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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 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>.

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 quoting:

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

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

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


Part of the git(1) suite

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