NAME

       git-update-ref - Update the object name stored in a ref safely

SYNOPSIS

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

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>.

       With --stdin, update-ref reads instructions from standard input and performs all
       modifications together. Specify commands of the form:

           update SP <ref> SP <newvalue> [SP <oldvalue>] LF
           create SP <ref> SP <newvalue> LF
           delete SP <ref> [SP <oldvalue>] LF
           verify SP <ref> [SP <oldvalue>] LF
           option SP <opt> LF

       Quote fields containing whitespace as if they were strings in C source code.
       Alternatively, use -z to specify commands without quoting:

           update SP <ref> NUL <newvalue> NUL [<oldvalue>] NUL
           create SP <ref> NUL <newvalue> NUL
           delete SP <ref> NUL [<oldvalue>] NUL
           verify SP <ref> NUL [<oldvalue>] NUL
           option SP <opt> NUL

       Lines of any other format or a repeated <ref> produce an error. Command meanings are:

       update
           Set <ref> to <newvalue> after verifying <oldvalue>, if given. Specify a zero
           <newvalue> to ensure the ref does not exist after the update and/or a zero <oldvalue>
           to make sure the ref does not exist before the update.

       create
           Create <ref> with <newvalue> after verifying it does not exist. The given <newvalue>
           may not be zero.

       delete
           Delete <ref> after verifying it exists with <oldvalue>, if given. If given, <oldvalue>
           may not be zero.

       verify
           Verify <ref> against <oldvalue> but do not change it. If <oldvalue> zero or missing,
           the ref must not exist.

       option
           Modify behavior of the next command naming a <ref>. The only valid option is no-deref
           to avoid dereferencing a symbolic ref.

       Use 40 "0" or the empty string to specify a zero value, except that with -z an empty
       <oldvalue> is considered missing.

       If all <ref>s can be locked with matching <oldvalue>s simultaneously, all modifications
       are performed. Otherwise, no modifications are performed. Note that while each individual
       <ref> is updated or deleted atomically, a concurrent reader may still see a subset of the
       modifications.

LOGGING UPDATES

       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.

GIT

       Part of the git(1) suite