git-update-ref

SYNOPSIS
       git update-ref [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref]
       <ref> <newvalue> [<oldvalue>])

DESCRIPTION
       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>.

LOGGING UPDATES
       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
        1. 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.

AUTHOR
       Written by Linus Torvalds <torvalds@osdl.org[1]>.

GIT
       Part of the git(1) suite

NOTES
        1. torvalds@osdl.org
           mailto:torvalds@osdl.org



Git 1.7.0.4                       04/12/2010                 GIT-UPDATE-REF(1)