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git-update-ref(1)
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=================
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NAME
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----
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git-update-ref - update the object name stored in a ref safely
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SYNOPSIS
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--------
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'git-update-ref' [-m <reason>] <ref> <newvalue> [<oldvalue>]
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DESCRIPTION
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-----------
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Given two arguments, stores the <newvalue> in the <ref>, possibly
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dereferencing the symbolic refs. E.g. `git-update-ref HEAD
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<newvalue>` updates the current branch head to the new object.
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Given three arguments, stores the <newvalue> in the <ref>,
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possibly dereferencing the symbolic refs, after verifying that
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the current value of the <ref> matches <oldvalue>.
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E.g. `git-update-ref refs/heads/master <newvalue> <oldvalue>`
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updates the master branch head to <newvalue> only if its current
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value is <oldvalue>.
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It also allows a "ref" file to be a symbolic pointer to another
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ref file by starting with the four-byte header sequence of
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"ref:".
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More importantly, it allows the update of a ref file to follow
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these symbolic pointers, whether they are symlinks or these
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"regular file symbolic refs". It follows *real* symlinks only
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if they start with "refs/": otherwise it will just try to read
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them and update them as a regular file (i.e. it will allow the
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filesystem to follow them, but will overwrite such a symlink to
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somewhere else with a regular filename).
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In general, using
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git-update-ref HEAD "$head"
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should be a _lot_ safer than doing
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echo "$head" > "$GIT_DIR/HEAD"
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both from a symlink following standpoint *and* an error checking
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standpoint. The "refs/" rule for symlinks means that symlinks
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that point to "outside" the tree are safe: they'll be followed
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for reading but not for writing (so we'll never write through a
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ref symlink to some other tree, if you have copied a whole
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archive by creating a symlink tree).
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Logging Updates
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---------------
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If config parameter "core.logAllRefUpdates" is true or the file
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"$GIT_DIR/logs/<ref>" exists then `git-update-ref` will append
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a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
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symbolic refs before creating the log name) describing the change
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in ref value. Log lines are formatted as:
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. oldsha1 SP newsha1 SP committer LF
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+
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Where "oldsha1" is the 40 character hexadecimal value previously
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stored in <ref>, "newsha1" is the 40 character hexadecimal value of
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<newvalue> and "committer" is the committer's name, email address
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and date in the standard GIT committer ident format.
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Optionally with -m:
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. oldsha1 SP newsha1 SP committer TAB message LF
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+
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Where all fields are as described above and "message" is the
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value supplied to the -m option.
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An update will fail (without changing <ref>) if the current user is
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unable to create a new log file, append to the existing log file
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or does not have committer information available.
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Author
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------
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Written by Linus Torvalds <torvalds@osdl.org>.
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GIT
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---
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Part of the gitlink:git[7] suite
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