This is Info file ../info/emacs, produced by Makeinfo-1.63 from the
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File: emacs,  Node: Backup,  Next: Interlocking,  Up: Saving

Backup Files
------------

   On most operating systems, rewriting a file automatically destroys
all record of what the file used to contain.  Thus, saving a file from
Emacs throws away the old contents of the file--or it would, except that
Emacs carefully copies the old contents to another file, called the
"backup" file, before actually saving.  (This assumes that the variable
`make-backup-files' is non-`nil'.  Backup files are not written if this
variable is `nil'.)  Emacs does not normally make backup files for
files in `/tmp'.

   At your option, Emacs can keep either a single backup file or a
series of numbered backup files for each file that you edit.

   Emacs makes a backup for a file only the first time the file is saved
from one buffer.  No matter how many times you save a file, its backup
file continues to contain the contents from before the file was visited.
Normally this means that the backup file contains the contents from
before the current editing session; however, if you kill the buffer and
then visit the file again, a new backup file will be made by the next
save.

* Menu:

* Names: Backup Names.		How backup files are named;
				  choosing single or numbered backup files.
* Deletion: Backup Deletion.	Emacs deletes excess numbered backups.
* Copying: Backup Copying.	Backups can be made by copying or renaming.


File: emacs,  Node: Backup Names,  Next: Backup Deletion,  Up: Backup

Single or Numbered Backups
..........................

   If you choose to have a single backup file (this is the default),
the backup file's name is constructed by appending `~' to the file name
being edited; thus, the backup file for `eval.c' would be `eval.c~'.

   If you choose to have a series of numbered backup files, backup file
names are made by appending `.~', the number, and another `~' to the
original file name.  Thus, the backup files of `eval.c' would be called
`eval.c.~1~', `eval.c.~2~', and so on, through names like
`eval.c.~259~' and beyond.

   If protection stops you from writing backup files under the usual
names, the backup file is written as `%backup%~' in your home directory.
Only one such file can exist, so only the most recently made such
backup is available.

   The choice of single backup or numbered backups is controlled by the
variable `version-control'.  Its possible values are

`t'
     Make numbered backups.

`nil'
     Make numbered backups for files that have numbered backups already.
     Otherwise, make single backups.

`never'
     Do not in any case make numbered backups; always make single
     backups.

You can set `version-control' locally in an individual buffer to
control the making of backups for that buffer's file.  For example,
Rmail mode locally sets `version-control' to `never' to make sure that
there is only one backup for an Rmail file.  *Note Locals::.

   If you set the environment variable `VERSION_CONTROL', to tell
various GNU utilities what to do with backup files, Emacs also obeys the
environment variable by setting the Lisp variable `version-control'
accordingly at startup.  If the environment variable's value is `t' or
`numbered', then `version-control' becomes `t'; if the value is `nil'
or `existing', then `version-control' becomes `nil'; if it is `never'
or `simple', then `version-control' becomes `never'.

   For files under version control (*note Version Control::.), the
variable `vc-make-backup-files' determines whether to make backup
files.  By default, it is `nil', since backup files are redundant when
you store all the previous versions in a version control system.  *Note
Editing with VC::.


File: emacs,  Node: Backup Deletion,  Next: Backup Copying,  Prev: Backup Names,  Up: Backup

Automatic Deletion of Backups
.............................

   To prevent unlimited consumption of disk space, Emacs can delete
numbered backup versions automatically.  Generally Emacs keeps the
first few backups and the latest few backups, deleting any in between.
This happens every time a new backup is made.

   The two variables `kept-old-versions' and `kept-new-versions'
control this deletion.  Their values are, respectively the number of
oldest (lowest-numbered) backups to keep and the number of newest
(highest-numbered) ones to keep, each time a new backup is made.
Recall that these values are used just after a new backup version is
made; that newly made backup is included in the count in
`kept-new-versions'.  By default, both variables are 2.

   If `delete-old-versions' is non-`nil', the excess middle versions
are deleted without a murmur.  If it is `nil', the default, then you
are asked whether the excess middle versions should really be deleted.

   Dired's `.' (Period) command can also be used to delete old versions.
*Note Dired Deletion::.


File: emacs,  Node: Backup Copying,  Prev: Backup Deletion,  Up: Backup

Copying vs. Renaming
....................

   Backup files can be made by copying the old file or by renaming it.
This makes a difference when the old file has multiple names.  If the
old file is renamed into the backup file, then the alternate names
become names for the backup file.  If the old file is copied instead,
then the alternate names remain names for the file that you are
editing, and the contents accessed by those names will be the new
contents.

   The method of making a backup file may also affect the file's owner
and group.  If copying is used, these do not change.  If renaming is
used, you become the file's owner, and the file's group becomes the
default (different operating systems have different defaults for the
group).

   Having the owner change is usually a good idea, because then the
owner always shows who last edited the file.  Also, the owners of the
backups show who produced those versions.  Occasionally there is a file
whose owner should not change; it is a good idea for such files to
contain local variable lists to set `backup-by-copying-when-mismatch'
locally (*note File Variables::.).

   The choice of renaming or copying is controlled by three variables.
Renaming is the default choice.  If the variable `backup-by-copying' is
non-`nil', copying is used.  Otherwise, if the variable
`backup-by-copying-when-linked' is non-`nil', then copying is used for
files that have multiple names, but renaming may still used when the
file being edited has only one name.  If the variable
`backup-by-copying-when-mismatch' is non-`nil', then copying is used if
renaming would cause the file's owner or group to change.


File: emacs,  Node: Interlocking,  Prev: Backup,  Up: Saving

Protection against Simultaneous Editing
---------------------------------------

   Simultaneous editing occurs when two users visit the same file, both
make changes, and then both save them.  If nobody were informed that
this was happening, whichever user saved first would later find that his
changes were lost.  On some systems, Emacs notices immediately when the
second user starts to change the file, and issues an immediate warning.

   For the sake of systems where that is not possible, and in case
someone else proceeds to change the file despite the warning, Emacs also
checks when the file is saved, and issues a second warning if you are
about to overwrite a file containing another user's changes.  You can
prevent loss of the other user's work by taking the proper corrective
action at that time.

   When you make the first modification in an Emacs buffer that is
visiting a file, Emacs records that  the file is "locked" by you.  (It
does this by writing another file in a directory reserved for this
purpose.)  The lock is removed when you save the changes.  The idea is
that the file is locked whenever an Emacs buffer visiting it has
unsaved changes.

   If you begin to modify the buffer while the visited file is locked by
someone else, this constitutes a "collision".  When Emacs detects a
collision, it asks you what to do, by calling the Lisp function
`ask-user-about-lock'.  You can redefine this function for the sake of
customization.  The standard definition of this function asks you a
question and accepts three possible answers:

`s'
     Steal the lock.  Whoever was already changing the file loses the
     lock, and you gain the lock.

`p'
     Proceed.  Go ahead and edit the file despite its being locked by
     someone else.

`q'
     Quit.  This causes an error (`file-locked') and the modification
     you were trying to make in the buffer does not actually take place.

   Note that locking works on the basis of a file name; if a file has
multiple names, Emacs does not realize that the two names are the same
file and cannot prevent two users from editing it simultaneously under
different names.  However, basing locking on names means that Emacs can
interlock the editing of new files that will not really exist until
they are saved.

   Some systems are not configured to allow Emacs to make locks.  On
these systems, Emacs cannot detect trouble in advance, but it still can
detect the collision when you try to save a file and overwrite someone
else's changes.

   Every time Emacs saves a buffer, it first checks the
last-modification date of the existing file on disk to verify that it
has not changed since the file was last visited or saved.  If the date
does not match, it implies that changes were made in the file in some
other way, and these changes are about to be lost if Emacs actually
does save.  To prevent this, Emacs prints a warning message and asks
for confirmation before saving.  Occasionally you will know why the
file was changed and know that it does not matter; then you can answer
`yes' and proceed.  Otherwise, you should cancel the save with `C-g'
and investigate the situation.

   The first thing you should do when notified that simultaneous editing
has already taken place is to list the directory with `C-u C-x C-d'
(*note Directories::.).  This shows the file's current author.  You
should attempt to contact him to warn him not to continue editing.
Often the next step is to save the contents of your Emacs buffer under a
different name, and use `diff' to compare the two files.

   Simultaneous editing checks are also made when you visit with `C-x
C-f' a file that is already visited and when you start to modify a
file.  This is not strictly necessary, but it can cause you to find out
about the collision earlier, when perhaps correction takes less work.


File: emacs,  Node: Reverting,  Next: Auto Save,  Prev: Saving,  Up: Files

Reverting a Buffer
==================

   If you have made extensive changes to a file and then change your
mind about them, you can get rid of them by reading in the previous
version of the file.  To do this, use `M-x revert-buffer', which
operates on the current buffer.  Since reverting a buffer
unintentionally could lose a lot of work, you must confirm this command
with `yes'.

   `revert-buffer' keeps point at the same distance (measured in
characters) from the beginning of the file.  If the file was edited only
slightly, you will be at approximately the same piece of text after
reverting as before.  If you have made drastic changes, the same value
of point in the old file may address a totally different piece of text.

   Reverting marks the buffer as "not modified" until another change is
made.

   Some kinds of buffers whose contents reflect data bases other than
files, such as Dired buffers, can also be reverted.  For them,
reverting means recalculating their contents from the appropriate data
base.  Buffers created randomly with `C-x b' cannot be reverted;
`revert-buffer' reports an error when asked to do so.


File: emacs,  Node: Auto Save,  Next: File Aliases,  Prev: Reverting,  Up: Files

Auto-Saving: Protection Against Disasters
=========================================

   Emacs saves all the visited files from time to time (based on
counting your keystrokes) without being asked.  This is called
"auto-saving".  It prevents you from losing more than a limited amount
of work if the system crashes.

   When Emacs determines that it is time for auto-saving, each buffer is
considered, and is auto-saved if auto-saving is turned on for it and it
has been changed since the last time it was auto-saved.  The message
`Auto-saving...' is displayed in the echo area during auto-saving, if
any files are actually auto-saved.  Errors occurring during auto-saving
are caught so that they do not interfere with the execution of commands
you have been typing.

* Menu:

* Files: Auto Save Files.       The file where auto-saved changes are
                                  actually made until you save the file.
* Control: Auto Save Control.   Controlling when and how often to auto-save.
* Recover::		        Recovering text from auto-save files.


File: emacs,  Node: Auto Save Files,  Next: Auto Save Control,  Up: Auto Save

Auto-Save Files
---------------

   Auto-saving does not normally save in the files that you visited,
because it can be very undesirable to save a program that is in an
inconsistent state when you have made half of a planned change.
Instead, auto-saving is done in a different file called the "auto-save
file", and the visited file is changed only when you request saving
explicitly (such as with `C-x C-s').

   Normally, the auto-save file name is made by appending `#' to the
front and rear of the visited file name.  Thus, a buffer visiting file
`foo.c' is auto-saved in a file `#foo.c#'.  Most buffers that are not
visiting files are auto-saved only if you request it explicitly; when
they are auto-saved, the auto-save file name is made by appending `#%'
to the front and `#' to the rear of buffer name.  For example, the
`*mail*' buffer in which you compose messages to be sent is auto-saved
in a file named `#%*mail*#'.  Auto-save file names are made this way
unless you reprogram parts of Emacs to do something different (the
functions `make-auto-save-file-name' and `auto-save-file-name-p').  The
file name to be used for auto-saving in a buffer is calculated when
auto-saving is turned on in that buffer.

   When you delete a substantial part of the text in a large buffer,
auto save turns off temporarily in that buffer.  This is because if you
deleted the text unintentionally, you might find the auto-save file more
useful if it contains the deleted text.  To reenable auto-saving after
this happens, save the buffer with `C-x C-s', or use `C-u 1 M-x
auto-save'.

   If you want auto-saving to be done in the visited file, set the
variable `auto-save-visited-file-name' to be non-`nil'.  In this mode,
there is really no difference between auto-saving and explicit saving.

   A buffer's auto-save file is deleted when you save the buffer in its
visited file.  To inhibit this, set the variable
`delete-auto-save-files' to `nil'.  Changing the visited file name with
`C-x C-w' or `set-visited-file-name' renames any auto-save file to go
with the new visited name.


File: emacs,  Node: Auto Save Control,  Next: Recover,  Prev: Auto Save Files,  Up: Auto Save

Controlling Auto-Saving
-----------------------

   Each time you visit a file, auto-saving is turned on for that file's
buffer if the variable `auto-save-default' is non-`nil' (but not in
batch mode; *note Entering Emacs::.).  The default for this variable is
`t', so auto-saving is the usual practice for file-visiting buffers.
Auto-saving can be turned on or off for any existing buffer with the
command `M-x auto-save-mode'.  Like other minor mode commands, `M-x
auto-save-mode' turns auto-saving on with a positive argument, off with
a zero or negative argument; with no argument, it toggles.

   Emacs does auto-saving periodically based on counting how many
characters you have typed since the last time auto-saving was done.
The variable `auto-save-interval' specifies how many characters there
are between auto-saves.  By default, it is 300.

   Auto-saving also takes place when you stop typing for a while.  The
variable `auto-save-timeout' says how many seconds Emacs should wait
before it does an auto save (and perhaps also a garbage collection).
(The actual time period is longer if the current buffer is long; this
is a heuristic which aims to keep out of your way when you are editing
long buffers in which auto-save takes an appreciable amount of time.)
Auto-saving during idle periods accomplishes two things: first, it
makes sure all your work is saved if you go away from the terminal for
a while; second, it may avoid some auto-saving while you are actually
typing.

   Emacs also does auto-saving whenever it gets a fatal error.  This
includes killing the Emacs job with a shell command such as `kill
%emacs', or disconnecting a phone line or network connection.

   You can request an auto-save explicitly with the command `M-x
do-auto-save'.


File: emacs,  Node: Recover,  Prev: Auto Save Control,  Up: Auto Save

Recovering Data from Auto-Saves
-------------------------------

   You can use the contents of an auto-save file to recover from a loss
of data with the command `M-x recover-file RET FILE RET'.  This visits
FILE and then (after your confirmation) restores the contents from from
its auto-save file `#FILE#'.  You can then save with `C-x C-s' to put
the recovered text into FILE itself.  For example, to recover file
`foo.c' from its auto-save file `#foo.c#', do:

     M-x recover-file RET foo.c RET
     yes RET
     C-x C-s

   Before asking for confirmation, `M-x recover-file' displays a
directory listing describing the specified file and the auto-save file,
so you can compare their sizes and dates.  If the auto-save file is
older, `M-x recover-file' does not offer to read it.

   If Emacs or the computer crashes, you can recover all the files you
were editing from their auto save files with the command `M-x
recover-session'.  This first shows you a list of recorded interrupted
sessions.  Move point to the one you choose, and type `C-c C-c'.

   Then `recover-session' asks about each of the files that were being
edited during that session, asking whether to recover that file.  If
you answer `y', it calls `recover-file', which works in its normal
fashion.  It shows the dates of the original file and its auto-save
file, and asks once again whether to recover that file.

   When `recover-session' is done, the files you've chosen to recover
are present in Emacs buffers.  You should then save them.  Only
this--saving them--updates the files themselves.

   Interrupted sessions are recorded for later recovery in files named
`~/.saves-PID-HOSTNAME'.  The `~/.saves' portion of these names comes
from the value of `auto-save-list-file-prefix'.  You can arrange to
record sessions in a different place by setting that variable in your
`.emacs' file, but you'll have to redefine `recover-session' as well to
make it look in the new place.  If you set `auto-save-list-file-prefix'
to `nil' in your `.emacs' file, sessions are not recorded for recovery.


File: emacs,  Node: File Aliases,  Next: Version Control,  Prev: Auto Save,  Up: Files

File Name Aliases
=================

   Symbolic links and hard links both make it possible for several file
names to refer to the same file.  Hard links are alternate names that
refer directly to the file; all the names are equally valid, and no one
of them is preferred.  By contrast, a symbolic link is a kind of defined
alias: when `foo' is a symbolic link to `bar', you can use either name
to refer to the file, but `bar' is the real name, while `foo' is just
an alias.  More complex cases occur when symbolic links point to
directories.

   If you visit two names for the same file, normally Emacs makes two
different buffers, but it warns you about the situation.

   If you wish to avoid visiting the same file in two buffers under
different names, set the variable `find-file-existing-other-name' to a
non-`nil' value.  Then `find-file' uses the existing buffer visiting
the file, no matter which of the file's names you specify.

   If the variable `find-file-visit-truename' is non-`nil', then the
file name recorded for a buffer is the file's "truename" (made by
replacing all symbolic links with their target names), rather than the
name you specify.  Setting `find-file-visit-truename' also implies the
effect of `find-file-existing-other-name'.


File: emacs,  Node: Version Control,  Next: Directories,  Prev: File Aliases,  Up: Files

Version Control
===============

   "Version control systems" are packages that can record multiple
versions of a source file, usually storing the unchanged parts of the
file just once.  Version control systems also record history information
such as the creation time of each version, who created it, and a
description of what was changed in that version.

   The Emacs version control commands work with three version control
systems--RCS, CVS and SCCS.  The GNU project recommends RCS and CVS,
which are free software and available from the Free Software Foundation.

* Menu:

* Version Systems::             Supported version control back end systems.
* VC Concepts::                 Basic version control information;
                                  checking files in and out.
* Editing with VC::             Commands for editing a file maintained
                                  with version control.
* Log Entries::                 Logging your changes.
* Change Logs and VC::          Generating a change log file from log entries.
* Old Versions::                Examining and comparing old versions.
* Branches::                    Selecting a branch to put your changes in,
                                  and creating a new branch.
* Status in VC::                Commands to view the VC status of files and
                                  look at log entries.
* Renaming and VC::             A command to rename both the source and
                                  master file correctly.
* Snapshots::                   How to make and use snapshots, a set of
                                  file versions that can be treated as a unit.
* Version Headers::             Inserting version control headers into
                                  working files.
* Customizing VC::              Variables to change VC's behavior.


File: emacs,  Node: Version Systems,  Next: VC Concepts,  Up: Version Control

Supported Version Control Systems
---------------------------------

   VC currently works with three different version control systems or
"back ends": RCS, CVS, and SCCS.

   RCS is a free version control system that is available from the Free
Software Foundation.  It is perhaps the most mature of the supported
back ends, and the VC commands are conceptually closest to RCS.  Almost
everything you can do with RCS can be done through VC.

   CVS is built on top of RCS, and extends the features of RCS, allowing
for more sophisticated release management, and concurrent multi-user
development.  VC supports basic editing operations under CVS, but for
some less common tasks you still need to call CVS from the command line.
Note also that before using CVS you must set up a repository, which is a
subject too complex to treat here.  *Note CVS and VC::.

   SCCS is a proprietary but widely used version control system.  In
terms of capabilities, it is the weakest of the the three that VC
supports.  VC compensates for certain features missing in SCCS
(snapshots, for example) by implementing them itself, but some other VC
features, such as multiple branches, are not available with SCCS.  You
should use SCCS only if for some reason you cannot use RCS.


File: emacs,  Node: VC Concepts,  Next: Editing with VC,  Prev: Version Systems,  Up: Version Control

Concepts of Version Control
---------------------------

   When a file is under version control, we also say that it is
"registered" in the version control system.  Each registered file has a
corresponding "master file" which represents the file's present state
plus its change history, so that you can reconstruct from it either the
current version or any specified earlier version.  Usually the master
file also records a "log entry" for each version describing what was
changed in that version.

   The file that is maintained under version control is sometimes called
the "work file" corresponding to its master file.

   To examine a file, you "check it out".  This extracts a version of
the source file (typically, the most recent) from the master file.  If
you want to edit the file, you must check it out "locked".  Only one
user can do this at a time for any given source file.  (This kind of
locking is completely unrelated to the locking that Emacs uses to detect
simultaneous editing of a file.)

   When you are done with your editing, you must "check in" the new
version.  This records the new version in the master file, and unlocks
the source file so that other people can lock it and thus modify it.

   Checkin and checkout are the basic operations of version control.
You can do both of them with a single Emacs command: `C-x C-q'
(`vc-toggle-read-only').

   There are variants of this basic pattern, though.  CVS, for example,
has no such thing as locking, and therefore you can normally edit files
right away, without having to check them out first.  *Note CVS and VC::.
With RCS, you can optionally select "non-strict locking" for a
particular source file; then you can edit the file in Emacs without
explicitly locking it.

   A "snapshot" is a coherent collection of versions of the various
files that make up a program.  *Note Snapshots::.


File: emacs,  Node: Editing with VC,  Next: Log Entries,  Prev: VC Concepts,  Up: Version Control

Editing with Version Control
----------------------------

   These are the commands for editing a file maintained with version
control:

`C-x C-q'
`C-x v v'
     Check the visited file in or out.

`C-x v u'
     Revert the buffer and the file to the last checked in version.

`C-x v c'
     Remove the last-entered change from the master for the visited
     file.  This undoes your last check-in.

`C-x v i'
     Register the visited file for version control.

(`C-x v' is the prefix key for version control commands; all of these
commands except for `C-x C-q' start with `C-x v'.)

* Menu:

* Check-Out::                   Checking out a file so you can edit it.
* Check-In::                    After you edit, you check in your changes
                                  to make a new version.
* Version Control Undo::        Canceling changes before or after checkin.
* Registering::                 How to start using version control
                                  for a file.
* VC Mode Line::                Mode line indicates version and lock status.
* CVS and VC::                  Checkout and checkin work differently in CVS.


File: emacs,  Node: Check-Out,  Next: Check-In,  Up: Editing with VC

Check-Out
.........

   When you want to modify a file maintained with version control, type
`C-x C-q' (`vc-toggle-read-only').  This "checks out" the file, and
tells RCS or SCCS to lock the file.  This means making the file
writable for you (but not for anyone else).

   If you specify a prefix argument (`C-u C-x C-q') for checkout, Emacs
asks you for a version number, and checks out that version *unlocked*.
This lets you move to old versions, or existing branches of the file
(*note Branches::.).  You can then start editing the selected version
by typing `C-x C-q' again.  (If you edit an old version of a file this
way, checking it in again creates a new branch.)

   Under CVS, you normally don't need to check out files explicitly.
CVS does not have locking; multiple users can edit their copies of a
file whenever they want.  (If two users make conflicting changes, they
need to reconcile their changes when checking them in.)  We therefore
say that an "implicit" check-out happens when you make the first change
in the file.

   CVS has an alternative mode in which explicit check-out is required.
And RCS has an alternative mode called "non-strict locking" in which
explicit check-out is not required.  Selecting these modes is done
outside of VC, but once you have selected them, VC obeys them.  With
RCS, you can select non-strict locking for a particular file using the
`rcs -U' command.  *Note CVS and VC::, for an explanation of how to do
this with CVS.


File: emacs,  Node: Check-In,  Next: Version Control Undo,  Prev: Check-Out,  Up: Editing with VC

Check-In
........

   When you are finished editing the file, type `C-x C-q' again.  When
used on a file that is checked out, this command checks the file in.
But check-in does not start immediately; first, you must enter the "log
entry"--a description of the changes in the new version.  `C-x C-q'
pops up a buffer for you to enter this in.  When you are finished
typing in the log entry, type `C-c C-c' to terminate it; this is when
actual check-in takes place.  *Note Log Entries::.

   With RCS and SCCS, a checked-out file is also "locked", which means
it is writable for you, but not for anyone else.  As long as you own
the lock on the file, nobody else can modify it, and nobody can check
in any changes to that particular version.  Checking in your changes
unlocks the file, so that other users can lock it and modify it.

   CVS, on the contrary, doesn't have a concept of locking.  The working
files are always modifiable, allowing concurrent development, with
possible conflicts being resolved at check-in time.  *Note CVS and VC::.

   To specify the version number for the new version, type `C-u C-x
C-q' to check in a file.  Then Emacs asks you for the new version number
in the minibuffer.  This can be used to create a new "branch" of the
file (*note Branches::.), or to increment the file's major version
number.

   It is not impossible to lock a file that someone else has locked.  If
you try to check out a file that is locked, `C-x C-q' asks you whether
you want to "steal the lock."  If you say yes, the file becomes locked
by you, but a message is sent to the person who had formerly locked the
file, to inform him of what has happened.  The mode line indicates that
a file is locked by someone else by displaying the login name of that
person, before the version number.


File: emacs,  Node: Registering,  Next: VC Mode Line,  Prev: Version Control Undo,  Up: Editing with VC

Registering a File for Version Control
......................................

`C-x v i'
     Register the visited file for version control.

   You can put any file under version control by simply visiting it, and
then typing `C-x v i' (`vc-register'). After `C-x v i', the file is
unlocked and read-only.  Type `C-x C-q' if you wish to start editing it.

   When you register the file, Emacs must choose which version control
system to use for it.  You can specify your choice explicitly by setting
`vc-default-back-end' to `RCS', `CVS' or `SCCS'.  Otherwise, if there
is a subdirectory named `RCS', `SCCS', or `CVS', Emacs uses the
corresponding version control system.  In the absence of any
specification, the default choice is RCS if RCS is installed, otherwise
SCCS.

   After registering a file with CVS, you must subsequently commit the
initial version by typing `C-x C-q'.  *Note CVS and VC::.

   The initial version number for a newly registered file is 1.1, by
default.  To specify a different number, give `C-x v i' a numeric
argument; then it reads the initial version number using the minibuffer.

   If `vc-initial-comment' is non-`nil', `C-x v i' reads an initial
comment (much like a log entry) to describe the purpose of this source
file.


File: emacs,  Node: Version Control Undo,  Next: Registering,  Prev: Check-In,  Up: Editing with VC

Undoing Version Control Actions
...............................

`C-x v u'
     Revert the buffer and the file to the last checked in version.

`C-x v c'
     Remove the last-entered change from the master for the visited
     file.  This undoes your last checkin.

   If you want to discard your current set of changes and revert to the
last version checked in, use `C-x v u' (`vc-revert-buffer').  This
cancels your last check-out, leaving the file unlocked.  If you want to
make a different set of changes, you must first check the file out
again.  `C-x v u' requires confirmation, unless it sees that you
haven't made any changes since the last checked-in version.

   `C-x v u' is also the command to use to unlock a file if you lock it
and then decide not to change it.

   You can cancel a change after checking it in, with `C-x v c'
(`vc-cancel-version').  This command discards all record of the most
recent checked in version.  `C-x v c' also offers to revert your work
file and buffer to the previous version (the one that precedes the
version that is deleted).  If you say `no', then VC keeps your changes
in the buffer and locks the file.

   The no-revert option is useful when you have checked in a change and
then discover a trivial error in it; you can cancel the erroneous
check-in, fix the error, and check the file in again.

   When `C-x v c' does not revert the buffer, it unexpands all version
control headers in the buffer instead (*note Version Headers::.).  This
is because the buffer no longer corresponds to any existing version.
If you check it in again, the checkin process will expand the headers
properly for the new version number.

   However, it is impossible to unexpand the RCS `$Log$' header
automatically.  If you use that header feature, you have to unexpand it
by hand--by deleting the entry for the version that you just canceled.

   Be careful when invoking `C-x v c', as it is easy to throw away a
lot of work with it.  To help you be careful, this command always
requires confirmation with `yes'.  Note also that this command is
disabled under CVS, because canceling versions is very dangerous and
discouraged with this back end.


File: emacs,  Node: VC Mode Line,  Next: CVS and VC,  Prev: Registering,  Up: Editing with VC

The VC Mode Line
................

   When you visit a file that is under version control, the mode line
indicates the current status of the file: the name of the version
control back end system, the locking state, and the version.

   The locking state is displayed as a single character, which can be
either `-' or `:'.  `-' means the file is not locked or not modified by
you.  Once you lock the file, the state indicator changes to `:'.  If
the file is locked by someone else, that user's name appears after the
version number.

   For example, `RCS-1.3' means you are looking at RCS version 1.3,
which is not locked.  `RCS:1.3' means that you have locked the file,
and possibly already changed it.  `RCS:jim:1.3' means that the file is
locked by jim.


File: emacs,  Node: CVS and VC,  Prev: VC Mode Line,  Up: Editing with VC

Using VC with CVS
.................

   In CVS, files are never locked.  Two users can check out the same
file at the same time; each user has a separate copy and can edit it.
Work files are always writable; once you have one, you need not type a
VC command to start editing the file.  You can edit it at any time.

   When using RCS and SCCS, you normally use `C-x C-q' twice for each
change; once before the change, for checkout, and once after, for
checkin.  With CVS, it's different: you normally use `C-x C-q' just
once for each change, to commit the change when it is done.  The work
file remains writable, so you can begin editing again with no special
commands.

   One way to understand this is that VC does an "implicit" check-out
when you save the modified file for the first time.  VC indicates this
on the mode line: the status indicator changes from `-' to `:' as soon
as you save a modified version, telling you that you are not in sync
with the repository anymore (*note VC Mode Line::.).  The file stays
"checked out" until you check it back in, even if you kill the buffer
and visit the file again.

   If, instead, you would like to use explicit check-out with CVS, set
the `CVSREAD' environment variable to some value.  (It does not matter
what value you use.)  CVS then makes your work files read-only by
default, and VC requires you to check them out explicitly with `C-x
C-q'.  When setting `CVSREAD' for the first time, make sure to check
out all your modules anew, so that the file protections are set
correctly.

   VC does not provide a way to check out a working copy of an existing
file in the repository.  You have to use the CVS shell commands to do
that.  Once you have a work file, you can start using VC for that file.

   CVS terminology speaks of "committing" a change rather than checking
it in.  But in practical terms they work the same way: Emacs asks you
to type in a log entry, and you finish it with `C-c C-c'.

   When you try to commit a change in a file, but someone else has
committed another change in the meanwhile, that creates a "conflict".
VC detects this situation and offers to "merge" your changes and those
of the other user, creating a new local version of the file, which you
can then commit to the repository.  This works smoothly if the changes
are in different parts of the file, although it is wise to check the
resulting file for semantic consistency.

   However, if you and the other user changed the same parts of the
file, the conflict cannot be resolved automatically.  In this case, CVS
inserts both variants of the conflicting regions into your working file,
and puts so-called "conflict markers" around them.  They indicate how
the region looks in the respective user's version.  You must resolve
the conflict manually, for example by choosing one of the two variants
and deleting the other one (and the conflict markers).  Then you can
commit the resulting file into the repository.  The example below shows
how a conflict region looks; the file is called `name' and the current
repository version with user B's changes in it is 1.11.

     <<<<<<< name
       USER A'S VERSION
     =======
       USER B'S VERSION
     >>>>>>> 1.11

   You can turn off use of VC for CVS-managed files by setting the
variable `vc-handle-cvs' to `nil'.  If you do this, Emacs treats these
files as if they were not managed, and the VC commands are not
available for them.  You must do all CVS operations manually.


File: emacs,  Node: Log Entries,  Next: Change Logs and VC,  Prev: Editing with VC,  Up: Version Control

Log Entries
-----------

   When you're editing an initial comment or log entry for inclusion in
a master file, finish your entry by typing `C-c C-c'.

`C-c C-c'
     Finish the comment edit normally (`vc-finish-logentry').  This
     finishes check-in.

   To abort check-in, just *don't* type `C-c C-c' in that buffer.  You
can switch buffers and do other editing.  As long as you don't try to
check in another file, the entry you were editing remains in its
buffer, and you can go back to that buffer at any time to complete the
check-in.

   If you change several source files for the same reason, it is often
convenient to specify the same log entry for many of the files.  To do
this, use the history of previous log entries.  The commands `M-n',
`M-p', `M-s' and `M-r' for doing this work just like the minibuffer
history commands (except that these versions are used outside the
minibuffer).

   Each time you check in a file, the log entry buffer is put into VC
Log mode, which involves running two hooks: `text-mode-hook' and
`vc-log-mode-hook'.  *Note Hooks::.


File: emacs,  Node: Change Logs and VC,  Next: Old Versions,  Prev: Log Entries,  Up: Version Control

Change Logs and VC
------------------

   If you use RCS for a program and also maintain a change log file for
it (*note Change Log::.), you can generate change log entries
automatically from the version control log entries:

`C-x v a'
     Visit the current directory's change log file and create new
     entries for versions checked in since the most recent entry in the
     change log file (`vc-update-change-log').

     This command works with RCS only; it does not work with CVS or
     SCCS.

   For example, suppose the first line of `ChangeLog' is dated 10 April
1992, and that the only check-in since then was by Nathaniel Bowditch
to `rcs2log' on 8 May 1992 with log text `Ignore log messages that
start with `#'.'.  Then `C-x v a' visits `ChangeLog' and inserts text
like this:

     Fri May  8 21:45:00 1992  Nathaniel Bowditch  <nat@apn.org>
     
             * rcs2log: Ignore log messages that start with `#'.

You can then edit the new change log entry further as you wish.

   Normally, the log entry for file `foo' is displayed as `* foo: TEXT
OF LOG ENTRY'.  The `:' after `foo' is omitted if the text of the log
entry starts with `(FUNCTIONNAME): '.  For example, if the log entry
for `vc.el' is `(vc-do-command): Check call-process status.', then the
text in `ChangeLog' looks like this:

     Wed May  6 10:53:00 1992  Nathaniel Bowditch  <nat@apn.org>
     
             * vc.el (vc-do-command): Check call-process status.

   When `C-x v a' adds several change log entries at once, it groups
related log entries together if they all are checked in by the same
author at nearly the same time.  If the log entries for several such
files all have the same text, it coalesces them into a single entry.
For example, suppose the most recent checkins have the following log
entries:

* For `vc.texinfo': `Fix expansion typos.'
* For `vc.el': `Don't call expand-file-name.'
* For `vc-hooks.el': `Don't call expand-file-name.'

They appear like this in `ChangeLog':

     Wed Apr  1 08:57:59 1992  Nathaniel Bowditch  <nat@apn.org>
     
             * vc.texinfo: Fix expansion typos.
     
             * vc.el, vc-hooks.el: Don't call expand-file-name.

   Normally, `C-x v a' separates log entries by a blank line, but you
can mark several related log entries to be clumped together (without an
intervening blank line) by starting the text of each related log entry
with a label of the form `{CLUMPNAME} '.  The label itself is not
copied to `ChangeLog'.  For example, suppose the log entries are:

* For `vc.texinfo': `{expand} Fix expansion typos.'
* For `vc.el': `{expand} Don't call expand-file-name.'
* For `vc-hooks.el': `{expand} Don't call expand-file-name.'

Then the text in `ChangeLog' looks like this:

     Wed Apr  1 08:57:59 1992  Nathaniel Bowditch  <nat@apn.org>
     
             * vc.texinfo: Fix expansion typos.
             * vc.el, vc-hooks.el: Don't call expand-file-name.

   A log entry whose text begins with `#' is not copied to `ChangeLog'.
For example, if you merely fix some misspellings in comments, you can
log the change with an entry beginning with `#' to avoid putting such
trivia into `ChangeLog'.


File: emacs,  Node: Old Versions,  Next: Branches,  Prev: Change Logs and VC,  Up: Version Control

Examining And Comparing Old Versions
------------------------------------

`C-u C-x C-q VERSION RET'
     Select version VERSION as the current work file version.

`C-x v ~ VERSION RET'
     Examine version VERSION of the visited file, in a buffer of its
     own.

`C-x v ='
     Compare the current buffer contents with the latest checked-in
     version of the file.

`C-u C-x v = FILE RET OLDVERS RET NEWVERS RET'
     Compare the specified two versions of FILE.

   There are two ways to work with an old version of a file.  You can
make the old version your current work file, for example if you want to
reproduce a former stage of development, or if you want to create a
branch from the old version (*note Branches::.).  To do this, visit the
file and type `C-u C-x C-q VERSION RET'.  (This works only with RCS.)

   If you want only to examine an old version, without changing your
work file, visit the file and then type `C-x v ~ VERSION RET'
(`vc-version-other-window').  This puts the text of version VERSION in
a file named `FILENAME.~VERSION~', and visits it in its own buffer in a
separate window.

   To compare two versions of a file, use the command `C-x v ='
(`vc-diff').  Plain `C-x v =' compares the current buffer contents
(saving them in the file if necessary) with the last checked-in version
of the file.  `C-u C-x v =', with a numeric argument, reads a file name
and two version numbers, then compares those versions of the specified
file.

   If you supply a directory name instead of the name of a work file,
this command compares the two specified versions of all registered files
in that directory and its subdirectories.  You can also specify a
snapshot name (*note Snapshots::.) instead of one or both version
numbers.

   You can specify a checked-in version by its number; an empty input
specifies the current contents of the work file (which may be different
from all the checked-in versions).

   This command works by running the `diff' utility, getting the
options from the variable `diff-switches'.  It displays the output in a
special buffer in another window.  Unlike the `M-x diff' command, `C-x
v =' does not try to locate the changes in the old and new versions.
This is because normally one or both versions do not exist as files
when you compare them; they exist only in the records of the master
file.  *Note Comparing Files::, for more information about `M-x diff'.


File: emacs,  Node: Branches,  Next: Status in VC,  Prev: Old Versions,  Up: Version Control

Multiple Branches of a File
---------------------------

   One use of version control is to maintain multiple "current"
versions of a file.  For example, you might have different versions of a
program in which you are gradually adding various unfinished new
features.  Each such independent line of development is called a
"branch".  VC allows you to create branches, and switch between
existing branches.  Note, however, that branches are supported only with
RCS.

   A file's main line of development is usually called the "trunk".
The versions on the trunk are normally numbered 1.1, 1.2, 1.3, etc.  At
any such version, you may start an independent branch.  A branch
starting at version 1.2 would have version number 1.2.1.1.  Consecutive
versions on this branch would have numbers 1.2.1.2, 1.2.1.3, 1.2.1.4,
and so on.  If there is a second branch also starting at version 1.2; it
would consist of versions 1.2.2.1, 1.2.2.2, 1.2.2.3, and so on.

   If you omit the final component of a version number, that is called a
"branch number".  It refers to the highest existing version on that
branch.  The branches in the example above have branch numbers 1.2.1 and
1.2.2.

   A version which is the last in its branch is called a "head" version.

* Menu:

* Switching Branches::    How to get to another existing branch.
* Creating Branches::     How to start a new branch.
* Multi-User Branching::  Multiple users working at multiple branches
                            in parallel.


File: emacs,  Node: Switching Branches,  Next: Creating Branches,  Up: Branches

Switching between Branches
..........................

   To switch between branches, type `C-u C-x C-q' and specify the
version number you want to select.  This version is then checked out
*unlocked* (write-protected), so you can examine it before really
checking it out.  Switching branches in this way is allowed only when
the file is not locked.

   You may omit the minor version number, thus giving only the branch
number; this takes you to the highest version on the indicated branch.
If you only type `RET', Emacs goes to the highest version on the trunk.

   After you have switched to any branch (including the main branch),
you stay on it for subsequent VC commands, until you explicitly select
some other branch.


File: emacs,  Node: Creating Branches,  Next: Multi-User Branching,  Prev: Switching Branches,  Up: Branches

Creating New Branches
.....................

   To create a new branch from a head version (one that is the latest in
the branch that contains it), first select that version if necessary,
lock it with `C-x C-q', and make whatever changes you want.  Then, when
you check in the changes, use `C-u C-x C-q'.  This lets you specify the
version number for the new version.  You should specify a suitable
branch number for a branch starting at the current version.  For
example, if the current version is 2.5, the branch number should be
2.5.1, 2.5.2, and so on, depending on the number of existing branches at
that point.

   To create a new branch at an older version (one that is no longer the
head of a branch), first select that version, then lock it with `C-x
C-q'.  You'll be asked to confirm, when you lock the old version, that
you really mean to create a new branch--if you say no, you'll be offered
a chance to lock the latest version instead.

   Then make your changes and type `C-x C-q' again to check in a new
version.  This automatically creates a new branch starting from the
selected version.  You need not specially request a new branch, because
that's the only way to add a new version at a point that is not the head
of a branch.

   After the branch is created, you "stay" on it.  That means that
subsequent checkouts and checkins create new versions on that branch.
To leave the branch, you must explicitly select a different version with
`C-u C-x C-q' for checkout.

