git-update-ref - Update the object name stored in a ref safely
git update-ref [-m <reason>] [--no-deref] (-d <ref> [<oldvalue>] | [--create-reflog] <ref> <newvalue> [<oldvalue>] | --stdin [-z])
Given two arguments, stores the in the , 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 in the , possibly dereferencing the symbolic refs, after verifying that the current value of the matches . E.g.
git update-ref refs/heads/master <newvalue> <oldvalue> updates the master branch head to only if its current value is . You can specify 40 "0" or an empty string as 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, 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).
-d flag, it deletes the named after verifying it still contains .
--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
--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.
-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 produce an error. Command meanings are:
- Set to after verifying , if given. Specify a zero to ensure the ref does not exist after the update and/or a zero to make sure the ref does not exist before the update.
- Create with after verifying it does not exist. The given may not be zero.
- Delete after verifying it exists with , if given. If given, may not be zero.
- Verify against but do not change it. If is zero or missing, the ref must not exist.
- Modify behavior of the next command naming a . The only valid option is
no-derefto avoid dereferencing a symbolic ref.
- 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 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 all reference updates queued for the transaction, ending the transaction.
- Abort the transaction, releasing all locks if the transaction is in prepared state.
If all s can be locked with matching s simultaneously, all modifications are performed. Otherwise, no modifications are performed. Note that while each individual is updated or deleted atomically, a concurrent reader may still see a subset of the modifications.
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/" exists then
git update-ref will append a line to the log file "$GIT_DIR/logs/" (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 , "newsha1" is the 40 character hexadecimal value of 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 ) 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.
© 2012–2021 Scott Chacon and others
Licensed under the MIT License.