In the example above, if you create an archive and exclude files that end with `*.o', but explicitly name the file `dir.o/foo' after all the options have been listed, `dir.o/foo' will be excluded from the archive. * You can sometimes confuse the meanings of `--exclude=PATTERN' and `--exclude-from=FILE-OF-PATTERNS' (`-X FILE-OF-PATTERNS'). Be careful: use `--exclude=PATTERN' when files to be excluded are given as a pattern on the command line. Use `--exclude-from=FILE-OF-PATTERNS' to introduce the name of a file which contains a list of patterns, one per line; each of these patterns can exclude zero, one, or many files. * When you use `--exclude=PATTERN', be sure to quote the PATTERN parameter, so GNU `tar' sees wildcard characters like `*'. If you do not do this, the shell might expand the `*' itself using files at hand, so `tar' might receive a list of files instead of one pattern, or none at all, making the command somewhat illegal. This might not correspond to what you want. For example, write: $ tar -c -f ARCHIVE.TAR --exclude '*.o' DIRECTORY rather than: $ tar -c -f ARCHIVE.TAR --exclude *.o DIRECTORY * You must use use shell syntax, or globbing, rather than `regexp' syntax, when using exclude options in `tar'. If you try to use `regexp' syntax to describe files to be excluded, your command might fail. * In earlier versions of `tar', what is now the `--exclude-from=FILE-OF-PATTERNS' option was called `--exclude=PATTERN' instead. Now, `--exclude=PATTERN' applies to patterns listed on the command line and `--exclude-from=FILE-OF-PATTERNS' applies to patterns listed in a file. File: tar.info, Node: Wildcards, Next: after, Prev: exclude, Up: Choosing 6.5 Wildcards Patterns and Matching =================================== "Globbing" is the operation by which "wildcard" characters, `*' or `?' for example, are replaced and expanded into all existing files matching the given pattern. However, `tar' often uses wildcard patterns for matching (or globbing) archive members instead of actual files in the filesystem. Wildcard patterns are also used for verifying volume labels of `tar' archives. This section has the purpose of explaining wildcard syntax for `tar'. A PATTERN should be written according to shell syntax, using wildcard characters to effect globbing. Most characters in the pattern stand for themselves in the matched string, and case is significant: `a' will match only `a', and not `A'. The character `?' in the pattern matches any single character in the matched string. The character `*' in the pattern matches zero, one, or more single characters in the matched string. The character `\' says to take the following character of the pattern _literally_; it is useful when one needs to match the `?', `*', `[' or `\' characters, themselves. The character `[', up to the matching `]', introduces a character class. A "character class" is a list of acceptable characters for the next single character of the matched string. For example, `[abcde]' would match any of the first five letters of the alphabet. Note that within a character class, all of the "special characters" listed above other than `\' lose their special meaning; for example, `[-\\[*?]]' would match any of the characters, `-', `\', `[', `*', `?', or `]'. (Due to parsing constraints, the characters `-' and `]' must either come _first_ or _last_ in a character class.) If the first character of the class after the opening `[' is `!' or `^', then the meaning of the class is reversed. Rather than listing character to match, it lists those characters which are _forbidden_ as the next single character of the matched string. Other characters of the class stand for themselves. The special construction `[A-E]', using an hyphen between two letters, is meant to represent all characters between A and E, inclusive. Periods (`.') or forward slashes (`/') are not considered special for wildcard matches. However, if a pattern completely matches a directory prefix of a matched string, then it matches the full matched string: excluding a directory also excludes all the files beneath it. File: tar.info, Node: after, Next: recurse, Prev: Wildcards, Up: Choosing 6.6 Operating Only on New Files =============================== _(This message will disappear, once this node revised.)_ The `--after-date=DATE' (`--newer=DATE', `-N DATE') option causes `tar' to only work on files whose modification or inode-changed times are newer than the DATE given. If DATE starts with `/' or `.', it is taken to be a file name; the last-modified time of that file is used as the date. If you use this option when creating or appending to an archive, the archive will only include new files. If you use `--after-date' when extracting an archive, `tar' will only extract files newer than the DATE you specify. If you only want `tar' to make the date comparison based on modification of the actual contents of the file (rather than inode changes), then use the `--newer-mtime=DATE' option. You may use these options with any operation. Note that these options differ from the `--update' (`-u') operation in that they allow you to specify a particular date against which `tar' can compare when deciding whether or not to archive the files. `--after-date=DATE' `--newer=DATE' `-N DATE' Only store files newer than DATE. Acts on files only if their modification or inode-changed times are later than DATE. Use in conjunction with any operation. If DATE starts with `/' or `.', it is taken to be a file name; the last-modified time of that file is used as the date. `--newer-mtime=DATE' Acts like `--after-date=DATE' (`--newer=DATE', `-N DATE'), but only looks at modification times. These options limit `tar' to only operating on files which have been modified after the date specified. A file is considered to have changed if the contents have been modified, or if the owner, permissions, and so forth, have been changed. (For more information on how to specify a date, see *Note Date input formats::; remember that the entire date argument must be quoted if it contains any spaces.) Gurus would say that `--after-date=DATE' (`--newer=DATE', `-N DATE') tests both the `mtime' (time the contents of the file were last modified) and `ctime' (time the file's status was last changed: owner, permissions, etc) fields, while `--newer-mtime=DATE' tests only `mtime' field. To be precise, `--after-date=DATE' (`--newer=DATE', `-N DATE') checks _both_ `mtime' and `ctime' and processes the file if either one is more recent than DATE, while `--newer-mtime=DATE' only checks `mtime' and disregards `ctime'. Neither uses `atime' (the last time the contents of the file were looked at). Date specifiers can have embedded spaces. Because of this, you may need to quote date arguments to keep the shell from parsing them as separate arguments. *Please Note:* `--after-date=DATE' (`--newer=DATE', `-N DATE') and `--newer-mtime=DATE' should not be used for incremental backups. Some files (such as those in renamed directories) are not selected properly by these options. *Note incremental and listed-incremental::. File: tar.info, Node: recurse, Next: one, Prev: after, Up: Choosing 6.7 Descending into Directories =============================== _(This message will disappear, once this node revised.)_ Usually, `tar' will recursively explore all directories (either those given on the command line or through the `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') option) for the various files they contain. However, you may not always want `tar' to act this way. The `--no-recursion' option inhibits `tar''s recursive descent into specified directories. If you specify `--no-recursion', you can use the `find' utility for hunting through levels of directories to construct a list of file names which you could then pass to `tar'. `find' allows you to be more selective when choosing which files to archive; see *Note files:: for more information on using `find' with `tar', or look. `--no-recursion' Prevents `tar' from recursively descending directories. `--recursion' Requires `tar' to recursively descend directories. This is the default. When you use `--no-recursion', GNU `tar' grabs directory entries themselves, but does not descend on them recursively. Many people use `find' for locating files they want to back up, and since `tar' _usually_ recursively descends on directories, they have to use the `! -d' option to `find' as they usually do not want all the files in a directory. They then use the `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES') option to archive the files located via `find'. The problem when restoring files archived in this manner is that the directories themselves are not in the archive; so the `--same-permissions' (`--preserve-permissions', `-p') option does not affect them--while users might really like it to. Specifying `--no-recursion' is a way to tell `tar' to grab only the directory entries given to it, adding no new files on its own. The `--no-recursion' option also applies when extracting: it causes `tar' to extract only the matched directory entries, not the files under those directories. The `--no-recursion' option also affects how exclude patterns are interpreted (*note controlling pattern-patching with exclude::). The `--no-recursion' and `--recursion' options apply to later options and operands, and can be overridden by later occurrences of `--no-recursion' and `--recursion'. For example: $ tar -cf jams.tar --norecursion grape --recursion grape/concord creates an archive with one entry for `grape', and the recursive contents of `grape/concord', but no entries under `grape' other than `grape/concord'. File: tar.info, Node: one, Prev: recurse, Up: Choosing 6.8 Crossing Filesystem Boundaries ================================== _(This message will disappear, once this node revised.)_ `tar' will normally automatically cross file system boundaries in order to archive files which are part of a directory tree. You can change this behavior by running `tar' and specifying `--one-file-system' (`-l'). This option only affects files that are archived because they are in a directory that is being archived; `tar' will still archive files explicitly named on the command line or through `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES'), regardless of where they reside. `--one-file-system' `-l' Prevents `tar' from crossing file system boundaries when archiving. Use in conjunction with any write operation. The `--one-file-system' option causes `tar' to modify its normal behavior in archiving the contents of directories. If a file in a directory is not on the same filesystem as the directory itself, then `tar' will not archive that file. If the file is a directory itself, `tar' will not archive anything beneath it; in other words, `tar' will not cross mount points. It is reported that using this option, the mount point is is archived, but nothing under it. This option is useful for making full or incremental archival backups of a file system. If this option is used in conjunction with `--verbose' (`-v'), files that are excluded are mentioned by name on the standard error. * Menu: * directory:: Changing Directory * absolute:: Absolute File Names File: tar.info, Node: directory, Next: absolute, Up: one 6.8.1 Changing the Working Directory ------------------------------------ _(This message will disappear, once this node revised.)_ To change the working directory in the middle of a list of file names, either on the command line or in a file specified using `--files-from=FILE-OF-NAMES' (`-T FILE-OF-NAMES'), use `--directory=DIRECTORY' (`-C DIRECTORY'). This will change the working directory to the directory DIRECTORY after that point in the list. `--directory=DIRECTORY' `-C DIRECTORY' Changes the working directory in the middle of a command line. For example, $ tar -c -f jams.tar grape prune -C food cherry will place the files `grape' and `prune' from the current directory into the archive `jams.tar', followed by the file `cherry' from the directory `food'. This option is especially useful when you have several widely separated files that you want to store in the same archive. Note that the file `cherry' is recorded in the archive under the precise name `cherry', _not_ `food/cherry'. Thus, the archive will contain three files that all appear to have come from the same directory; if the archive is extracted with plain `tar --extract', all three files will be written in the current directory. Contrast this with the command, $ tar -c -f jams.tar grape prune -C food red/cherry which records the third file in the archive under the name `red/cherry' so that, if the archive is extracted using `tar --extract', the third file will be written in a subdirectory named `orange-colored'. You can use the `--directory' option to make the archive independent of the original name of the directory holding the files. The following command places the files `/etc/passwd', `/etc/hosts', and `/lib/libc.a' into the archive `foo.tar': $ tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a However, the names of the archive members will be exactly what they were on the command line: `passwd', `hosts', and `libc.a'. They will not appear to be related by file name to the original directories where those files were located. Note that `--directory' options are interpreted consecutively. If `--directory' specifies a relative file name, it is interpreted relative to the then current directory, which might not be the same as the original current working directory of `tar', due to a previous `--directory' option. When using `--files-from' (*note files::), you can put `-C' options in the file list. Notice, however, that in this case the option and its argument must occupy two consecutive lines. For instance, the file list for the above example will be: -C /etc passwd hosts -C /lib libc.a To use it, you would invoke `tar' as follows: $ tar -c -f foo.tar --files-from list Notice also that you can only use the short option variant in the file list, i.e. always use `-C', not `--directory'. The interpretation of `--directory=DIRECTORY' (`-C DIRECTORY') is disabled by `--null' option. File: tar.info, Node: absolute, Prev: directory, Up: one 6.8.2 Absolute File Names ------------------------- _(This message will disappear, once this node revised.)_ `-P' `--absolute-names' Do not strip leading slashes from file names, and permit file names containing a `..' file name component. By default, GNU `tar' drops a leading `/' on input or output, and complains about file names containing a `..' component. This option turns off this behavior. When `tar' extracts archive members from an archive, it strips any leading slashes (`/') from the member name. This causes absolute member names in the archive to be treated as relative file names. This allows you to have such members extracted wherever you want, instead of being restricted to extracting the member in the exact directory named in the archive. For example, if the archive member has the name `/etc/passwd', `tar' will extract it as if the name were really `etc/passwd'. File names containing `..' can cause problems when extracting, so `tar' normally warns you about such files when creating an archive, and rejects attempts to extracts such files. Other `tar' programs do not do this. As a result, if you create an archive whose member names start with a slash, they will be difficult for other people with a non-GNU `tar' program to use. Therefore, GNU `tar' also strips leading slashes from member names when putting members into the archive. For example, if you ask `tar' to add the file `/bin/ls' to an archive, it will do so, but the member name will be `bin/ls'. If you use the `--absolute-names' (`-P') option, `tar' will do none of these transformations. To archive or extract files relative to the root directory, specify the `--absolute-names' (`-P') option. Normally, `tar' acts on files relative to the working directory--ignoring superior directory names when archiving, and ignoring leading slashes when extracting. When you specify `--absolute-names' (`-P'), `tar' stores file names including all superior directory names, and preserves leading slashes. If you only invoked `tar' from the root directory you would never need the `--absolute-names' (`-P') option, but using this option may be more convenient than switching to root. `--absolute-names' Preserves full file names (including superior directory names) when archiving files. Preserves leading slash when extracting files. `tar' prints out a message about removing the `/' from file names. This message appears once per GNU `tar' invocation. It represents something which ought to be told; ignoring what it means can cause very serious surprises, later. Some people, nevertheless, do not want to see this message. Wanting to play really dangerously, one may of course redirect `tar' standard error to the sink. For example, under `sh': $ tar -c -f archive.tar /home 2> /dev/null Another solution, both nicer and simpler, would be to change to the `/' directory first, and then avoid absolute notation. For example: $ (cd / && tar -c -f archive.tar home) $ tar -c -f archive.tar -C / home File: tar.info, Node: Date input formats, Next: Formats, Prev: Choosing, Up: Top 7 Date input formats ******************** First, a quote: Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done better than handing down our present system. It is like a set of trapezoidal building blocks, with no vertical or horizontal surfaces, like a language in which the simplest thought demands ornate constructions, useless particles and lengthy circumlocutions. Unlike the more successful patterns of language and science, which enable us to face experience boldly or at least level-headedly, our system of temporal calculation silently and persistently encourages our terror of time. ... It is as though architects had to measure length in feet, width in meters and height in ells; as though basic instruction manuals demanded a knowledge of five different languages. It is no wonder then that we often look into our own immediate past or future, last Tuesday or a week from Sunday, with feelings of helpless confusion. ... -- Robert Grudin, `Time and the Art of Living'. This section describes the textual date representations that GNU programs accept. These are the strings you, as a user, can supply as arguments to the various programs. The C interface (via the `get_date' function) is not described here. * Menu: * General date syntax:: Common rules. * Calendar date items:: 19 Dec 1994. * Time of day items:: 9:20pm. * Time zone items:: EST, PDT, GMT, ... * Day of week items:: Monday and others. * Relative items in date strings:: next tuesday, 2 years ago. * Pure numbers in date strings:: 19931219, 1440. * Seconds since the Epoch:: @1078100502. * Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al. File: tar.info, Node: General date syntax, Next: Calendar date items, Up: Date input formats 7.1 General date syntax ======================= A "date" is a string, possibly empty, containing many items separated by whitespace. The whitespace may be omitted when no ambiguity arises. The empty string means the beginning of today (i.e., midnight). Order of the items is immaterial. A date string may contain many flavors of items: * calendar date items * time of the day items * time zone items * day of the week items * relative items * pure numbers. We describe each of these item types in turn, below. A few numbers may be written out in words in most contexts. This is most useful for specifying day of the week items or relative items (see below). Here is the list: `first' for 1, `next' for 2, `third' for 3, `fourth' for 4, `fifth' for 5, `sixth' for 6, `seventh' for 7, `eighth' for 8, `ninth' for 9, `tenth' for 10, `eleventh' for 11 and `twelfth' for 12. Also, `last' means exactly -1. When a month is written this way, it is still considered to be written numerically, instead of being "spelled in full"; this changes the allowed strings. In the current implementation, only English is supported for words and abbreviations like `AM', `DST', `EST', `first', `January', `Sunday', `tomorrow', and `year'. The output of `date' is not always acceptable as a date string, not only because of the language problem, but also because there is no standard meaning for time zone items like `IST'. When using `date' to generate a date string intended to be parsed later, specify a date format that is independent of language and that does not use time zone items other than `UTC' and `Z'. Here are some ways to do this: $ LC_ALL=C TZ=UTC0 date Mon Mar 1 00:21:42 UTC 2004 $ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ' 2004-03-01 00:21:42Z $ date --iso-8601=ns # a GNU extension 2004-02-29T16:21:42,692722128-0800 $ date --rfc-2822 # a GNU extension Sun, 29 Feb 2004 16:21:42 -0800 $ date +'%Y-%m-%d %H:%M:%S %z' # %z is a GNU extension. 2004-02-29 16:21:42 -0800 $ date +'@%s.%N' # %s and %N are GNU extensions. @1078100502.692722128 Alphabetic case is completely ignored in dates. Comments may be introduced between round parentheses, as long as included parentheses are properly nested. Hyphens not followed by a digit are currently ignored. Leading zeros on numbers are ignored. File: tar.info, Node: Calendar date items, Next: Time of day items, Prev: General date syntax, Up: Date input formats 7.2 Calendar date items ======================= A "calendar date item" specifies a day of the year. It is specified differently, depending on whether the month is specified numerically or literally. All these strings specify the same calendar date: 1972-09-24 # ISO 8601. 72-9-24 # Assume 19xx for 69 through 99, # 20xx for 00 through 68. 72-09-24 # Leading zeros are ignored. 9/24/72 # Common U.S. writing. 24 September 1972 24 Sept 72 # September has a special abbreviation. 24 Sep 72 # Three-letter abbreviations always allowed. Sep 24, 1972 24-sep-72 24sep72 The year can also be omitted. In this case, the last specified year is used, or the current year if none. For example: 9/24 sep 24 Here are the rules. For numeric months, the ISO 8601 format `YEAR-MONTH-DAY' is allowed, where YEAR is any positive number, MONTH is a number between 01 and 12, and DAY is a number between 01 and 31. A leading zero must be present if a number is less than ten. If YEAR is 68 or smaller, then 2000 is added to it; otherwise, if YEAR is less than 100, then 1900 is added to it. The construct `MONTH/DAY/YEAR', popular in the United States, is accepted. Also `MONTH/DAY', omitting the year. Literal months may be spelled out in full: `January', `February', `March', `April', `May', `June', `July', `August', `September', `October', `November' or `December'. Literal months may be abbreviated to their first three letters, possibly followed by an abbreviating dot. It is also permitted to write `Sept' instead of `September'. When months are written literally, the calendar date may be given as any of the following: DAY MONTH YEAR DAY MONTH MONTH DAY YEAR DAY-MONTH-YEAR Or, omitting the year: MONTH DAY File: tar.info, Node: Time of day items, Next: Time zone items, Prev: Calendar date items, Up: Date input formats 7.3 Time of day items ===================== A "time of day item" in date strings specifies the time on a given day. Here are some examples, all of which represent the same time: 20:02:00.000000 20:02 8:02pm 20:02-0500 # In EST (U.S. Eastern Standard Time). More generally, the time of the day may be given as `HOUR:MINUTE:SECOND', where HOUR is a number between 0 and 23, MINUTE is a number between 0 and 59, and SECOND is a number between 0 and 59 possibly followed by `.' or `,' and a fraction containing one or more digits. Alternatively, `:SECOND' can be omitted, in which case it is taken to be zero. If the time is followed by `am' or `pm' (or `a.m.' or `p.m.'), HOUR is restricted to run from 1 to 12, and `:MINUTE' may be omitted (taken to be zero). `am' indicates the first half of the day, `pm' indicates the second half of the day. In this notation, 12 is the predecessor of 1: midnight is `12am' while noon is `12pm'. (This is the zero-oriented interpretation of `12am' and `12pm', as opposed to the old tradition derived from Latin which uses `12m' for noon and `12pm' for midnight.) The time may alternatively be followed by a time zone correction, expressed as `SHHMM', where S is `+' or `-', HH is a number of zone hours and MM is a number of zone minutes. When a time zone correction is given this way, it forces interpretation of the time relative to Coordinated Universal Time (UTC), overriding any previous specification for the time zone or the local time zone. The MINUTE part of the time of the day may not be elided when a time zone correction is used. This is the best way to specify a time zone correction by fractional parts of an hour. Either `am'/`pm' or a time zone correction may be specified, but not both. File: tar.info, Node: Time zone items, Next: Day of week items, Prev: Time of day items, Up: Date input formats 7.4 Time zone items =================== A "time zone item" specifies an international time zone, indicated by a small set of letters, e.g., `UTC' or `Z' for Coordinated Universal Time. Any included periods are ignored. By following a non-daylight-saving time zone by the string `DST' in a separate word (that is, separated by some white space), the corresponding daylight saving time zone may be specified. Time zone items other than `UTC' and `Z' are obsolescent and are not recommended, because they are ambiguous; for example, `EST' has a different meaning in Australia than in the United States. Instead, it's better to use unambiguous numeric time zone corrections like `-0500', as described in the previous section. File: tar.info, Node: Day of week items, Next: Relative items in date strings, Prev: Time zone items, Up: Date input formats 7.5 Day of week items ===================== The explicit mention of a day of the week will forward the date (only if necessary) to reach that day of the week in the future. Days of the week may be spelled out in full: `Sunday', `Monday', `Tuesday', `Wednesday', `Thursday', `Friday' or `Saturday'. Days may be abbreviated to their first three letters, optionally followed by a period. The special abbreviations `Tues' for `Tuesday', `Wednes' for `Wednesday' and `Thur' or `Thurs' for `Thursday' are also allowed. A number may precede a day of the week item to move forward supplementary weeks. It is best used in expression like `third monday'. In this context, `last DAY' or `next DAY' is also acceptable; they move one week before or after the day that DAY by itself would represent. A comma following a day of the week item is ignored. File: tar.info, Node: Relative items in date strings, Next: Pure numbers in date strings, Prev: Day of week items, Up: Date input formats 7.6 Relative items in date strings ================================== "Relative items" adjust a date (or the current date if none) forward or backward. The effects of relative items accumulate. Here are some examples: 1 year 1 year ago 3 years 2 days The unit of time displacement may be selected by the string `year' or `month' for moving by whole years or months. These are fuzzy units, as years and months are not all of equal duration. More precise units are `fortnight' which is worth 14 days, `week' worth 7 days, `day' worth 24 hours, `hour' worth 60 minutes, `minute' or `min' worth 60 seconds, and `second' or `sec' worth one second. An `s' suffix on these units is accepted and ignored. The unit of time may be preceded by a multiplier, given as an optionally signed number. Unsigned numbers are taken as positively signed. No number at all implies 1 for a multiplier. Following a relative item by the string `ago' is equivalent to preceding the unit by a multiplier with value -1. The string `tomorrow' is worth one day in the future (equivalent to `day'), the string `yesterday' is worth one day in the past (equivalent to `day ago'). The strings `now' or `today' are relative items corresponding to zero-valued time displacement, these strings come from the fact a zero-valued time displacement represents the current time when not otherwise changed by previous items. They may be used to stress other items, like in `12:00 today'. The string `this' also has the meaning of a zero-valued time displacement, but is preferred in date strings like `this thursday'. When a relative item causes the resulting date to cross a boundary where the clocks were adjusted, typically for daylight-saving time, the resulting date and time are adjusted accordingly. The fuzz in units can cause problems with relative items. For example, `2003-07-31 -1 month' might evaluate to 2003-07-01, because 2003-06-31 is an invalid date. To determine the previous month more reliably, you can ask for the month before the 15th of the current month. For example: $ date -R Thu, 31 Jul 2003 13:02:39 -0700 $ date --date='-1 month' +'Last month was %B?' Last month was July? $ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!' Last month was June! Also, take care when manipulating dates around clock changes such as daylight saving leaps. In a few cases these have added or subtracted as much as 24 hours from the clock, so it is often wise to adopt universal time by setting the `TZ' environment variable to `UTC0' before embarking on calendrical calculations. File: tar.info, Node: Pure numbers in date strings, Next: Seconds since the Epoch, Prev: Relative items in date strings, Up: Date input formats 7.7 Pure numbers in date strings ================================ The precise interpretation of a pure decimal number depends on the context in the date string. If the decimal number is of the form YYYYMMDD and no other calendar date item (*note Calendar date items::) appears before it in the date string, then YYYY is read as the year, MM as the month number and DD as the day of the month, for the specified calendar date. If the decimal number is of the form HHMM and no other time of day item appears before it in the date string, then HH is read as the hour of the day and MM as the minute of the hour, for the specified time of the day. MM can also be omitted. If both a calendar date and a time of day appear to the left of a number in the date string, but no relative item, then the number overrides the year. File: tar.info, Node: Seconds since the Epoch, Next: Authors of get_date, Prev: Pure numbers in date strings, Up: Date input formats 7.8 Seconds since the Epoch =========================== If you precede a number with `@', it represents an internal time stamp as a count of seconds. The number can contain an internal decimal point (either `.' or `,'); any excess precision not supported by the internal representation is truncated toward minus infinity. Internally, computer times are represented as a count of seconds since an epoch--a well-defined point of time. On GNU and POSIX systems, the epoch is 1970-01-01 00:00:00 UTC, so `@0' represents this time, `@1' represents 1970-01-01 00:00:01 UTC, and so forth. GNU and most other POSIX-compliant systems support such times as an extension to POSIX, using negative counts, so that `@-1' represents 1969-12-31 23:59:59 UTC. Traditional Unix systems count seconds with 32-bit two's-complement integers and can represent times from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 UTC. More modern systems use 64-bit counts of seconds with nanosecond subcounts, and can represent all the times in the known lifetime of the universe to a resolution of 1 nanosecond. On most systems, these counts ignore the presence of leap seconds. For example, on most systems `@915148799' represents 1998-12-31 23:59:59 UTC, `@915148800' represents 1999-01-01 00:00:00 UTC, and there is no way to represent the intervening leap second 1998-12-31 23:59:60 UTC. File: tar.info, Node: Authors of get_date, Prev: Seconds since the Epoch, Up: Date input formats 7.9 Authors of `get_date' ========================= `get_date' was originally implemented by Steven M. Bellovin (<smb@research.att.com>) while at the University of North Carolina at Chapel Hill. The code was later tweaked by a couple of people on Usenet, then completely overhauled by Rich $alz (<rsalz@bbn.com>) and Jim Berets (<jberets@bbn.com>) in August, 1990. Various revisions for the GNU system were made by David MacKenzie, Jim Meyering, Paul Eggert and others. This chapter was originally produced by Franc,ois Pinard (<pinard@iro.umontreal.ca>) from the `getdate.y' source code, and then edited by K. Berry (<kb@cs.umb.edu>). File: tar.info, Node: Formats, Next: Media, Prev: Date input formats, Up: Top 8 Controlling the Archive Format ******************************** Due to historical reasons, there are several formats of tar archives. All of them are based on the same principles, but have some subtle differences that often make them incompatible with each other. GNU tar is able to create and handle archives in a variety of formats. The most frequently used formats are (in alphabetical order): gnu Format used by GNU `tar' versions up to 1.13.25. This format derived from an early POSIX standard, adding some improvements such as sparse file handling and incremental archives. Unfortunately these features were implemented in a way incompatible with other archive formats. Archives in `gnu' format are able to hold pathnames of unlimited length. oldgnu Format used by GNU `tar' of versions prior to 1.12. v7 Archive format, compatible with the V7 implementation of tar. This format imposes a number of limitations. The most important of them are: 1. The maximum length of a file name is limited to 99 characters. 2. The maximum length of a symbolic link is limited to 99 characters. 3. It is impossible to store special files (block and character devices, fifos etc.) 4. Maximum value of user or group ID is limited to 2097151 (7777777 octal) 5. V7 archives do not contain symbolic ownership information (user and group name of the file owner). This format has traditionally been used by Automake when producing Makefiles. This practice will change in the future, in the meantime, however this means that projects containing filenames more than 99 characters long will not be able to use GNU `tar' 1.14.90 and Automake prior to 1.9. ustar Archive format defined by POSIX.1-1988 specification. It stores symbolic ownership information. It is also able to store special files. However, it imposes several restrictions as well: 1. The maximum length of a file name is limited to 256 characters, provided that the filename can be split at directory separator in two parts, first of them being at most 155 bytes long. So, in most cases the maximum file name length will be shorter than 256 characters. 2. The maximum length of a symbolic link name is limited to 100 characters. 3. Maximum size of a file the archive is able to accomodate is 8GB 4. Maximum value of UID/GID is 2097151. 5. Maximum number of bits in device major and minor numbers is 21. star Format used by Jo"rg Schilling `star' implementation. GNU `tar' is able to read `star' archives but currently does not produce them. posix Archive format defined by POSIX.1-2001 specification. This is the most flexible and feature-rich format. It does not impose any restrictions on file sizes or filename lengths. This format is quite recent, so not all tar implementations are able to handle it properly. However, this format is designed in such a way that any tar implementation able to read `ustar' archives will be able to read most `posix' archives as well, with the only exception that any additional information (such as long file names etc.) will in such case be extracted as plain text files along with the files it refers to. This archive format will be the default format for future versions of GNU `tar'. The following table summarizes the limitations of each of these formats: Format UID File Size Path Name Devn gnu 1.8e19 Unlimited Unlimited 63 oldgnu 1.8e19 Unlimited Unlimited 63 v7 2097151 8GB 99 n/a ustar 2097151 8GB 256 21 posix Unlimited Unlimited Unlimited Unlimited The default format for GNU `tar' is defined at compilation time. You may check it by running `tar --help', and examining the last lines of its output. Usually, GNU `tar' is configured to create archives in `gnu' format, however, future version will switch to `posix'. * Menu: * Portability:: Making `tar' Archives More Portable * Compression:: Using Less Space through Compression * Attributes:: Handling File Attributes * Standard:: The Standard Format * Extensions:: GNU Extensions to the Archive Format * cpio:: Comparison of `tar' and `cpio' File: tar.info, Node: Portability, Next: Compression, Up: Formats 8.1 Making `tar' Archives More Portable ======================================= Creating a `tar' archive on a particular system that is meant to be useful later on many other machines and with other versions of `tar' is more challenging than you might think. `tar' archive formats have been evolving since the first versions of Unix. Many such formats are around, and are not always compatible with each other. This section discusses a few problems, and gives some advice about making `tar' archives more portable. One golden rule is simplicity. For example, limit your `tar' archives to contain only regular files and directories, avoiding other kind of special files. Do not attempt to save sparse files or contiguous files as such. Let's discuss a few more problems, in turn. * Menu: * Portable Names:: Portable Names * dereference:: Symbolic Links * old:: Old V7 Archives * ustar:: Ustar Archives * gnu:: GNU and old GNU format archives. * posix:: POSIX archives * Checksumming:: Checksumming Problems * Large or Negative Values:: Large files, negative time stamps, etc. File: tar.info, Node: Portable Names, Next: dereference, Up: Portability 8.1.1 Portable Names -------------------- Use portable file and member names. A name is portable if it contains only ASCII letters and digits, `/', `.', `_', and `-'; it cannot be empty, start with `-' or `//', or contain `/-'. Avoid deep directory nesting. For portability to old Unix hosts, limit your file name components to 14 characters or less. If you intend to have your `tar' archives to be read under MSDOS, you should not rely on case distinction for file names, and you might use the GNU `doschk' program for helping you further diagnosing illegal MSDOS names, which are even more limited than System V's. File: tar.info, Node: dereference, Next: old, Prev: Portable Names, Up: Portability 8.1.2 Symbolic Links -------------------- Normally, when `tar' archives a symbolic link, it writes a block to the archive naming the target of the link. In that way, the `tar' archive is a faithful record of the filesystem contents. `--dereference' (`-h') is used with `--create' (`-c'), and causes `tar' to archive the files symbolic links point to, instead of the links themselves. When this option is used, when `tar' encounters a symbolic link, it will archive the linked-to file, instead of simply recording the presence of a symbolic link. The name under which the file is stored in the file system is not recorded in the archive. To record both the symbolic link name and the file name in the system, archive the file under both names. If all links were recorded automatically by `tar', an extracted file might be linked to a file name that no longer exists in the file system. If a linked-to file is encountered again by `tar' while creating the same archive, an entire second copy of it will be stored. (This _might_ be considered a bug.) So, for portable archives, do not archive symbolic links as such, and use `--dereference' (`-h'): many systems do not support symbolic links, and moreover, your distribution might be unusable if it contains unresolved symbolic links. File: tar.info, Node: old, Next: ustar, Prev: dereference, Up: Portability 8.1.3 Old V7 Archives --------------------- Certain old versions of `tar' cannot handle additional information recorded by newer `tar' programs. To create an archive in V7 format (not ANSI), which can be read by these old versions, specify the `--format=v7' option in conjunction with the `--create' (`-c') (`tar' also accepts `--portability' or `op-old-archive' for this option). When you specify it, `tar' leaves out information about directories, pipes, fifos, contiguous files, and device files, and specifies file ownership by group and user IDs instead of group and user names. When updating an archive, do not use `--format=v7' unless the archive was created using this option. In most cases, a _new_ format archive can be read by an _old_ `tar' program without serious trouble, so this option should seldom be needed. On the other hand, most modern `tar's are able to read old format archives, so it might be safer for you to always use `--format=v7' for your distributions. File: tar.info, Node: ustar, Next: gnu, Prev: old, Up: Portability 8.1.4 Ustar Archive Format -------------------------- Archive format defined by POSIX.1-1988 specification is called `ustar'. Although it is more flexible than the V7 format, it still has many restrictions (*Note ustar: Formats, for the detailed description of `ustar' format). Along with V7 format, `ustar' format is a good choice for archives intended to be read with other implementations of `tar'. To create archive in `ustar' format, use `--format=ustar' option in conjunction with the `--create' (`-c'). File: tar.info, Node: gnu, Next: posix, Prev: ustar, Up: Portability 8.1.5 GNU and old GNU `tar' format ---------------------------------- GNU `tar' was based on an early draft of the POSIX 1003.1 `ustar' standard. GNU extensions to `tar', such as the support for file names longer than 100 characters, use portions of the `tar' header record which were specified in that POSIX draft as unused. Subsequent changes in POSIX have allocated the same parts of the header record for other purposes. As a result, GNU `tar' format is incompatible with the current POSIX specification, and with `tar' programs that follow it. In the majority of cases, `tar' will be configured to create this format by default. This will change in the future releases, since we plan to make `posix' format the default. To force creation a GNU `tar' archive, use option `--format=gnu'. Some `tar' options are currently basing on GNU `tar' format, and can therefore be used only with `gnu' or `oldgnu' archive formats. The list of such options follows: * `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL'), when used with `--create' (`-c'). * `--incremental' (`-G') * `--multi-volume' (`-M') These options will be re-implemented for the `posix' archive format in the future. File: tar.info, Node: posix, Next: Checksumming, Prev: gnu, Up: Portability 8.1.6 GNU `tar' and POSIX `tar' ------------------------------- The version 1.14.90 of GNU `tar' is able to read and create archives conforming to POSIX.1-2001 standard. A POSIX conformant archive will be created if `tar' was given `--format=posix' option. Notice, that currently GNU extensions are not allowed with this format. Following is the list of options that cannot be used with `--format=posix': * `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL'), when used with `--create' (`-c'). * `--incremental' (`-G') * `--multi-volume' (`-M') This restriction will disappear in the future versions. File: tar.info, Node: Checksumming, Next: Large or Negative Values, Prev: posix, Up: Portability 8.1.7 Checksumming Problems --------------------------- SunOS and HP-UX `tar' fail to accept archives created using GNU `tar' and containing non-ASCII file names, that is, file names having characters with the eight bit set, because they use signed checksums, while GNU `tar' uses unsigned checksums while creating archives, as per POSIX standards. On reading, GNU `tar' computes both checksums and accept any. It is somewhat worrying that a lot of people may go around doing backup of their files using faulty (or at least non-standard) software, not learning about it until it's time to restore their missing files with an incompatible file extractor, or vice versa. GNU `tar' compute checksums both ways, and accept any on read, so GNU tar can read Sun tapes even with their wrong checksums. GNU `tar' produces the standard checksum, however, raising incompatibilities with Sun. That is to say, GNU `tar' has not been modified to _produce_ incorrect archives to be read by buggy `tar''s. I've been told that more recent Sun `tar' now read standard archives, so maybe Sun did a similar patch, after all? The story seems to be that when Sun first imported `tar' sources on their system, they recompiled it without realizing that the checksums were computed differently, because of a change in the default signing of `char''s in their compiler. So they started computing checksums wrongly. When they later realized their mistake, they merely decided to stay compatible with it, and with themselves afterwards. Presumably, but I do not really know, HP-UX has chosen that their `tar' archives to be compatible with Sun's. The current standards do not favor Sun `tar' format. In any case, it now falls on the shoulders of SunOS and HP-UX users to get a `tar' able to read the good archives they receive. File: tar.info, Node: Large or Negative Values, Prev: Checksumming, Up: Portability 8.1.8 Large or Negative Values ------------------------------ POSIX `tar' format uses fixed-sized unsigned octal strings to represent numeric values. User and group IDs and device major and minor numbers have unsigned 21-bit representations, and file sizes and times have unsigned 33-bit representations. GNU `tar' generates POSIX representations when possible, but for values outside the POSIX range it generates two's-complement base-256 strings: uids, gids, and device numbers have signed 57-bit representations, and file sizes and times have signed 89-bit representations. These representations are an extension to POSIX `tar' format, so they are not universally portable. The most common portability problems with out-of-range numeric values are large files and future or negative time stamps. Portable archives should avoid members of 8 GB or larger, as POSIX `tar' format cannot represent them. Portable archives should avoid time stamps from the future. POSIX `tar' format can represent time stamps in the range 1970-01-01 00:00:00 through 2242-03-16 12:56:31 UTC. However, many current hosts use a signed 32-bit `time_t', or internal time stamp format, and cannot represent time stamps after 2038-01-19 03:14:07 UTC; so portable archives must avoid these time stamps for many years to come. Portable archives should also avoid time stamps before 1970. These time stamps are a common POSIX extension but their `time_t' representations are negative. Many traditional `tar' implementations generate a two's complement representation for negative time stamps that assumes a signed 32-bit `time_t'; hence they generate archives that are not portable to hosts with differing `time_t' representations. GNU `tar' recognizes this situation when it is run on host with a signed 32-bit `time_t', but it issues a warning, as these time stamps are nonstandard and unportable. File: tar.info, Node: Compression, Next: Attributes, Prev: Portability, Up: Formats 8.2 Using Less Space through Compression ======================================== * Menu: * gzip:: Creating and Reading Compressed Archives * sparse:: Archiving Sparse Files File: tar.info, Node: gzip, Next: sparse, Up: Compression 8.2.1 Creating and Reading Compressed Archives ---------------------------------------------- _(This message will disappear, once this node revised.)_ `-z' `--gzip' `--ungzip' Filter the archive through `gzip'. Some format parameters must be taken into consideration when modifying an archive. Compressed archives cannot be modified. You can use `--gzip' and `--gunzip' on physical devices (tape drives, etc.) and remote files as well as on normal files; data to or from such devices or remote files is reblocked by another copy of the `tar' program to enforce the specified (or default) record size. The default compression parameters are used; if you need to override them, avoid the `--gzip' (`--gunzip', `--ungzip', `-z') option and run `gzip' explicitly. (Or set the `GZIP' environment variable.) The `--gzip' (`--gunzip', `--ungzip', `-z') option does not work with the `--multi-volume' (`-M') option, or with the `--update' (`-u'), `--append' (`-r'), `--concatenate' (`--catenate', `-A'), or `--delete' operations. It is not exact to say that GNU `tar' is to work in concert with `gzip' in a way similar to `zip', say. Surely, it is possible that `tar' and `gzip' be done with a single call, like in: $ tar cfz archive.tar.gz subdir to save all of `subdir' into a `gzip''ed archive. Later you can do: $ tar xfz archive.tar.gz to explode and unpack. The difference is that the whole archive is compressed. With `zip', archive members are archived individually. `tar''s method yields better compression. On the other hand, one can view the contents of a `zip' archive without having to decompress it. As for the `tar' and `gzip' tandem, you need to decompress the archive to see its contents. However, this may be done without needing disk space, by using pipes internally: $ tar tfz archive.tar.gz About corrupted compressed archives: `gzip''ed files have no redundancy, for maximum compression. The adaptive nature of the compression scheme means that the compression tables are implicitly spread all over the archive. If you lose a few blocks, the dynamic construction of the compression tables becomes unsynchronized, and there is little chance that you could recover later in the archive. There are pending suggestions for having a per-volume or per-file compression in GNU `tar'. This would allow for viewing the contents without decompression, and for resynchronizing decompression at every volume or file, in case of corrupted archives. Doing so, we might lose some compressibility. But this would have make recovering easier. So, there are pros and cons. We'll see! `-j' `--bzip2' Filter the archive through `bzip2'. Otherwise like `--gzip' (`--gunzip', `--ungzip', `-z'). `-Z' `--compress' `--uncompress' Filter the archive through `compress'. Otherwise like `--gzip' (`--gunzip', `--ungzip', `-z'). `--use-compress-program=PROG' Filter through PROG (must accept `-d'). `--compress' (`--uncompress', `-Z') stores an archive in compressed format. This option is useful in saving time over networks and space in pipes, and when storage space is at a premium. `--compress' (`--uncompress', `-Z') causes `tar' to compress when writing the archive, or to uncompress when reading the archive. To perform compression and uncompression on the archive, `tar' runs the `compress' utility. `tar' uses the default compression parameters; if you need to override them, avoid the `--compress' (`--uncompress', `-Z') option and run the `compress' utility explicitly. It is useful to be able to call the `compress' utility from within `tar' because the `compress' utility by itself cannot access remote tape drives. The `--compress' (`--uncompress', `-Z') option will not work in conjunction with the `--multi-volume' (`-M') option or the `--append' (`-r'), `--update' (`-u') and `--delete' operations. *Note Operations::, for more information on these operations. If there is no compress utility available, `tar' will report an error. *Please note* that the `compress' program may be covered by a patent, and therefore we recommend you stop using it. `--bzip2' (`-j') acts like `--compress' (`--uncompress', `-Z'), except that it uses the `bzip2' utility. `--compress' `--uncompress' `-z' `-Z' When this option is specified, `tar' will compress (when writing an archive), or uncompress (when reading an archive). Used in conjunction with the `--create' (`-c'), `--extract' (`--get', `-x'), `--list' (`-t') and `--compare' (`--diff', `-d') operations. You can have archives be compressed by using the `--gzip' (`--gunzip', `--ungzip', `-z') option. This will arrange for `tar' to use the `gzip' program to be used to compress or uncompress the archive wren writing or reading it. To use the older, obsolete, `compress' program, use the `--compress' (`--uncompress', `-Z') option. The GNU Project recommends you not use `compress', because there is a patent covering the algorithm it uses. You could be sued for patent infringement merely by running `compress'. File: tar.info, Node: sparse, Prev: gzip, Up: Compression 8.2.2 Archiving Sparse Files ---------------------------- _(This message will disappear, once this node revised.)_ `-S' `--sparse' Handle sparse files efficiently. This option causes all files to be put in the archive to be tested for sparseness, and handled specially if they are. The `--sparse' (`-S') option is useful when many `dbm' files, for example, are being backed up. Using this option dramatically decreases the amount of space needed to store such a file. In later versions, this option may be removed, and the testing and treatment of sparse files may be done automatically with any special GNU options. For now, it is an option needing to be specified on the command line with the creation or updating of an archive. Files in the filesystem occasionally have "holes." A hole in a file is a section of the file's contents which was never written. The contents of a hole read as all zeros. On many operating systems, actual disk storage is not allocated for holes, but they are counted in the length of the file. If you archive such a file, `tar' could create an archive longer than the original. To have `tar' attempt to recognize the holes in a file, use `--sparse' (`-S'). When you use the `--sparse' (`-S') option, then, for any file using less disk space than would be expected from its length, `tar' searches the file for consecutive stretches of zeros. It then records in the archive for the file where the consecutive stretches of zeros are, and only archives the "real contents" of the file. On extraction (using `--sparse' (`-S') is not needed on extraction) any such files have holes created wherever the continuous stretches of zeros were found. Thus, if you use `--sparse' (`-S'), `tar' archives won't take more space than the original. A file is sparse if it contains blocks of zeros whose existence is recorded, but that have no space allocated on disk. When you specify the `--sparse' (`-S') option in conjunction with the `--create' (`-c') operation, `tar' tests all files for sparseness while archiving. If `tar' finds a file to be sparse, it uses a sparse representation of the file in the archive. *Note create::, for more information about creating archives. `--sparse' (`-S') is useful when archiving files, such as dbm files, likely to contain many nulls. This option dramatically decreases the amount of space needed to store such an archive. *Please Note:* Always use `--sparse' (`-S') when performing file system backups, to avoid archiving the expanded forms of files stored sparsely in the system. Even if your system has no sparse files currently, some may be created in the future. If you use `--sparse' (`-S') while making file system backups as a matter of course, you can be assured the archive will never take more space on the media than the files take on disk (otherwise, archiving a disk filled with sparse files might take hundreds of tapes). `tar' ignores the `--sparse' (`-S') option when reading an archive. `--sparse' `-S' Files stored sparsely in the file system are represented sparsely in the archive. Use in conjunction with write operations. However, users should be well aware that at archive creation time, GNU `tar' still has to read whole disk file to locate the "holes", and so, even if sparse files use little space on disk and in the archive, they may sometimes require inordinate amount of time for reading and examining all-zero blocks of a file. Although it works, it's painfully slow for a large (sparse) file, even though the resulting tar archive may be small. (One user reports that dumping a `core' file of over 400 megabytes, but with only about 3 megabytes of actual data, took about 9 minutes on a Sun Sparcstation ELC, with full CPU utilization.) This reading is required in all cases and is not related to the fact the `--sparse' (`-S') option is used or not, so by merely _not_ using the option, you are not saving time(1). Programs like `dump' do not have to read the entire file; by examining the file system directly, they can determine in advance exactly where the holes are and thus avoid reading through them. The only data it need read are the actual allocated data blocks. GNU `tar' uses a more portable and straightforward archiving approach, it would be fairly difficult that it does otherwise. Elizabeth Zwicky writes to `comp.unix.internals', on 1990-12-10: What I did say is that you cannot tell the difference between a hole and an equivalent number of nulls without reading raw blocks. `st_blocks' at best tells you how many holes there are; it doesn't tell you _where_. Just as programs may, conceivably, care what `st_blocks' is (care to name one that does?), they may also care where the holes are (I have no examples of this one either, but it's equally imaginable). I conclude from this that good archivers are not portable. One can arguably conclude that if you want a portable program, you can in good conscience restore files with as many holes as possible, since you can't get it right. ---------- Footnotes ---------- (1) Well! We should say the whole truth, here. When `--sparse' (`-S') is selected while creating an archive, the current `tar' algorithm requires sparse files to be read twice, not once. We hope to develop a new archive format for saving sparse files in which one pass will be sufficient. File: tar.info, Node: Attributes, Next: Standard, Prev: Compression, Up: Formats 8.3 Handling File Attributes ============================ _(This message will disappear, once this node revised.)_ When `tar' reads files, this causes them to have the access times updated. To have `tar' attempt to set the access times back to what they were before they were read, use the `--atime-preserve' option. Handling of file attributes `--atime-preserve' Preserve access times on files that are read. This doesn't work for files that you don't own, unless you're root, and it doesn't interact with incremental dumps nicely (*note Backups::), and it can set access or modification times incorrectly if other programs access the file while `tar' is running; but it is good enough for some purposes. `-m' `--touch' Do not extract file modified time. When this option is used, `tar' leaves the modification times of the files it extracts as the time when the files were extracted, instead of setting it to the time recorded in the archive. This option is meaningless with `--list' (`-t'). `--same-owner' Create extracted files with the same ownership they have in the archive. This is the default behavior for the superuser, so this option is meaningful only for non-root users, when `tar' is executed on those systems able to give files away. This is considered as a security flaw by many people, at least because it makes quite difficult to correctly account users for the disk space they occupy. Also, the `suid' or `sgid' attributes of files are easily and silently lost when files are given away. When writing an archive, `tar' writes the user id and user name separately. If it can't find a user name (because the user id is not in `/etc/passwd'), then it does not write one. When restoring, and doing a `chmod' like when you use `--same-permissions' (`--preserve-permissions', `-p'), it tries to look the name (if one was written) up in `/etc/passwd'. If it fails, then it uses the user id stored in the archive instead. `--no-same-owner' `-o' Do not attempt to restore ownership when extracting. This is the default behavior for ordinary users, so this option has an effect only for the superuser. `--numeric-owner' The `--numeric-owner' option allows (ANSI) archives to be written without user/group name information or such information to be ignored when extracting. It effectively disables the generation and/or use of user/group name information. This option forces extraction using the numeric ids from the archive, ignoring the names. This is useful in certain circumstances, when restoring a backup from an emergency floppy with different passwd/group files for example. It is otherwise impossible to extract files with the right ownerships if the password file in use during the extraction does not match the one belonging to the filesystem(s) being extracted. This occurs, for example, if you are restoring your files after a major crash and had booted from an emergency floppy with no password file or put your disk into another machine to do the restore. The numeric ids are _always_ saved into `tar' archives. The identifying names are added at create time when provided by the system, unless `--old-archive' (`-o') is used. Numeric ids could be used when moving archives between a collection of machines using a centralized management for attribution of numeric ids to users and groups. This is often made through using the NIS capabilities. When making a `tar' file for distribution to other sites, it is sometimes cleaner to use a single owner for all files in the distribution, and nicer to specify the write permission bits of the files as stored in the archive independently of their actual value on the file system. The way to prepare a clean distribution is usually to have some Makefile rule creating a directory, copying all needed files in that directory, then setting ownership and permissions as wanted (there are a lot of possible schemes), and only then making a `tar' archive out of this directory, before cleaning everything out. Of course, we could add a lot of options to GNU `tar' for fine tuning permissions and ownership. This is not the good way, I think. GNU `tar' is already crowded with options and moreover, the approach just explained gives you a great deal of control already. `-p' `--same-permissions' `--preserve-permissions' Extract all protection information. This option causes `tar' to set the modes (access permissions) of extracted files exactly as recorded in the archive. If this option is not used, the current `umask' setting limits the permissions on extracted files. This option is meaningless with `--list' (`-t'). `--preserve' Same as both `--same-permissions' (`--preserve-permissions', `-p') and `--same-order' (`--preserve-order', `-s'). The `--preserve' option has no equivalent short option name. It is equivalent to `--same-permissions' (`--preserve-permissions', `-p') plus `--same-order' (`--preserve-order', `-s'). File: tar.info, Node: Standard, Next: Extensions, Prev: Attributes, Up: Formats 8.4 Basic Tar Format ==================== _(This message will disappear, once this node revised.)_ While an archive may contain many files, the archive itself is a single ordinary file. Like any other file, an archive file can be written to a storage device such as a tape or disk, sent through a pipe or over a network, saved on the active file system, or even stored in another archive. An archive file is not easy to read or manipulate without using the `tar' utility or Tar mode in GNU Emacs. Physically, an archive consists of a series of file entries terminated by an end-of-archive entry, which consists of two 512 blocks of zero bytes. A file entry usually describes one of the files in the archive (an "archive member"), and consists of a file header and the contents of the file. File headers contain file names and statistics, checksum information which `tar' uses to detect file corruption, and information about file types. Archives are permitted to have more than one member with the same member name. One way this situation can occur is if more than one version of a file has been stored in the archive. For information about adding new versions of a file to an archive, see *Note update::. In addition to entries describing archive members, an archive may contain entries which `tar' itself uses to store information. *Note label::, for an example of such an archive entry. A `tar' archive file contains a series of blocks. Each block contains `BLOCKSIZE' bytes. Although this format may be thought of as being on magnetic tape, other media are often used. Each file archived is represented by a header block which describes the file, followed by zero or more blocks which give the contents of the file. At the end of the archive file there are two 512-byte blocks filled with binary zeros as an end-of-file marker. A reasonable system should write such end-of-file marker at the end of an archive, but must not assume that such a block exists when reading an archive. In particular GNU `tar' always issues a warning if it does not encounter it. The blocks may be "blocked" for physical I/O operations. Each record of N blocks (where N is set by the `--blocking-factor=512-SIZE' (`-b 512-SIZE') option to `tar') is written with a single `write ()' operation. On magnetic tapes, the result of such a write is a single record. When writing an archive, the last record of blocks should be written at the full size, with blocks after the zero block containing all zeros. When reading an archive, a reasonable system should properly handle an archive whose last record is shorter than the rest, or which contains garbage records after a zero block. The header block is defined in C as follows. In the GNU `tar' distribution, this is part of file `src/tar.h': /* GNU tar Archive Format description. Copyright (C) 1988, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 2000, 2001, 2003 Free Software Foundation, Inc. This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */ /* tar Header Block, from POSIX 1003.1-1990. */ /* POSIX header. */ struct posix_header { /* byte offset */ char name[100]; /* 0 */ char mode[8]; /* 100 */ char uid[8]; /* 108 */ char gid[8]; /* 116 */ char size[12]; /* 124 */ char mtime[12]; /* 136 */ char chksum[8]; /* 148 */ char typeflag; /* 156 */ char linkname[100]; /* 157 */ char magic[6]; /* 257 */ char version[2]; /* 263 */ char uname[32]; /* 265 */ char gname[32]; /* 297 */ char devmajor[8]; /* 329 */ char devminor[8]; /* 337 */ char prefix[155]; /* 345 */ /* 500 */ }; #define TMAGIC "ustar" /* ustar and a null */ #define TMAGLEN 6 #define TVERSION "00" /* 00 and no null */ #define TVERSLEN 2 /* Values used in typeflag field. */ #define REGTYPE '0' /* regular file */ #define AREGTYPE '\0' /* regular file */ #define LNKTYPE '1' /* link */ #define SYMTYPE '2' /* reserved */ #define CHRTYPE '3' /* character special */ #define BLKTYPE '4' /* block special */ #define DIRTYPE '5' /* directory */ #define FIFOTYPE '6' /* FIFO special */ #define CONTTYPE '7' /* reserved */ #define XHDTYPE 'x' /* Extended header referring to the next file in the archive */ #define XGLTYPE 'g' /* Global extended header */ /* Bits used in the mode field, values in octal. */ #define TSUID 04000 /* set UID on execution */ #define TSGID 02000 /* set GID on execution */ #define TSVTX 01000 /* reserved */ /* file permissions */ #define TUREAD 00400 /* read by owner */ #define TUWRITE 00200 /* write by owner */ #define TUEXEC 00100 /* execute/search by owner */ #define TGREAD 00040 /* read by group */ #define TGWRITE 00020 /* write by group */ #define TGEXEC 00010 /* execute/search by group */ #define TOREAD 00004 /* read by other */ #define TOWRITE 00002 /* write by other */ #define TOEXEC 00001 /* execute/search by other */ /* tar Header Block, GNU extensions. */ /* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for contiguous files, so maybe disobeying the `reserved' comment in POSIX header description. I suspect these were meant to be used this way, and should not have really been `reserved' in the published standards. */ /* *BEWARE* *BEWARE* *BEWARE* that the following information is still boiling, and may change. Even if the OLDGNU format description should be accurate, the so-called GNU format is not yet fully decided. It is surely meant to use only extensions allowed by POSIX, but the sketch below repeats some ugliness from the OLDGNU format, which should rather go away. Sparse files should be saved in such a way that they do *not* require two passes at archive creation time. Huge files get some POSIX fields to overflow, alternate solutions have to be sought for this. */ /* Descriptor for a single file hole. */ struct sparse { /* byte offset */ char offset[12]; /* 0 */ char numbytes[12]; /* 12 */ /* 24 */ }; /* Sparse files are not supported in POSIX ustar format. For sparse files with a POSIX header, a GNU extra header is provided which holds overall sparse information and a few sparse descriptors. When an old GNU header replaces both the POSIX header and the GNU extra header, it holds some sparse descriptors too. Whether POSIX or not, if more sparse descriptors are still needed, they are put into as many successive sparse headers as necessary. The following constants tell how many sparse descriptors fit in each kind of header able to hold them. */ #define SPARSES_IN_EXTRA_HEADER 16 #define SPARSES_IN_OLDGNU_HEADER 4 #define SPARSES_IN_SPARSE_HEADER 21 /* Extension header for sparse files, used immediately after the GNU extra header, and used only if all sparse information cannot fit into that extra header. There might even be many such extension headers, one after the other, until all sparse information has been recorded. */ struct sparse_header { /* byte offset */ struct sparse sp[SPARSES_IN_SPARSE_HEADER]; /* 0 */ char isextended; /* 504 */ /* 505 */ }; /* The old GNU format header conflicts with POSIX format in such a way that POSIX archives may fool old GNU tar's, and POSIX tar's might well be fooled by old GNU tar archives. An old GNU format header uses the space used by the prefix field in a POSIX header, and cumulates information normally found in a GNU extra header. With an old GNU tar header, we never see any POSIX header nor GNU extra header. Supplementary sparse headers are allowed, however. */ struct oldgnu_header { /* byte offset */ char unused_pad1[345]; /* 0 */ char atime[12]; /* 345 Incr. archive: atime of the file */ char ctime[12]; /* 357 Incr. archive: ctime of the file */ char offset[12]; /* 369 Multivolume archive: the offset of the start of this volume */ char longnames[4]; /* 381 Not used */ char unused_pad2; /* 385 */ struct sparse sp[SPARSES_IN_OLDGNU_HEADER]; /* 386 */ char isextended; /* 482 Sparse file: Extension sparse header follows */ char realsize[12]; /* 483 Sparse file: Real size*/ /* 495 */ }; /* OLDGNU_MAGIC uses both magic and version fields, which are contiguous. Found in an archive, it indicates an old GNU header format, which will be hopefully become obsolescent. With OLDGNU_MAGIC, uname and gname are valid, though the header is not truly POSIX conforming. */ #define OLDGNU_MAGIC "ustar " /* 7 chars and a null */ /* The standards committee allows only capital A through capital Z for user-defined expansion. Other letters in use include: 'A' Solaris Access Control List 'E' Solaris Extended Attribute File 'I' Inode only, as in 'star' 'X' POSIX 1003.1-2001 eXtended (VU version) */ /* This is a dir entry that contains the names of files that were in the dir at the time the dump was made. */ #define GNUTYPE_DUMPDIR 'D' /* Identifies the *next* file on the tape as having a long linkname. */ #define GNUTYPE_LONGLINK 'K' /* Identifies the *next* file on the tape as having a long name. */ #define GNUTYPE_LONGNAME 'L' /* This is the continuation of a file that began on another volume. */ #define GNUTYPE_MULTIVOL 'M' /* For storing filenames that do not fit into the main header. */ #define GNUTYPE_NAMES 'N' /* This is for sparse files. */ #define GNUTYPE_SPARSE 'S' /* This file is a tape/volume header. Ignore it on extraction. */ #define GNUTYPE_VOLHDR 'V' /* Jörg Schilling star header */ struct star_header { /* byte offset */ char name[100]; /* 0 */ char mode[8]; /* 100 */ char uid[8]; /* 108 */ char gid[8]; /* 116 */ char size[12]; /* 124 */ char mtime[12]; /* 136 */ char chksum[8]; /* 148 */ char typeflag; /* 156 */ char linkname[100]; /* 157 */ char magic[6]; /* 257 */ char version[2]; /* 263 */ char uname[32]; /* 265 */ char gname[32]; /* 297 */ char devmajor[8]; /* 329 */ char devminor[8]; /* 337 */ char prefix[131]; /* 345 */ char atime[12]; /* 476 */ char ctime[12]; /* 488 */ /* 500 */ }; #define SPARSES_IN_STAR_HEADER 4 #define SPARSES_IN_STAR_EXT_HEADER 21 struct star_in_header { char fill[345]; /* 0 Everything that is before t_prefix */ char prefix[1]; /* 345 t_name prefix */ char fill2; /* 346 */ char fill3[8]; /* 347 */ char isextended; /* 355 */ struct sparse sp[SPARSES_IN_STAR_HEADER]; /* 356 */ char realsize[12]; /* 452 Actual size of the file */ char offset[12]; /* 464 Offset of multivolume contents */ char atime[12]; /* 476 */ char ctime[12]; /* 488 */ char mfill[8]; /* 500 */ char xmagic[4]; /* 508 "tar" */ }; struct star_ext_header { struct sparse sp[SPARSES_IN_STAR_EXT_HEADER]; char isextended; }; /* tar Header Block, overall structure. */ /* tar files are made in basic blocks of this size. */ #define BLOCKSIZE 512 enum archive_format { DEFAULT_FORMAT, /* format to be decided later */ V7_FORMAT, /* old V7 tar format */ OLDGNU_FORMAT, /* GNU format as per before tar 1.12 */ USTAR_FORMAT, /* POSIX.1-1988 (ustar) format */ POSIX_FORMAT, /* POSIX.1-2001 format */ STAR_FORMAT, /* Star format defined in 1994 */ GNU_FORMAT /* Same as OLDGNU_FORMAT with one exception: see FIXME note for to_chars() function (create.c:189) */ }; /* Information about a sparse file. */ struct sp_array { off_t offset; size_t numbytes; }; struct tar_stat_info { char *orig_file_name; /* name of file read from the archive header */ char *file_name; /* name of file for the current archive entry after being normalized. */ int had_trailing_slash; /* nonzero if the current archive entry had a trailing slash before it was normalized. */ char *link_name; /* name of link for the current archive entry. */ unsigned int devminor; /* device minor number */ unsigned int devmajor; /* device major number */ char *uname; /* user name of owner */ char *gname; /* group name of owner */ struct stat stat; /* regular filesystem stat */ /* Nanosecond parts of file timestamps (if available) */ unsigned long atime_nsec; unsigned long mtime_nsec; unsigned long ctime_nsec; off_t archive_file_size; /* Size of file as stored in the archive. Equals stat.st_size for non-sparse files */ bool is_sparse; /* Is the file sparse */ size_t sparse_map_avail; /* Index to the first unused element in sparse_map array. Zero if the file is not sparse */ size_t sparse_map_size; /* Size of the sparse map */ struct sp_array *sparse_map; }; union block { char buffer[BLOCKSIZE]; struct posix_header header; struct star_header star_header; struct oldgnu_header oldgnu_header; struct sparse_header sparse_header; struct star_in_header star_in_header; struct star_ext_header star_ext_header; }; /* End of Format description. */ All characters in header blocks are represented by using 8-bit characters in the local variant of ASCII. Each field within the structure is contiguous; that is, there is no padding used within the structure. Each character on the archive medium is stored contiguously. Bytes representing the contents of files (after the header block of each file) are not translated in any way and are not constrained to represent characters in any character set. The `tar' format does not distinguish text files from binary files, and no translation of file contents is performed. The `name', `linkname', `magic', `uname', and `gname' are null-terminated character strings. All other fields are zero-filled octal numbers in ASCII. Each numeric field of width W contains W minus 1 digits, and a null. The `name' field is the file name of the file, with directory names (if any) preceding the file name, separated by slashes. The `mode' field provides nine bits specifying file permissions and three bits to specify the Set UID, Set GID, and Save Text ("sticky") modes. Values for these bits are defined above. When special permissions are required to create a file with a given mode, and the user restoring files from the archive does not hold such permissions, the mode bit(s) specifying those special permissions are ignored. Modes which are not supported by the operating system restoring files from the archive will be ignored. Unsupported modes should be faked up when creating or updating an archive; e.g. the group permission could be copied from the _other_ permission. The `uid' and `gid' fields are the numeric user and group ID of the file owners, respectively. If the operating system does not support numeric user or group IDs, these fields should be ignored. The `size' field is the size of the file in bytes; linked files are archived with this field specified as zero. The `mtime' field is the modification time of the file at the time it was archived. It is the ASCII representation of the octal value of the last time the file was modified, represented as an integer number of seconds since January 1, 1970, 00:00 Coordinated Universal Time. The `chksum' field is the ASCII representation of the octal value of the simple sum of all bytes in the header block. Each 8-bit byte in the header is added to an unsigned integer, initialized to zero, the precision of which shall be no less than seventeen bits. When calculating the checksum, the `chksum' field is treated as if it were all blanks. The `typeflag' field specifies the type of file archived. If a particular implementation does not recognize or permit the specified type, the file will be extracted as if it were a regular file. As this action occurs, `tar' issues a warning to the standard error. The `atime' and `ctime' fields are used in making incremental backups; they store, respectively, the particular file's access time and last inode-change time. The `offset' is used by the `--multi-volume' (`-M') option, when making a multi-volume archive. The offset is number of bytes into the file that we need to restart at to continue the file on the next tape, i.e., where we store the location that a continued file is continued at. The following fields were added to deal with sparse files. A file is "sparse" if it takes in unallocated blocks which end up being represented as zeros, i.e., no useful data. A test to see if a file is sparse is to look at the number blocks allocated for it versus the number of characters in the file; if there are fewer blocks allocated for the file than would normally be allocated for a file of that size, then the file is sparse. This is the method `tar' uses to detect a sparse file, and once such a file is detected, it is treated differently from non-sparse files. Sparse files are often `dbm' files, or other database-type files which have data at some points and emptiness in the greater part of the file. Such files can appear to be very large when an `ls -l' is done on them, when in truth, there may be a very small amount of important data contained in the file. It is thus undesirable to have `tar' think that it must back up this entire file, as great quantities of room are wasted on empty blocks, which can lead to running out of room on a tape far earlier than is necessary. Thus, sparse files are dealt with so that these empty blocks are not written to the tape. Instead, what is written to the tape is a description, of sorts, of the sparse file: where the holes are, how big the holes are, and how much data is found at the end of the hole. This way, the file takes up potentially far less room on the tape, and when the file is extracted later on, it will look exactly the way it looked beforehand. The following is a description of the fields used to handle a sparse file: The `sp' is an array of `struct sparse'. Each `struct sparse' contains two 12-character strings which represent an offset into the file and a number of bytes to be written at that offset. The offset is absolute, and not relative to the offset in preceding array element. The header can hold four of these `struct sparse' at the moment; if more are needed, they are not stored in the header. The `isextended' flag is set when an `extended_header' is needed to deal with a file. Note that this means that this flag can only be set when dealing with a sparse file, and it is only set in the event that the description of the file will not fit in the allotted room for sparse structures in the header. In other words, an extended_header is needed. The `extended_header' structure is used for sparse files which need more sparse structures than can fit in the header. The header can fit 4 such structures; if more are needed, the flag `isextended' gets set and the next block is an `extended_header'. Each `extended_header' structure contains an array of 21 sparse structures, along with a similar `isextended' flag that the header had. There can be an indeterminate number of such `extended_header's to describe a sparse file. `REGTYPE' `AREGTYPE' These flags represent a regular file. In order to be compatible with older versions of `tar', a `typeflag' value of `AREGTYPE' should be silently recognized as a regular file. New archives should be created using `REGTYPE'. Also, for backward compatibility, `tar' treats a regular file whose name ends with a slash as a directory. `LNKTYPE' This flag represents a file linked to another file, of any type, previously archived. Such files are identified in Unix by each file having the same device and inode number. The linked-to name is specified in the `linkname' field with a trailing null. `SYMTYPE' This represents a symbolic link to another file. The linked-to name is specified in the `linkname' field with a trailing null. `CHRTYPE' `BLKTYPE' These represent character special files and block special files respectively. In this case the `devmajor' and `devminor' fields will contain the major and minor device numbers respectively. Operating systems may map the device specifications to their own local specification, or may ignore the entry. `DIRTYPE' This flag specifies a directory or sub-directory. The directory name in the `name' field should end with a slash. On systems where disk allocation is performed on a directory basis, the `size' field will contain the maximum number of bytes (which may be rounded to the nearest disk block allocation unit) which the directory may hold. A `size' field of zero indicates no such limiting. Systems which do not support limiting in this manner should ignore the `size' field. `FIFOTYPE' This specifies a FIFO special file. Note that the archiving of a FIFO file archives the existence of this file and not its contents. `CONTTYPE' This specifies a contiguous file, which is the same as a normal file except that, in operating systems which support it, all its space is allocated contiguously on the disk. Operating systems which do not allow contiguous allocation should silently treat this type as a normal file. `A' ... `Z' These are reserved for custom implementations. Some of these are used in the GNU modified format, as described below. Other values are reserved for specification in future revisions of the P1003 standard, and should not be used by any `tar' program. The `magic' field indicates that this archive was output in the P1003 archive format. If this field contains `TMAGIC', the `uname' and `gname' fields will contain the ASCII representation of the owner and group of the file respectively. If found, the user and group IDs are used rather than the values in the `uid' and `gid' fields. For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages 169-173 (section 10.1) for `Archive/Interchange File Format'; and IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940 (section E.4.48) for `pax - Portable archive interchange'. File: tar.info, Node: Extensions, Next: cpio, Prev: Standard, Up: Formats 8.5 GNU Extensions to the Archive Format ======================================== _(This message will disappear, once this node revised.)_ The GNU format uses additional file types to describe new types of files in an archive. These are listed below. `GNUTYPE_DUMPDIR' `'D'' This represents a directory and a list of files created by the `--incremental' (`-G') option. The `size' field gives the total size of the associated list of files. Each file name is preceded by either a `Y' (the file should be in this archive) or an `N'. (The file is a directory, or is not stored in the archive.) Each file name is terminated by a null. There is an additional null after the last file name. `GNUTYPE_MULTIVOL' `'M'' This represents a file continued from another volume of a multi-volume archive created with the `--multi-volume' (`-M') option. The original type of the file is not given here. The `size' field gives the maximum size of this piece of the file (assuming the volume does not end before the file is written out). The `offset' field gives the offset from the beginning of the file where this part of the file begins. Thus `size' plus `offset' should equal the original size of the file. `GNUTYPE_SPARSE' `'S'' This flag indicates that we are dealing with a sparse file. Note that archiving a sparse file requires special operations to find holes in the file, which mark the positions of these holes, along with the number of bytes of data to be found after the hole. `GNUTYPE_VOLHDR' `'V'' This file type is used to mark the volume header that was given with the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option when the archive was created. The `name' field contains the `name' given after the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option. The `size' field is zero. Only the first file in each volume of an archive should have this type. You may have trouble reading a GNU format archive on a non-GNU system if the options `--incremental' (`-G'), `--multi-volume' (`-M'), `--sparse' (`-S'), or `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') were used when writing the archive. In general, if `tar' does not use the GNU-added fields of the header, other versions of `tar' should be able to read the archive. Otherwise, the `tar' program will give an error, the most likely one being a checksum error.