NAME

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

SYNOPSIS

       git update-ref [-m <reason>] [--no-deref] (-d <ref> [<oldvalue>] | [--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
           start LF
           prepare LF
           commit LF
           abort 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
           start NUL
           prepare NUL
           commit NUL
           abort 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> is 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.

       start
           Start a transaction. In contrast to a non-transactional session, a transaction will
           automatically abort if the session ends without an explicit commit. This command may
           create a new empty transaction when the current one has been committed or aborted
           already.

       prepare
           Prepare to commit the transaction. This will create lock files for all queued
           reference updates. If one reference could not be locked, the transaction will be
           aborted.

       commit
           Commit all reference updates queued for the transaction, ending the transaction.

       abort
           Abort the transaction, releasing all locks if the transaction is in prepared state.

       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 a pseudoref like HEAD or ORIG_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:

           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.

GIT

       Part of the git(1) suite