GNU Info
(emacs)Local Version Control
Local Version Control
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When you make many changes to a file that comes from a remote
repository, it can be convenient to have version control on your local
machine as well. You can then record intermediate versions, revert to
a previous state, etc., before you actually commit your changes to the
remote server.
VC lets you do this by putting a file under a second, local version
control system, so that the file is effectively registered in two
systems at the same time. For the description here, we will assume
that the remote system is CVS, and you use RCS locally, although the
mechanism works with any combination of version control systems ("back
ends").
To make it work with other back ends, you must make sure that the
"more local" back end comes before the "more remote" back end in the
setting of `vc-handled-backends' (Note: Customizing VC). By default,
this variable is set up so that you can use remote CVS and local RCS as
described here.
To start using local RCS for a file that comes from a remote CVS
server, you must _register the file in RCS_, by typing `C-u C-x v v rcs
<RET>'. (In other words, use `vc-next-action' with a prefix argument,
and specify RCS as the back end.)
You can do this at any time; it does not matter whether you have
already modified the file with respect to the version in the CVS
repository. If possible, VC tries to make the RCS master start with
the unmodified repository version, then checks in any local changes as
a new version. This works if you have not made any changes yet, or if
the unmodified repository version exists locally as a version backup
(Note: Version Backups). If the unmodified version is not available
locally, the RCS master starts with the modified version; the only
drawback to this is that you cannot compare your changes locally to
what is stored in the repository.
The version number of the RCS master is derived from the current CVS
version, starting a branch from it. For example, if the current CVS
version is 1.23, the local RCS branch will be 1.23.1. Version 1.23 in
the RCS master will be identical to version 1.23 under CVS; your first
changes are checked in as 1.23.1.1. (If the unmodified file is not
available locally, VC will check in the modified file twice, both as
1.23 and 1.23.1.1, to make the revision numbers consistent.)
If you do not use locking under CVS (the default), locking is also
disabled for RCS, so that editing under RCS works exactly as under CVS.
When you are done with local editing, you can commit the final
version back to the CVS repository by typing `C-u C-x v v cvs <RET>'.
This initializes the log entry buffer (Note: Log Buffer) to contain
all the log entries you have recorded in the RCS master; you can edit
them as you wish, and then commit in CVS by typing `C-c C-c'. If the
commit is successful, VC removes the RCS master, so that the file is
once again registered under CVS only. (The RCS master is not actually
deleted, just renamed by appending `~' to the name, so that you can
refer to it later if you wish.)
While using local RCS, you can pick up recent changes from the CVS
repository into your local file, or commit some of your changes back to
CVS, without terminating local RCS version control. To do this, switch
to the CVS back end temporarily, with the `C-x v b' command:
`C-x v b'
Switch to another back end that the current file is registered
under (`vc-switch-backend').
`C-u C-x v b BACKEND <RET>'
Switch to BACKEND for the current file.
`C-x v b' does not change the buffer contents, or any files; it only
changes VC's perspective on how to handle the file. Any subsequent VC
commands for that file will operate on the back end that is currently
selected.
If the current file is registered in more than one back end, typing
`C-x v b' "cycles" through all of these back ends. With a prefix
argument, it asks for the back end to use in the minibuffer.
Thus, if you are using local RCS, and you want to pick up some recent
changes in the file from remote CVS, first visit the file, then type
`C-x v b' to switch to CVS, and finally use `C-x v m <RET>' to merge
the news (Note: Merging). You can then switch back to RCS by typing
`C-x v b' again, and continue to edit locally.
But if you do this, the revision numbers in the RCS master no longer
correspond to those of CVS. Technically, this is not a problem, but it
can become difficult to keep track of what is in the CVS repository and
what is not. So we suggest that you return from time to time to
CVS-only operation, using `C-u C-x v v cvs <RET>'.
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