GIT-UPDATE-REF(1) Git Manual GIT-UPDATE-REF(1)
git-update-ref - Update the object name stored in a ref safely
git update-ref [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] [--create-reflog] <ref> <newvalue> [<oldvalue>] | --stdin [-z])
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
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
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
With -d flag, it deletes the named <ref> after verifying it still
With --stdin, update-ref reads instructions from standard input and
performs all modifications together. Specify commands of the form:
update SP <ref> SP <newvalue> [SP <oldvalue>] LF
create SP <ref> SP <newvalue> LF
delete SP <ref> [SP <oldvalue>] LF
verify SP <ref> [SP <oldvalue>] LF
option SP <opt> LF
With --create-reflog, update-ref will create a reflog for each ref even
if one would not ordinarily be created.
Quote fields containing whitespace as if they were strings in C source
code; i.e., surrounded by double-quotes and with backslash escapes. Use
40 "0" characters or the empty string to specify a zero value. To
specify a missing value, omit the value and its preceding SP entirely.
Alternatively, use -z to specify in NUL-terminated format, without
update SP <ref> NUL <newvalue> NUL [<oldvalue>] NUL
create SP <ref> NUL <newvalue> NUL
delete SP <ref> NUL [<oldvalue>] NUL
verify SP <ref> NUL [<oldvalue>] NUL
option SP <opt> NUL
In this format, use 40 "0" to specify a zero value, and use the empty
string to specify a missing value.
In either format, values can be specified in any form that Git
recognizes as an object name. Commands in any other format or a
repeated <ref> produce an error. Command meanings are:
Set <ref> to <newvalue> after verifying <oldvalue>, if given.
Specify a zero <newvalue> to ensure the ref does not exist after
the update and/or a zero <oldvalue> to make sure the ref does not
exist before the update.
Create <ref> with <newvalue> after verifying it does not exist. The
given <newvalue> may not be zero.
Delete <ref> after verifying it exists with <oldvalue>, if given.
If given, <oldvalue> may not be zero.
Verify <ref> against <oldvalue> but do not change it. If <oldvalue>
zero or missing, the ref must not exist.
Modify behavior of the next command naming a <ref>. The only valid
option is no-deref to avoid dereferencing a symbolic ref.
If all <ref>s can be locked with matching <oldvalue>s simultaneously,
all modifications are performed. Otherwise, no modifications are
performed. Note that while each individual <ref> is updated or deleted
atomically, a concurrent reader may still see a subset of the
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:
1. 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.
Part of the git(1) suite
Git 2.19.1 10/05/2018 GIT-UPDATE-REF(1)