Man page 1 : git-update-ref

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

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

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(cqll be followed for reading but not for writing (so we(cqll 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>.

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:


    .ie n \ \h'-04' 1.\h'+01'\c .

1. 4.2 . 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(cqs name, email address and date in the standard GIT committer ident format.

Optionally with -m:


    .ie n \ \h'-04' 1.\h'+01'\c .

1. 4.2 . 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.

Written by Linus Torvalds <\m[blue]\\fR\m[]\s-2\u[1]\d\s+2>.

Part of the git(1) suite

1. 4
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