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>] [--no-deref] (-d <ref> [<oldvalue>] | [--create-reflog] <ref> <newvalue> [<oldvalue>] | --stdin [-z])
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>.
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
start LF
prepare LF
commit LF
abort 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
start NUL
prepare NUL
commit NUL
abort 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:
update
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
Create <ref> with <newvalue> after verifying it does not exist. The given <newvalue> may not be zero.
delete
Delete <ref> after verifying it exists with <oldvalue>, if given. If given, <oldvalue> may not be zero.
verify
Verify <ref> against <oldvalue> but do not change it. If <oldvalue> is zero or missing, the ref must not exist.
option
Modify behavior of the next command naming a <ref>. The only valid option is no-deref to avoid dereferencing a symbolic
ref.
start
Start a transaction. In contrast to a non-transactional session, a transaction will automatically abort if the session
ends without an explicit commit. This command may create a new empty transaction when the current one has been committed
or aborted already.
prepare
Prepare to commit the transaction. This will create lock files for all queued reference updates. If one reference could
not be locked, the transaction will be aborted.
commit
Commit all reference updates queued for the transaction, ending the transaction.
abort
Abort the transaction, releasing all locks if the transaction is in prepared state.
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 modifications.
LOGGING UPDATES
If config parameter "core.logAllRefUpdates" is true and the ref is one under "refs/heads/", "refs/remotes/", "refs/notes/",
or a pseudoref like HEAD or ORIG_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:
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:
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.
GIT
Part of the git(1) suite
Git 2.39.2 04/24/2023 GIT-UPDATE-REF(1)