git-update-ref(1)

NAME

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

SYNOPSIS

   git update-ref [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] [--create-reflog] <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

   With --create-reflog, update-ref will create a reflog for each ref even
   if one would not ordinarily be created.

   Quote fields containing whitespace as if they were strings in C source
   code; i.e., surrounded by double-quotes and with backslash escapes. Use
   40 "0" characters or the empty string to specify a zero value. To
   specify a missing value, omit the value and its preceding SP entirely.

   Alternatively, use -z to specify in NUL-terminated format, 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

   In this format, use 40 "0" to specify a zero value, and use the empty
   string to specify a missing value.

   In either format, values can be specified in any form that Git
   recognizes as an object name. Commands in 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.

   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

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