# git-update-ref(1) Manual Page

## 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.

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