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
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
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
flag, it deletes the named <ref> after verifying it still
, 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
, 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
Verify <ref> against <oldvalue>
but do not change it. If <oldvalue> zero or missing, the ref must not
Modify behavior of the next command naming a
<ref>. The only valid option is no-deref to avoid dereferencing a
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
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
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)