Git Update Ref Command Details and Logging

<old-oid>, if given. If given, <old-oid> may not be zero. symref-update:: Set <ref> to <new-target> after verifying <old-target> or <old-oid>, if given. Specify a zero <old-oid> to ensure that the ref does not exist before the update. verify:: Verify <ref> against <old-oid> but do not change it. If <old-oid> is zero or missing, the ref must not exist. symref-create: Create symbolic ref <ref> with <new-target> after verifying that it does not exist. symref-delete:: Delete <ref> after verifying that it exists with <old-target>, if given. symref-verify:: Verify symbolic <ref> against <old-target> but do not change it. If <old-target> is missing, the ref must not exist. Can only be used in `no-deref` mode. option:: Modify the 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 <old-oid>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 <new-oid> 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. NOTES ----- Symbolic refs were initially implemented using symbolic links. This is now deprecated since not all filesystems support symbolic links. This command 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). SEE ALSO -------- linkgit:git-symbolic-ref[1] GIT --- Part of the linkgit:git[1] suite

The git update-ref command provides various options for updating references, including verifying old object IDs, creating symbolic refs, and logging updates. The command supports transactions, which can be started, prepared, committed, or aborted, ensuring atomic updates. Additionally, the command logs updates to ref values in log files, providing a record of changes made to refs, and follows specific rules for handling symbolic links and refs.