git/Documentation/git-update-ref.txt

git-update-ref(1)
=================

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

SYNOPSIS
--------
[verse]
'git update-ref' [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] <ref> <newvalue> [<oldvalue>])

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>.


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:

    . 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 linkgit:git[1] suite