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4 <article lang="en" id="git-update-ref(1)">
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6 <title>git-update-ref(1)</title>
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8 <primary>git-update-ref(1)</primary>
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11 <simplesect id="_name">
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13 <simpara>git-update-ref - Update the object name stored in a ref safely</simpara>
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15 <simplesect id="_synopsis">
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16 <title>SYNOPSIS</title>
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18 <literallayout><emphasis>git update-ref</emphasis> [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] <ref> <newvalue> [<oldvalue>])</literallayout>
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21 <simplesect id="_description">
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22 <title>DESCRIPTION</title>
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23 <simpara>Given two arguments, stores the <newvalue> in the <ref>, possibly
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24 dereferencing the symbolic refs. E.g. <emphasis>git update-ref HEAD
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25 <newvalue></emphasis> updates the current branch head to the new object.</simpara>
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26 <simpara>Given three arguments, stores the <newvalue> in the <ref>,
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27 possibly dereferencing the symbolic refs, after verifying that
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28 the current value of the <ref> matches <oldvalue>.
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29 E.g. <emphasis>git update-ref refs/heads/master <newvalue> <oldvalue></emphasis>
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30 updates the master branch head to <newvalue> only if its current
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31 value is <oldvalue>. You can specify 40 "0" or an empty string
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32 as <oldvalue> to make sure that the ref you are creating does
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33 not exist.</simpara>
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34 <simpara>It also allows a "ref" file to be a symbolic pointer to another
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35 ref file by starting with the four-byte header sequence of
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37 <simpara>More importantly, it allows the update of a ref file to follow
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38 these symbolic pointers, whether they are symlinks or these
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39 "regular file symbolic refs". It follows <emphasis role="strong">real</emphasis> symlinks only
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40 if they start with "refs/": otherwise it will just try to read
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41 them and update them as a regular file (i.e. it will allow the
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42 filesystem to follow them, but will overwrite such a symlink to
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43 somewhere else with a regular filename).</simpara>
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44 <simpara>If --no-deref is given, <ref> itself is overwritten, rather than
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45 the result of following the symbolic pointers.</simpara>
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46 <simpara>In general, using</simpara>
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47 <literallayout class="monospaced">git update-ref HEAD "$head"</literallayout>
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48 <simpara>should be a <emphasis>lot</emphasis> safer than doing</simpara>
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49 <literallayout class="monospaced">echo "$head" > "$GIT_DIR/HEAD"</literallayout>
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50 <simpara>both from a symlink following standpoint <emphasis role="strong">and</emphasis> an error checking
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51 standpoint. The "refs/" rule for symlinks means that symlinks
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52 that point to "outside" the tree are safe: they'll be followed
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53 for reading but not for writing (so we'll never write through a
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54 ref symlink to some other tree, if you have copied a whole
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55 archive by creating a symlink tree).</simpara>
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56 <simpara>With <emphasis>-d</emphasis> flag, it deletes the named <ref> after verifying it
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57 still contains <oldvalue>.</simpara>
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59 <simplesect id="_logging_updates">
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60 <title>Logging Updates</title>
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61 <simpara>If config parameter "core.logAllRefUpdates" is true and the ref is one under
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62 "refs/heads/", "refs/remotes/", "refs/notes/", or the symbolic ref HEAD; or
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63 the file "$GIT_DIR/logs/<ref>" exists then <emphasis>git update-ref</emphasis> will append
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64 a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
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65 symbolic refs before creating the log name) describing the change
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66 in ref value. Log lines are formatted as:</simpara>
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67 <orderedlist numeration="arabic">
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70 oldsha1 SP newsha1 SP committer LF
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72 <simpara>Where "oldsha1" is the 40 character hexadecimal value previously
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73 stored in <ref>, "newsha1" is the 40 character hexadecimal value of
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74 <newvalue> and "committer" is the committer's name, email address
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75 and date in the standard GIT committer ident format.</simpara>
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78 <simpara>Optionally with -m:</simpara>
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79 <orderedlist numeration="arabic">
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82 oldsha1 SP newsha1 SP committer TAB message LF
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84 <simpara>Where all fields are as described above and "message" is the
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85 value supplied to the -m option.</simpara>
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88 <simpara>An update will fail (without changing <ref>) if the current user is
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89 unable to create a new log file, append to the existing log file
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90 or does not have committer information available.</simpara>
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92 <simplesect id="_git">
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94 <simpara>Part of the <xref linkend="git(1)" /> suite</simpara>
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