SYNOPSIS
--------
-'git-update-ref' <ref> <newvalue> [<oldvalue>] [-m <reason>]
+'git-update-ref' [-m <reason>] (-d <ref> <oldvalue> | <ref> <newvalue> [<oldvalue>])
DESCRIPTION
-----------
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>.
+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 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>.
+
+
+Logging Updates
+---------------
+If config parameter "core.logAllRefUpdates" is true 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.
+
Author
------
Written by Linus Torvalds <torvalds@osdl.org>.