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GIT-UPDATE-REF(1)                 Git Manual                 GIT-UPDATE-REF(1)

       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 and the ref is one under "refs/heads/", "refs/remotes/",
       "refs/notes/", or the symbolic ref HEAD; 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:

        1. 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:

        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.

       Part of the git(1) suite

Git                      08/29/2012                 GIT-UPDATE-REF(1)