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] <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:
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:
Git 1.8.3.1 03/23/2016 GIT-UPDATE-REF(1)
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