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git-update-ref(1) Manual Page
git-update-ref - Update the object name stored in a ref safely
git update-ref [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] <ref> <newvalue> [<oldvalue>])
Given two arguments, stores the <newvalue> in the <ref>, possibly dereferencing the symbolic refs. E.g. git update-ref HEAD <newvalue> updates the current branch head to the new object.
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. git update-ref refs/heads/master <newvalue> <oldvalue> 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.
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:".
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 real 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).
If --no-deref is given, <ref> itself is overwritten, rather than the result of following the symbolic pointers.
In general, using
git update-ref HEAD "$head"
should be a lot safer than doing
echo "$head" > "$GIT_DIR/HEAD"
both from a symlink following standpoint and an error checking standpoint. The "refs/" rule for symlinks means that symlinks that point to "outside" the tree are safe: they'll be followed for reading but not for writing (so we'll never write through a ref symlink to some other tree, if you have copied a whole archive by creating a symlink tree).
With -d flag, it deletes the named <ref> after verifying it still contains <oldvalue>.
If config parameter "core.logAllRefUpdates" is true or the file "$GIT_DIR/logs/<ref>" exists then git update-ref 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:
oldsha1 SP newsha1 SP committer LF
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.
Optionally with -m:
oldsha1 SP newsha1 SP committer TAB message LF
Where all fields are as described above and "message" is the value supplied to the -m option.
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.
Part of the git(1) suite