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<div id="header">
<h1>
git-update-ref(1) Manual Page
</h1>
<h2>NAME</h2>
<div class="sectionbody">
<p>git-update-ref -
Update the object name stored in a ref safely
</p>
</div>
</div>
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="paragraph"><p><em>git update-ref</em> [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] <ref> <newvalue> [<oldvalue>])</p></div>
</div>
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph"><p>Given two arguments, stores the <newvalue> in the <ref>, possibly
dereferencing the symbolic refs. E.g. <tt>git update-ref HEAD
<newvalue></tt> updates the current branch head to the new object.</p></div>
<div class="paragraph"><p>Given three arguments, stores the <newvalue> in the <ref>,
possibly dereferencing the symbolic refs, after verifying that
the current value of the <ref> matches <oldvalue>.
E.g. <tt>git update-ref refs/heads/master <newvalue> <oldvalue></tt>
updates the master branch head to <newvalue> only if its current
value is <oldvalue>. You can specify 40 "0" or an empty string
as <oldvalue> to make sure that the ref you are creating does
not exist.</p></div>
<div class="paragraph"><p>It also allows a "ref" file to be a symbolic pointer to another
ref file by starting with the four-byte header sequence of
"ref:".</p></div>
<div class="paragraph"><p>More importantly, it allows the update of a ref file to follow
these symbolic pointers, whether they are symlinks or these
"regular file symbolic refs". It follows <strong>real</strong> symlinks only
if they start with "refs/": otherwise it will just try to read
them and update them as a regular file (i.e. it will allow the
filesystem to follow them, but will overwrite such a symlink to
somewhere else with a regular filename).</p></div>
<div class="paragraph"><p>If --no-deref is given, <ref> itself is overwritten, rather than
the result of following the symbolic pointers.</p></div>
<div class="paragraph"><p>In general, using</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>git update-ref HEAD "$head"</tt></pre>
</div></div>
<div class="paragraph"><p>should be a <em>lot</em> safer than doing</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>echo "$head" > "$GIT_DIR/HEAD"</tt></pre>
</div></div>
<div class="paragraph"><p>both from a symlink following standpoint <strong>and</strong> an error checking
standpoint. The "refs/" rule for symlinks means that symlinks
that point to "outside" the tree are safe: they’ll be followed
for reading but not for writing (so we’ll never write through a
ref symlink to some other tree, if you have copied a whole
archive by creating a symlink tree).</p></div>
<div class="paragraph"><p>With <tt>-d</tt> flag, it deletes the named <ref> after verifying it
still contains <oldvalue>.</p></div>
</div>
<h2 id="_logging_updates">Logging Updates</h2>
<div class="sectionbody">
<div class="paragraph"><p>If config parameter "core.logAllRefUpdates" is true or the file
"$GIT_DIR/logs/<ref>" exists then <tt>git update-ref</tt> will append
a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
symbolic refs before creating the log name) describing the change
in ref value. Log lines are formatted as:</p></div>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
oldsha1 SP newsha1 SP committer LF
</p>
<div class="paragraph"><p>Where "oldsha1" is the 40 character hexadecimal value previously
stored in <ref>, "newsha1" is the 40 character hexadecimal value of
<newvalue> and "committer" is the committer’s name, email address
and date in the standard GIT committer ident format.</p></div>
</li>
</ol></div>
<div class="paragraph"><p>Optionally with -m:</p></div>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
oldsha1 SP newsha1 SP committer TAB message LF
</p>
<div class="paragraph"><p>Where all fields are as described above and "message" is the
value supplied to the -m option.</p></div>
</li>
</ol></div>
<div class="paragraph"><p>An update will fail (without changing <ref>) if the current user is
unable to create a new log file, append to the existing log file
or does not have committer information available.</p></div>
</div>
<h2 id="_git">GIT</h2>
<div class="sectionbody">
<div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
</div>
<div id="footer">
<div id="footer-text">
Last updated 2011-04-19 22:26:58 UTC
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