git/Documentation/git-update-ref.txt

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

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

SYNOPSIS
--------
'git-update-ref' <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>.

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).

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).

Author
------
Written by Linus Torvalds <torvalds@osdl.org>.

GIT
---
Part of the gitlink:git[7] suite