git-update-ref






git‐update‐ref − Update the object name stored in a ref
safely



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




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).









                             ‐2‐


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.











                             ‐3‐


     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.

     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.



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:










                             ‐4‐


     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.



Part of the git(1) suite