Most version control systems allow you to apply a revision tag to a specific version of a version-controlled tree. On modern changeset-based version control systems, a revision tag is simply a symbolic name for a particular revision. On older file-based systems like CVS, each tag is added to the entire set of version-controlled files, allowing them to be handled as a unit. Revision tags are commonly used to identify releases that are distributed to users.
There are two basic commands for tags; one makes a tag with a given name, the other retrieves a named tag.
C-x v sname
C-x v rname
This command reports an error if any files are locked at or below the current directory, without changing anything; this is to avoid overwriting work in progress.
You can give a tag or branch name as an argument to C-x v = or C-x v ~ (see Old Revisions). Thus, you can use it to compare a tagged version against the current files, or two tagged versions against each other.
On SCCS, VC implements tags itself; these tags are visible only through VC. Most later systems (including CVS, Subversion, bzr, git, and hg) have a native tag facility, and VC uses it where available; those tags will be visible even when you bypass VC.
In a file-oriented VCS, when you rename a registered file you need
to rename its master along with it; the command
will do this automatically. If you are using SCCS, you must also
update the records of the tag, to mention the file by its new name
vc-rename-file does this, too). An old tag that refers to a
master file that no longer exists under the recorded name is invalid;
VC can no longer retrieve it. It would be beyond the scope of this
manual to explain enough about RCS and SCCS to explain how to update
the tags by hand. Using
vc-rename-file makes the tag remain
valid for retrieval, but it does not solve all problems. For example,
some of the files in your program probably refer to others by name.
At the very least, the makefile probably mentions the file that you
renamed. If you retrieve an old tag, the renamed file is retrieved
under its new name, which is not the name that the makefile expects.
So the program won't really work as retrieved.