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GIT-UPDATE-REF(1)		  Git Manual		     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

Git 2.13.2			  06/24/2017		     GIT-UPDATE-REF(1)

NAME | SYNOPSIS | DESCRIPTION | LOGGING UPDATES | GIT

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