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[emacs.git] / doc / emacs / files.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1997, 1999, 2000,
@c   2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Files, Buffers, Keyboard Macros, Top
@chapter File Handling
@cindex files

  The operating system stores data permanently in named @dfn{files}, so
most of the text you edit with Emacs comes from a file and is ultimately
stored in a file.

  To edit a file, you must tell Emacs to read the file and prepare a
buffer containing a copy of the file's text.  This is called
@dfn{visiting} the file.  Editing commands apply directly to text in the
buffer; that is, to the copy inside Emacs.  Your changes appear in the
file itself only when you @dfn{save} the buffer back into the file.

  In addition to visiting and saving files, Emacs can delete, copy,
rename, and append to files, keep multiple versions of them, and operate
on file directories.

@menu
* File Names::          How to type and edit file-name arguments.
* Visiting::            Visiting a file prepares Emacs to edit the file.
* Saving::              Saving makes your changes permanent.
* Reverting::           Reverting cancels all the changes not saved.
@ifnottex
* Autorevert::          Auto Reverting non-file buffers.
@end ifnottex
* Auto Save::           Auto Save periodically protects against loss of data.
* File Aliases::        Handling multiple names for one file.
* Version Control::     Version control systems (RCS, CVS and SCCS).
* Directories::         Creating, deleting, and listing file directories.
* Comparing Files::     Finding where two files differ.
* Diff Mode::           Mode for editing file differences.
* Misc File Ops::       Other things you can do on files.
* Compressed Files::    Accessing compressed files.
* File Archives::       Operating on tar, zip, jar etc. archive files.
* Remote Files::        Accessing files on other sites.
* Quoted File Names::   Quoting special characters in file names.
* File Name Cache::     Completion against a list of files you often use.
* File Conveniences::   Convenience Features for Finding Files.
* Filesets::            Handling sets of files.
@end menu

@node File Names
@section File Names
@cindex file names

  Many Emacs commands that operate on a file require you to specify
the file name, using the minibuffer (@pxref{Minibuffer}).  You can use
@dfn{completion} to specify long file names (@pxref{Completion}).
Note that file name completion ignores file names whose extensions
appear in the variable @code{completion-ignored-extensions}
(@pxref{Completion Options}).

  For most operations, there is a @dfn{default file name} which is
used if you type just @key{RET} to enter an empty argument.  Normally,
the default file name is the name of the file visited in the current
buffer.

@vindex default-directory
@vindex insert-default-directory
  Each buffer has a @dfn{default directory} which is normally the same
as the directory of the file visited in that buffer.  For example, if
the default file name is @file{/u/rms/gnu/gnu.tasks}, the default
directory is normally @file{/u/rms/gnu/}.  The default directory is
kept in the variable @code{default-directory}, which has a separate
value in every buffer.  When a command reads a file name using the
minibuffer, the default directory usually serves as the initial
contents of the minibuffer.  To inhibit the insertion of the default
directory, set the variable @code{insert-default-directory} to
@code{nil}.

  If you enter a file name without a directory, that specifies a file
in the default directory.  If you specify a directory in a relative
fashion, with a name that does not start with a slash, it is
interpreted with respect to the default directory.  For example,
suppose the default directory is @file{/u/rms/gnu/}.  Entering just
@samp{foo} in the minibuffer, with a directory omitted, specifies the
file @file{/u/rms/gnu/foo}; entering @samp{../.login} specifies
@file{/u/rms/.login}; and entering @samp{new/foo} specifies
@file{/u/rms/gnu/new/foo}.

  When typing a file name into the minibuffer, you can make use of a
couple of shortcuts: a double slash is interpreted as ``ignore
everything before the second slash in the pair,'' and @samp{~/} is
interpreted as your home directory.  @xref{Minibuffer File}, for more
information about these shortcuts.

@findex cd
@findex pwd
  The command @kbd{M-x pwd} displays the default directory, and the
command @kbd{M-x cd} sets it to a value read using the minibuffer.  A
buffer's default directory changes only when the @code{cd} command is
used.  A file-visiting buffer's default directory is initialized to
the directory of the file it visits.  If you create a buffer with
@kbd{C-x b}, its default directory is copied from that of the buffer
that was current at the time (@pxref{Select Buffer}).

@cindex environment variables in file names
@cindex expansion of environment variables
@cindex @code{$} in file names
  @anchor{File Names with $}The character @samp{$} is used to
substitute an environment variable into a file name.  The name of the
environment variable consists of all the alphanumeric characters after
the @samp{$}; alternatively, it can be enclosed in braces after the
@samp{$}.  For example, if you have used the shell command
@command{export FOO=rms/hacks} to set up an environment variable named
@env{FOO}, then both @file{/u/$FOO/test.c} and
@file{/u/$@{FOO@}/test.c} are abbreviations for
@file{/u/rms/hacks/test.c}.  If the environment variable is not
defined, no substitution occurs, so that the character @samp{$} stands
for itself.

  Note that environment variables affect Emacs only if they are
applied before Emacs is started.

  To access a file with @samp{$} in its name, if the @samp{$} causes
expansion, type @samp{$$}.  This pair is converted to a single
@samp{$} at the same time that variable substitution is performed for
a single @samp{$}.  Alternatively, quote the whole file name with
@samp{/:} (@pxref{Quoted File Names}).  File names which begin with a
literal @samp{~} should also be quoted with @samp{/:}.

  You can include non-@acronym{ASCII} characters in file names if you set the
variable @code{file-name-coding-system} to a non-@code{nil} value.
@xref{File Name Coding}.

@node Visiting
@section Visiting Files
@cindex visiting files
@cindex open file

@table @kbd
@item C-x C-f
Visit a file (@code{find-file}).
@item C-x C-r
Visit a file for viewing, without allowing changes to it
(@code{find-file-read-only}).
@item C-x C-v
Visit a different file instead of the one visited last
(@code{find-alternate-file}).
@item C-x 4 f
Visit a file, in another window (@code{find-file-other-window}).  Don't
alter what is displayed in the selected window.
@item C-x 5 f
Visit a file, in a new frame (@code{find-file-other-frame}).  Don't
alter what is displayed in the selected frame.
@item M-x find-file-literally
Visit a file with no conversion of the contents.
@end table

@cindex files, visiting and saving
@cindex saving files
  @dfn{Visiting} a file means reading its contents into an Emacs
buffer so you can edit them.  Emacs makes a new buffer for each file
that you visit.

  Emacs normally constructs the buffer name from the file name,
omitting the directory name.  For example, a file named
@file{/usr/rms/emacs.tex} is visited in a buffer named
@samp{emacs.tex}.  If there is already a buffer with that name, Emacs
constructs a unique name; the normal method is to append @samp{<2>},
@samp{<3>}, and so on, but you can select other methods.
@xref{Uniquify}.

  Each window's mode line shows the name of the buffer that is being
displayed in that window, so you can always tell what buffer you are
editing.  @pxref{Mode Line}.

  The changes you make with editing commands are made in the Emacs
buffer.  They do not take effect in the file that you visited, or any
permanent place, until you @dfn{save} the buffer (@pxref{Saving}).

@cindex modified (buffer)
  If a buffer contains changes that have not been saved, we say the
buffer is @dfn{modified}.  This implies that some changes will be lost
if the buffer is not saved.  The mode line displays two stars near the
left margin to indicate that the buffer is modified.

@kindex C-x C-f
@findex find-file
  To visit a file, type @kbd{C-x C-f} (@code{find-file}) and use the
minibuffer to enter the name of the desired file.  The usual
defaulting and completion behavior is available in this minibuffer
(@pxref{Minibuffer File}).  Note, also, that completion ignores
certain file names (@pxref{Completion Options}).  While in the
minibuffer, you can abort @kbd{C-x C-f} by typing @kbd{C-g}.

  Your can tell that @kbd{C-x C-f} has completed successfully by the
appearance of new text on the screen and a new buffer name in the mode
line.  If the specified file does not exist and you could not create
it, or exists but you can't read it, an error message is displayed in
the echo area.

  If you visit a file that is already in Emacs, @kbd{C-x C-f} does not make
another copy.  It selects the existing buffer containing that file.
However, before doing so, it checks whether the file itself has changed
since you visited or saved it last.  If the file has changed, Emacs offers
to reread it.

@vindex large-file-warning-threshold
@cindex maximum buffer size exceeded, error message
  If you try to visit a file larger than
@code{large-file-warning-threshold} (the default is 10000000, which is
about 10 megabytes), Emacs asks you for confirmation first.  You can
answer @kbd{y} to proceed with visiting the file.  Note, however, that
Emacs cannot visit files that are larger than the maximum Emacs buffer
size, which is around 256 megabytes on 32-bit machines
(@pxref{Buffers}).  If you try, Emacs will display an error message
saying that the maximum buffer size has been exceeded.

@cindex wildcard characters in file names
@vindex find-file-wildcards
  If the file name you specify contains shell-style wildcard
characters, Emacs visits all the files that match it.  (On
case-insensitive filesystems, Emacs matches the wildcards disregarding
the letter case.)  Wildcards include @samp{?}, @samp{*}, and
@samp{[@dots{}]} sequences.  To enter the wild card @samp{?} in a file
name in the minibuffer, you need to type @kbd{C-q ?}.  @xref{Quoted
File Names}, for information on how to visit a file whose name
actually contains wildcard characters.  You can disable the wildcard
feature by customizing @code{find-file-wildcards}.

@cindex file selection dialog
  On graphical displays, there are two additional methods for visiting
files.  Firstly, when Emacs is built with a suitable GUI toolkit,
commands invoked with the mouse (by clicking on the menu bar or tool
bar) use the toolkit's standard ``File Selection'' dialog instead of
prompting for the file name in the minibuffer.  On GNU/Linux and Unix
platforms, Emacs does this when built with GTK, LessTif, and Motif
toolkits; on MS-Windows and Mac, the GUI version does that by default.
For information on how to customize this, see @ref{Dialog Boxes}.

  Secondly, Emacs supports ``drag and drop'': dropping a file into an
ordinary Emacs window visits the file using that window.  As an
exception, dropping a file into a window displaying a Dired buffer
moves or copies the file into the displayed directory.  For details,
see @ref{Drag and Drop}, and @ref{Misc Dired Features}.

@cindex creating files
@vindex find-file-confirm-nonexistent-file
  What if you want to create a new file?  Just visit it.  Emacs
displays @samp{(New file)} in the echo area, but in other respects
behaves as if you had visited an existing empty file.  If you make
changes and save them, the file is created.  If you change the
variable @code{find-file-confirm-nonexistent-file} to @code{t}, then
Emacs prompts you for confirmation before visiting a non-existent
file.

@kindex C-x C-v
@findex find-alternate-file
  If you visit a nonexistent file unintentionally (because you typed
the wrong file name), type @kbd{C-x C-v} (@code{find-alternate-file})
to visit the file you really wanted.  @kbd{C-x C-v} is similar to
@kbd{C-x C-f}, but it kills the current buffer (after first offering
to save it if it is modified).  When @kbd{C-x C-v} reads the file name
to visit, it inserts the entire default file name in the buffer, with
point just after the directory part; this is convenient if you made a
slight error in typing the name.

@vindex find-file-run-dired
  If you ``visit'' a file that is actually a directory, Emacs invokes
Dired, the Emacs directory browser; this lets you you ``edit'' the
contents of the directory.  @xref{Dired}.  You can disable this
behavior by setting the variable @code{find-file-run-dired} to
@code{nil}; in that case, it is an error to try to visit a directory.

  Files which are actually collections of other files, or @dfn{file
archives}, are visited in special modes which invoke a Dired-like
environment to allow operations on archive members.  @xref{File
Archives}, for more about these features.

  If you visit a file that the operating system won't let you modify,
or that is marked read-only, Emacs makes the buffer read-only too, so
that you won't go ahead and make changes that you'll have trouble
saving afterward.  You can make the buffer writable with @kbd{C-x C-q}
(@code{toggle-read-only}).  @xref{Misc Buffer}.

@kindex C-x C-r
@findex find-file-read-only
  If you want to visit a file as read-only in order to protect
yourself from entering changes accidentally, visit it with the command
@kbd{C-x C-r} (@code{find-file-read-only}) instead of @kbd{C-x C-f}.

@kindex C-x 4 f
@findex find-file-other-window
  @kbd{C-x 4 f} (@code{find-file-other-window}) is like @kbd{C-x C-f}
except that the buffer containing the specified file is selected in another
window.  The window that was selected before @kbd{C-x 4 f} continues to
show the same buffer it was already showing.  If this command is used when
only one window is being displayed, that window is split in two, with one
window showing the same buffer as before, and the other one showing the
newly requested file.  @xref{Windows}.

@kindex C-x 5 f
@findex find-file-other-frame
  @kbd{C-x 5 f} (@code{find-file-other-frame}) is similar, but opens a
new frame, or makes visible any existing frame showing the file you
seek.  This feature is available only when you are using a window
system.  @xref{Frames}.

  Emacs recognizes from the contents of a file which end-of-line
convention it uses to separate lines---newline (used on GNU/Linux and
on Unix), carriage-return linefeed (used on Microsoft systems), or
just carriage-return (used on the Macintosh)---and automatically
converts the contents to the normal Emacs convention, which is that
the newline character separates lines.  This is a part of the general
feature of coding system conversion (@pxref{Coding Systems}), and
makes it possible to edit files imported from different operating
systems with equal convenience.  If you change the text and save the
file, Emacs performs the inverse conversion, changing newlines back
into carriage-return linefeed or just carriage-return if appropriate.

@findex find-file-literally
  If you wish to edit a file as a sequence of @acronym{ASCII}
characters with no special encoding or conversion, use the @kbd{M-x
find-file-literally} command.  This visits a file, like @kbd{C-x C-f},
but does not do format conversion (@pxref{Formatted Text}), character
code conversion (@pxref{Coding Systems}), or automatic uncompression
(@pxref{Compressed Files}), and does not add a final newline because
of @code{require-final-newline} (@pxref{Customize Save}).  If you have
already visited the same file in the usual (non-literal) manner, this
command asks you whether to visit it literally instead.

@vindex find-file-hook
@vindex find-file-not-found-functions
  Two special hook variables allow extensions to modify the operation of
visiting files.  Visiting a file that does not exist runs the functions
in the list @code{find-file-not-found-functions}; this variable holds a list
of functions, and the functions are called one by one (with no
arguments) until one of them returns non-@code{nil}.  This is not a
normal hook, and the name ends in @samp{-functions} rather than @samp{-hook}
to indicate that fact.

  Successful visiting of any file, whether existing or not, calls the
functions in the list @code{find-file-hook}, with no arguments.
This variable is a normal hook.  In the case of a nonexistent file, the
@code{find-file-not-found-functions} are run first.  @xref{Hooks}.

  There are several ways to specify automatically the major mode for
editing the file (@pxref{Choosing Modes}), and to specify local
variables defined for that file (@pxref{File Variables}).

@node Saving
@section Saving Files

  @dfn{Saving} a buffer in Emacs means writing its contents back into the file
that was visited in the buffer.

@menu
* Save Commands::       Commands for saving files.
* Backup::              How Emacs saves the old version of your file.
* Customize Save::      Customizing the saving of files.
* Interlocking::        How Emacs protects against simultaneous editing
                          of one file by two users.
* Shadowing: File Shadowing.  Copying files to "shadows" automatically.
* Time Stamps::         Emacs can update time stamps on saved files.
@end menu

@node Save Commands
@subsection Commands for Saving Files

  These are the commands that relate to saving and writing files.

@table @kbd
@item C-x C-s
Save the current buffer in its visited file on disk (@code{save-buffer}).
@item C-x s
Save any or all buffers in their visited files (@code{save-some-buffers}).
@item M-~
Forget that the current buffer has been changed (@code{not-modified}).
With prefix argument (@kbd{C-u}), mark the current buffer as changed.
@item C-x C-w
Save the current buffer with a specified file name (@code{write-file}).
@item M-x set-visited-file-name
Change the file name under which the current buffer will be saved.
@end table

@kindex C-x C-s
@findex save-buffer
  When you wish to save the file and make your changes permanent, type
@kbd{C-x C-s} (@code{save-buffer}).  After saving is finished, @kbd{C-x C-s}
displays a message like this:

@example
Wrote /u/rms/gnu/gnu.tasks
@end example

@noindent
If the selected buffer is not modified (no changes have been made in it
since the buffer was created or last saved), saving is not really done,
because it would have no effect.  Instead, @kbd{C-x C-s} displays a message
like this in the echo area:

@example
(No changes need to be saved)
@end example

With a prefix argument, @kbd{C-u C-x C-s}, Emacs also marks the buffer
to be backed up when the next save is done.  @xref{Backup}.

@kindex C-x s
@findex save-some-buffers
  The command @kbd{C-x s} (@code{save-some-buffers}) offers to save any
or all modified buffers.  It asks you what to do with each buffer.  The
possible responses are analogous to those of @code{query-replace}:

@table @kbd
@item y
Save this buffer and ask about the rest of the buffers.
@item n
Don't save this buffer, but ask about the rest of the buffers.
@item !
Save this buffer and all the rest with no more questions.
@c following generates acceptable underfull hbox
@item @key{RET}
Terminate @code{save-some-buffers} without any more saving.
@item .
Save this buffer, then exit @code{save-some-buffers} without even asking
about other buffers.
@item C-r
View the buffer that you are currently being asked about.  When you exit
View mode, you get back to @code{save-some-buffers}, which asks the
question again.
@item d
Diff the buffer against its corresponding file, so you can see
what changes you would be saving.
@item C-h
Display a help message about these options.
@end table

  @kbd{C-x C-c}, the key sequence to exit Emacs, invokes
@code{save-some-buffers} and therefore asks the same questions.

@kindex M-~
@findex not-modified
  If you have changed a buffer but do not wish to save the changes,
you should take some action to prevent it.  Otherwise, each time you
use @kbd{C-x s} or @kbd{C-x C-c}, you are liable to save this buffer
by mistake.  One thing you can do is type @kbd{M-~}
(@code{not-modified}), which clears out the indication that the buffer
is modified.  If you do this, none of the save commands will believe
that the buffer needs to be saved.  (@samp{~} is often used as a
mathematical symbol for `not'; thus @kbd{M-~} is `not', metafied.)
Alternatively, you can cancel all the changes made since the file was
visited or saved, by reading the text from the file again.  This is
called @dfn{reverting}.  @xref{Reverting}.  (You could also undo all
the changes by repeating the undo command @kbd{C-x u} until you have
undone all the changes; but reverting is easier.)

@findex set-visited-file-name
  @kbd{M-x set-visited-file-name} alters the name of the file that the
current buffer is visiting.  It reads the new file name using the
minibuffer.  Then it marks the buffer as visiting that file name, and
changes the buffer name correspondingly.  @code{set-visited-file-name}
does not save the buffer in the newly visited file; it just alters the
records inside Emacs in case you do save later.  It also marks the
buffer as ``modified'' so that @kbd{C-x C-s} in that buffer
@emph{will} save.

@kindex C-x C-w
@findex write-file
  If you wish to mark the buffer as visiting a different file and save
it right away, use @kbd{C-x C-w} (@code{write-file}).  This is
equivalent to @code{set-visited-file-name} followed by @kbd{C-x C-s},
except that @kbd{C-x C-w} asks for confirmation if the file exists.
@kbd{C-x C-s} used on a buffer that is not visiting a file has the
same effect as @kbd{C-x C-w}; that is, it reads a file name, marks the
buffer as visiting that file, and saves it there.  The default file
name in a buffer that is not visiting a file is made by combining the
buffer name with the buffer's default directory (@pxref{File Names}).

  If the new file name implies a major mode, then @kbd{C-x C-w} switches
to that major mode, in most cases.  The command
@code{set-visited-file-name} also does this.  @xref{Choosing Modes}.

  If Emacs is about to save a file and sees that the date of the latest
version on disk does not match what Emacs last read or wrote, Emacs
notifies you of this fact, because it probably indicates a problem caused
by simultaneous editing and requires your immediate attention.
@xref{Interlocking,, Simultaneous Editing}.

@node Backup
@subsection Backup Files
@cindex backup file
@vindex make-backup-files
@vindex vc-make-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
@dfn{backup} file, before actually saving.

  Emacs makes a backup for a file only the first time the file is
saved from a buffer.  No matter how many times you subsequently save
the file, its backup remains unchanged.  However, if you kill the
buffer and then visit the file again, a new backup file will be made.

  For most files, the variable @code{make-backup-files} determines
whether to make backup files.  On most operating systems, its default
value is @code{t}, so that Emacs does write backup files.

  For files managed by a version control system (@pxref{Version
Control}), the variable @code{vc-make-backup-files} determines whether
to make backup files.  By default it is @code{nil}, since backup files
are redundant when you store all the previous versions in a version
control system.
@iftex
@xref{General VC Options,,,emacs-xtra, Specialized Emacs Features}.
@end iftex
@ifnottex
@xref{General VC Options}.
@end ifnottex

  At your option, Emacs can keep either a single backup for each file,
or make a series of numbered backup files for each file that you edit.
@xref{Backup Names}.

@vindex backup-enable-predicate
@vindex temporary-file-directory
@vindex small-temporary-file-directory
  The default value of the @code{backup-enable-predicate} variable
prevents backup files being written for files in the directories used
for temporary files, specified by @code{temporary-file-directory} or
@code{small-temporary-file-directory}.

  You can explicitly tell Emacs to make another backup file from a
buffer, even though that buffer has been saved before.  If you save
the buffer with @kbd{C-u C-x C-s}, the version thus saved will be made
into a backup file if you save the buffer again.  @kbd{C-u C-u C-x
C-s} saves the buffer, but first makes the previous file contents into
a new backup file.  @kbd{C-u C-u C-u C-x C-s} does both things: it
makes a backup from the previous contents, and arranges to make
another from the newly saved contents if you save again.

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

@node Backup Names
@subsubsection Single or Numbered Backups

  When Emacs makes a backup file, its name is normally constructed by
appending @samp{~} to the file name being edited; thus, the backup
file for @file{eval.c} would be @file{eval.c~}.

  If access control stops Emacs from writing backup files under the usual
names, it writes the backup file as @file{%backup%~} in your home
directory.  Only one such file can exist, so only the most recently
made such backup is available.

  Emacs can also make @dfn{numbered backup files}.  Numbered backup
file names contain @samp{.~}, the number, and another @samp{~} after
the original file name.  Thus, the backup files of @file{eval.c} would
be called @file{eval.c.~1~}, @file{eval.c.~2~}, and so on, all the way
through names like @file{eval.c.~259~} and beyond.

@vindex version-control
  The variable @code{version-control} determines whether to make
single backup files or multiple numbered backup files.  Its possible
values are:

@table @code
@item nil
Make numbered backups for files that have numbered backups already.
Otherwise, make single backups.  This is the default.
@item t
Make numbered backups.
@item never
Never make numbered backups; always make single backups.
@end table

@noindent
The usual way to set this variable is globally, through your
@file{.emacs} file or the customization buffer.  However, you can set
@code{version-control} locally in an individual buffer to control the
making of backups for that buffer's file (@pxref{Locals}).  You can
have Emacs set @code{version-control} locally whenever you visit a
given file (@pxref{File Variables}).  Some modes, such as Rmail mode,
set this variable.

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

@vindex backup-directory-alist
  You can customize the variable @code{backup-directory-alist} to
specify that files matching certain patterns should be backed up in
specific directories.  This variable applies to both single and
numbered backups.  A typical use is to add an element @code{("."
. @var{dir})} to make all backups in the directory with absolute name
@var{dir}; Emacs modifies the backup file names to avoid clashes
between files with the same names originating in different
directories.  Alternatively, adding, @code{("." . ".~")} would make
backups in the invisible subdirectory @file{.~} of the original file's
directory.  Emacs creates the directory, if necessary, to make the
backup.

@vindex make-backup-file-name-function
  If you define the variable @code{make-backup-file-name-function} to
a suitable Lisp function, that overrides the usual way Emacs
constructs backup file names.

@node Backup Deletion
@subsubsection Automatic Deletion of Backups

  To prevent excessive 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.

@vindex kept-old-versions
@vindex kept-new-versions
  The two variables @code{kept-old-versions} and
@code{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.  The backups in the middle (excluding those oldest
and newest) are the excess middle versions---those backups are
deleted.  These variables' values are used when it is time to delete
excess versions, just after a new backup version is made; the newly
made backup is included in the count in @code{kept-new-versions}.  By
default, both variables are 2.

@vindex delete-old-versions
  If @code{delete-old-versions} is @code{t}, Emacs deletes the excess
backup files silently.  If it is @code{nil}, the default, Emacs asks
you whether it should delete the excess backup versions.  If it has
any other value, then Emacs never automatically deletes backups.

  Dired's @kbd{.} (Period) command can also be used to delete old versions.
@xref{Dired Deletion}.

@node Backup Copying
@subsubsection 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 (hard
links).  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 @code{backup-by-copying-when-mismatch}
locally (@pxref{File Variables}).

@vindex backup-by-copying
@vindex backup-by-copying-when-linked
@vindex backup-by-copying-when-mismatch
@vindex backup-by-copying-when-privileged-mismatch
@cindex file ownership, and backup
@cindex backup, and user-id
  The choice of renaming or copying is controlled by four variables.
Renaming is the default choice.  If the variable
@code{backup-by-copying} is non-@code{nil}, copying is used.  Otherwise,
if the variable @code{backup-by-copying-when-linked} is non-@code{nil},
then copying is used for files that have multiple names, but renaming
may still be used when the file being edited has only one name.  If the
variable @code{backup-by-copying-when-mismatch} is non-@code{nil}, then
copying is used if renaming would cause the file's owner or group to
change.  @code{backup-by-copying-when-mismatch} is @code{t} by default
if you start Emacs as the superuser.  The fourth variable,
@code{backup-by-copying-when-privileged-mismatch}, gives the highest
numeric user-id for which @code{backup-by-copying-when-mismatch} will be
forced on.  This is useful when low-numbered user-ids are assigned to
special system users, such as @code{root}, @code{bin}, @code{daemon},
etc., which must maintain ownership of files.

  When a file is managed with a version control system (@pxref{Version
Control}), Emacs does not normally make backups in the usual way for
that file.  But check-in and check-out are similar in some ways to
making backups.  One unfortunate similarity is that these operations
typically break hard links, disconnecting the file name you visited from
any alternate names for the same file.  This has nothing to do with
Emacs---the version control system does it.

@node Customize Save
@subsection Customizing Saving of Files

@vindex require-final-newline
  If the value of the variable @code{require-final-newline} is
@code{t}, saving or writing a file silently puts a newline at the end
if there isn't already one there.  If the value is @code{visit}, Emacs
adds a newline at the end of any file that doesn't have one, just
after it visits the file.  (This marks the buffer as modified, and you
can undo it.)  If the value is @code{visit-save}, that means to add
newlines both on visiting and on saving.  If the value is @code{nil},
Emacs leaves the end of the file unchanged; if it's neither @code{nil}
nor @code{t}, Emacs asks you whether to add a newline.  The default is
@code{nil}.

@vindex mode-require-final-newline
  Many major modes are designed for specific kinds of files that are
always supposed to end in newlines.  These major modes set the
variable @code{require-final-newline} according to
@code{mode-require-final-newline}.  By setting the latter variable,
you can control how these modes handle final newlines.

@vindex write-region-inhibit-fsync
  When Emacs saves a file, it invokes the @code{fsync} system call to
force the data immediately out to disk.  This is important for safety
if the system crashes or in case of power outage.  However, it can be
disruptive on laptops using power saving, because it requires the disk
to spin up each time you save a file.  Setting
@code{write-region-inhibit-fsync} to a non-@code{nil} value disables
this synchronization.  Be careful---this means increased risk of data
loss.

@node Interlocking
@subsection Protection against Simultaneous Editing

@cindex file dates
@cindex 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.  On all systems,
Emacs checks when you save the file, and warns if you are about to
overwrite another user's changes.  You can prevent loss of the other
user's work by taking the proper corrective action instead of saving the
file.

@findex ask-user-about-lock
@cindex locking files
  When you make the first modification in an Emacs buffer that is
visiting a file, Emacs records that the file is @dfn{locked} by you.
(It does this by creating a specially-named symbolic link in the same
directory.)  Emacs removes the lock when you save the changes.  The
idea is that the file is locked whenever an Emacs buffer visiting it
has unsaved changes.

@cindex collision
  If you begin to modify the buffer while the visited file is locked by
someone else, this constitutes a @dfn{collision}.  When Emacs detects a
collision, it asks you what to do, by calling the Lisp function
@code{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:

@table @kbd
@item s
Steal the lock.  Whoever was already changing the file loses the lock,
and you gain the lock.
@item p
Proceed.  Go ahead and edit the file despite its being locked by someone else.
@item q
Quit.  This causes an error (@code{file-locked}), and the buffer
contents remain unchanged---the modification you were trying to make
does not actually take place.
@end table

  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, and
there are cases where lock files cannot be written.  In these cases,
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 displays 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
@kbd{yes} and proceed.  Otherwise, you should cancel the save with
@kbd{C-g} and investigate the situation.

  If Emacs or the operating system crashes, this may leave behind lock
files which are stale, so you may occasionally get warnings about
spurious collisions.  When you determine that the collision is spurious,
just use @kbd{p} to tell Emacs to go ahead anyway.

  The first thing you should do when notified that simultaneous editing
has already taken place is to list the directory with @kbd{C-u C-x C-d}
(@pxref{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 @code{diff} to compare the two files.@refill

@node File Shadowing
@subsection Shadowing Files
@cindex shadow files
@cindex file shadows
@findex shadow-initialize

@table @kbd
@item M-x shadow-initialize
Set up file shadowing.
@item M-x shadow-define-literal-group
Declare a single file to be shared between sites.
@item M-x shadow-define-regexp-group
Make all files that match each of a group of files be shared between hosts.
@item M-x shadow-define-cluster @key{RET} @var{name} @key{RET}
Define a shadow file cluster @var{name}.
@item M-x shadow-copy-files
Copy all pending shadow files.
@item M-x shadow-cancel
Cancel the instruction to shadow some files.
@end table

You can arrange to keep identical @dfn{shadow} copies of certain files
in more than one place---possibly on different machines.  To do this,
first you must set up a @dfn{shadow file group}, which is a set of
identically-named files shared between a list of sites.  The file
group is permanent and applies to further Emacs sessions as well as
the current one.  Once the group is set up, every time you exit Emacs,
it will copy the file you edited to the other files in its group.  You
can also do the copying without exiting Emacs, by typing @kbd{M-x
shadow-copy-files}.

To set up a shadow file group, use @kbd{M-x
shadow-define-literal-group} or @kbd{M-x shadow-define-regexp-group}.
See their documentation strings for further information.

Before copying a file to its shadows, Emacs asks for confirmation.
You can answer ``no'' to bypass copying of this file, this time.  If
you want to cancel the shadowing permanently for a certain file, use
@kbd{M-x shadow-cancel} to eliminate or change the shadow file group.

A @dfn{shadow cluster} is a group of hosts that share directories, so
that copying to or from one of them is sufficient to update the file
on all of them.  Each shadow cluster has a name, and specifies the
network address of a primary host (the one we copy files to), and a
regular expression that matches the host names of all the other hosts
in the cluster.  You can define a shadow cluster with @kbd{M-x
shadow-define-cluster}.

@node Time Stamps
@subsection Updating Time Stamps Automatically
@cindex time stamps
@cindex modification dates
@cindex locale, date format

You can arrange to put a time stamp in a file, so that it will be updated
automatically each time you edit and save the file.  The time stamp
has to be in the first eight lines of the file, and you should
insert it like this:

@example
Time-stamp: <>
@end example

@noindent
or like this:

@example
Time-stamp: " "
@end example

@findex time-stamp
  Then add the hook function @code{time-stamp} to the hook
@code{before-save-hook}; that hook function will automatically update
the time stamp, inserting the current date and time when you save the
file.  You can also use the command @kbd{M-x time-stamp} to update the
time stamp manually.  For other customizations, see the Custom group
@code{time-stamp}.  Note that non-numeric fields in the time stamp are
formatted according to your locale setting (@pxref{Environment}).

@node Reverting
@section Reverting a Buffer
@findex revert-buffer
@cindex drastic changes
@cindex reread a file

  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 @kbd{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 @kbd{yes}.

  @code{revert-buffer} tries to position point in such a way that, if
the file was edited only slightly, you will be at approximately the
same piece of text after reverting as before.  However, if you have made
drastic changes, point may wind up in a totally different piece of text.

  Reverting marks the buffer as ``not modified''.

  Some kinds of buffers that are not associated with files, such as
Dired buffers, can also be reverted.  For them, reverting means
recalculating their contents.  Buffers created explicitly with
@kbd{C-x b} cannot be reverted; @code{revert-buffer} reports an error
if you try.

@vindex revert-without-query
  When you edit a file that changes automatically and frequently---for
example, a log of output from a process that continues to run---it may
be useful for Emacs to revert the file without querying you.  To
request this behavior, set the variable @code{revert-without-query} to
a list of regular expressions.  When a file name matches one of these
regular expressions, @code{find-file} and @code{revert-buffer} will
revert it automatically if it has changed---provided the buffer itself
is not modified.  (If you have edited the text, it would be wrong to
discard your changes.)

@cindex Global Auto-Revert mode
@cindex mode, Global Auto-Revert
@cindex Auto-Revert mode
@cindex mode, Auto-Revert
@findex global-auto-revert-mode
@findex auto-revert-mode
@findex auto-revert-tail-mode
@vindex auto-revert-interval

  In addition, you can tell Emacs to periodically revert a buffer by
typing @kbd{M-x auto-revert-mode}.  This turns on Auto-Revert mode, a
minor mode that makes Emacs automatically revert the current buffer
every five seconds.  You can change this interval through the variable
@code{auto-revert-interval}.  Typing @kbd{M-x global-auto-revert-mode}
enables Global Auto-Revert mode, which does the same for all file
buffers.  Auto-Revert mode and Global Auto-Revert modes do not check
or revert remote files, because that is usually too slow.

  One use of Auto-Revert mode is to ``tail'' a file such as a system
log, so that changes made to that file by other programs are
continuously displayed.  To do this, just move the point to the end of
the buffer, and it will stay there as the file contents change.
However, if you are sure that the file will only change by growing at
the end, use Auto-Revert Tail mode instead
(@code{auto-revert-tail-mode}).  It is more efficient for this.

  @xref{VC Mode Line}, for Auto Revert peculiarities in buffers that
visit files under version control.

@ifnottex
@include arevert-xtra.texi
@end ifnottex

@node Auto Save
@section Auto-Saving: Protection Against Disasters
@cindex Auto Save mode
@cindex mode, Auto Save
@cindex crashes

  From time to time, Emacs automatically saves each visited file in a
separate file, without altering the file you actually use.  This is
called @dfn{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, it considers
each buffer, and each is auto-saved if auto-saving is enabled for it
and it has been changed since the last time it was auto-saved.  The
message @samp{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.
@end menu

@node Auto Save Files
@subsection Auto-Save Files

  Auto-saving does not normally save in the files that you visited,
because it can be very undesirable to save a change that you did not
want to make permanent.  Instead, auto-saving is done in a different
file called the @dfn{auto-save file}, and the visited file is changed
only when you request saving explicitly (such as with @kbd{C-x C-s}).

  Normally, the auto-save file name is made by appending @samp{#} to the
front and rear of the visited file name.  Thus, a buffer visiting file
@file{foo.c} is auto-saved in a file @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
@samp{#} to the front and rear of buffer name, then
adding digits and letters at the end for uniqueness.  For
example, the @samp{*mail*} buffer in which you compose messages to be
sent might be auto-saved in a file named @file{#*mail*#704juu}.  Auto-save file
names are made this way unless you reprogram parts of Emacs to do
something different (the functions @code{make-auto-save-file-name} and
@code{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.

@cindex auto-save for remote files
@vindex auto-save-file-name-transforms
  The variable @code{auto-save-file-name-transforms} allows a degree
of control over the auto-save file name.  It lets you specify a series
of regular expressions and replacements to transform the auto save
file name.  The default value puts the auto-save files for remote
files (@pxref{Remote Files}) into the temporary file directory on the
local machine.

  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 @kbd{C-x C-s}, or use @kbd{C-u 1 M-x
auto-save-mode}.

@vindex auto-save-visited-file-name
  If you want auto-saving to be done in the visited file rather than
in a separate auto-save file, set the variable
@code{auto-save-visited-file-name} to a non-@code{nil} value.  In this
mode, there is no real difference between auto-saving and explicit
saving.

@vindex delete-auto-save-files
  A buffer's auto-save file is deleted when you save the buffer in its
visited file.  (You can inhibit this by setting the variable
@code{delete-auto-save-files} to @code{nil}.)  Changing the visited
file name with @kbd{C-x C-w} or @code{set-visited-file-name} renames
any auto-save file to go with the new visited name.

@node Auto Save Control
@subsection Controlling Auto-Saving

@vindex auto-save-default
@findex auto-save-mode
  Each time you visit a file, auto-saving is turned on for that file's
buffer if the variable @code{auto-save-default} is non-@code{nil} (but not
in batch mode; @pxref{Entering Emacs}).  The default for this variable is
@code{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 @kbd{M-x auto-save-mode}.  Like other minor mode commands, @kbd{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.

@vindex auto-save-interval
  Emacs does auto-saving periodically based on counting how many characters
you have typed since the last time auto-saving was done.  The variable
@code{auto-save-interval} specifies how many characters there are between
auto-saves.  By default, it is 300.  Emacs doesn't accept values that are
too small: if you customize @code{auto-save-interval} to a value less
than 20, Emacs will behave as if the value is 20.

@vindex auto-save-timeout
  Auto-saving also takes place when you stop typing for a while.  The
variable @code{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 @samp{kill
%emacs}, or disconnecting a phone line or network connection.

@findex do-auto-save
  You can request an auto-save explicitly with the command @kbd{M-x
do-auto-save}.

@node Recover
@subsection Recovering Data from Auto-Saves

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

@example
M-x recover-file @key{RET} foo.c @key{RET}
yes @key{RET}
C-x C-s
@end example

  Before asking for confirmation, @kbd{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, @kbd{M-x recover-file} does not offer to read it.

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

  Then @code{recover-session} asks about each of the files that were
being edited during that session, asking whether to recover that file.
If you answer @kbd{y}, it calls @code{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 @code{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.

@vindex auto-save-list-file-prefix
  Emacs records information about interrupted sessions for later
recovery in files named
@file{~/.emacs.d/auto-save-list/.saves-@var{pid}-@var{hostname}}.  The
directory used, @file{~/.emacs.d/auto-save-list/}, is determined by
the variable @code{auto-save-list-file-prefix}.  You can record
sessions in a different place by customizing that variable.  If you
set @code{auto-save-list-file-prefix} to @code{nil} in your
@file{.emacs} file, sessions are not recorded for recovery.

@node File Aliases
@section File Name Aliases
@cindex symbolic links (visiting)
@cindex hard links (visiting)

  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 @file{foo} is a symbolic link to @file{bar}, you can use
either name to refer to the file, but @file{bar} is the real name, while
@file{foo} is just an alias.  More complex cases occur when symbolic
links point to directories.

@vindex find-file-existing-other-name
@vindex find-file-suppress-same-file-warnings

  Normally, if you visit a file which Emacs is already visiting under
a different name, Emacs displays a message in the echo area and uses
the existing buffer visiting that file.  This can happen on systems
that support hard or symbolic links, or if you use a long file name on
a system that truncates long file names, or on a case-insensitive file
system.  You can suppress the message by setting the variable
@code{find-file-suppress-same-file-warnings} to a non-@code{nil}
value.  You can disable this feature entirely by setting the variable
@code{find-file-existing-other-name} to @code{nil}: then if you visit
the same file under two different names, you get a separate buffer for
each file name.

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

@node Version Control
@section Version Control
@cindex version control

  A @dfn{version control system} is a package that can record multiple
versions of a source file, storing 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 interface is called VC.  Its commands work
with several different version control systems; currently, it supports
GNU Arch, Bazaar, CVS, Git, Mercurial, Monotone, RCS, SCCS/CSSC, and
Subversion.  Of these, the GNU project distributes CVS, GNU Arch, RCS,
and Bazaar.

  VC is enabled automatically whenever you visit a file that is
governed by a version control system.  To disable VC entirely, set the
customizable variable @code{vc-handled-backends} to @code{nil}
@iftex
(@pxref{Customizing VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Customizing VC}).
@end ifnottex


@menu
* Introduction to VC::  How version control works in general.
* VC Mode Line::        How the mode line shows version control status.
* Basic VC Editing::    How to edit a file under version control.
* Old Revisions::       Examining and comparing old versions.
* Secondary VC Commands::    The commands used a little less frequently.
* VC Directory Mode::   Listing files managed by version control.
* Branches::            Multiple lines of development.
@ifnottex
* Remote Repositories:: Efficient access to remote CVS servers.
* Revision Tags::       Symbolic names for revisions
* Miscellaneous VC::    Various other commands and features of VC.
* Customizing VC::      Variables that change VC's behavior.
@end ifnottex
@end menu

@node Introduction to VC
@subsection Introduction to Version Control

  VC allows you to use a version control system from within Emacs,
integrating the version control operations smoothly with editing.
Though VC cannot completely bridge the gaps between version control
systems with widely differing capabilities, it does provide a uniform
interface to many version control operations. Regardless of which
version control system is in use, you will be able to do basic
operations in much the same way.

  This section provides a general overview of version control, and
describes the version control systems that VC supports.  You can skip
this section if you are already familiar with the version control system
you want to use.

@menu
* Why Version Control?::    Understanding the problems it addresses
* Version Control Systems:: Supported version control back-end systems.
* VCS Concepts::            Words and concepts related to version control.
* Types of Log File::       The VCS log in contrast to the ChangeLog.
@end menu

@node Why Version Control?
@subsubsection Understanding the problems it addresses

  Version control systems provide you with three important
capabilities:

@itemize @bullet
@item
@dfn{Reversibility}: the ability to back up to a previous state if you
discover that some modification you did was a mistake or a bad idea.

@item
@dfn{Concurrency}: the ability to have many people modifying the same
collection of files knowing that conflicting modifications can be
detected and resolved.

@item
@dfn{History}: the ability to attach historical data to your data,
such as explanatory comments about the intention behind each change to
it.  Even for a programmer working solo, change histories are an
important aid to memory; for a multi-person project, they are a
vitally important form of communication among developers.
@end itemize

@node Version Control Systems
@subsubsection Supported Version Control Systems

@cindex back end (version control)
  VC currently works with many different version control systems or
@dfn{back ends}:

@itemize @bullet

@cindex SCCS
@item
SCCS was the first version control system ever built, and was long ago
superseded by more advanced ones.  VC compensates for certain features
missing in SCCS (e.g., tag names for releases) by implementing them
itself.  Other VC features, such as multiple branches, are simply
unavailable.  Since SCCS is non-free, we recommend avoiding it.

@cindex CSSC
@item
CSSC is a free replacement for SCCS.  You should use CSSC only if, for
some reason, you cannot use a more recent and better-designed version
control system.

@cindex RCS
@item
RCS is the free version control system around which VC was initially
built.  Almost everything you can do with RCS can be done through VC.
However, you cannot use RCS over the network, and it only works at the
level of individual files rather than projects.

@cindex CVS
@item
CVS is the free version control system that was, until recently (circa
2008), used by the majority of free software projects.  Nowadays, it
is slowly being superseded by newer systems.  CVS allows concurrent
multi-user development either locally or over the network.  It lacks
support for atomic commits or file moving/renaming.  VC supports all
basic editing operations under CVS.  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.

@cindex SVN
@cindex Subversion
@item
Subversion (SVN) is a free version control system designed to be
similar to CVS but without its problems.  It supports atomic commits
of filesets, and versioning of directories, symbolic links, meta-data,
renames, copies, and deletes.

@cindex GNU Arch
@cindex Arch
@item
GNU Arch is a version control system designed for distributed work.
It differs in many ways from older systems like CVS and RCS.  It
provides different methods for interoperating between users, support
for offline operations, and good branching and merging features.  It
also supports atomic commits of filesets and file moving/renaming.  VC
does not support all operations provided by GNU Arch, so you must
sometimes invoke it from the command line.

@cindex git
@item
Git is a distributed version control system invented by Linus Torvalds to support
Linux kernel development.  It supports atomic commits of filesets and
file moving/renaming.  One significant feature of git is that it
largely abolishes the notion of a single centralized repository;
instead, each working copy of a git project is its own repository and
coordination is done through repository-sync operations.  VC supports
most git operations, with the exception of news merges and repository
syncing; these must be done from the command line.

@cindex hg
@cindex Mercurial
@item
Mercurial (hg) is a distributed version control system broadly
resembling GNU Arch and git, with atomic fileset commits and file
moving/renaming.  Like git, it is fully decentralized.  VC supports
most Mercurial commands, with the exception of repository sync
operations; this needs to be done from the command line.

@cindex bzr
@cindex Bazaar
@item
Bazaar (bzr) is a distributed version control system that supports both
repository-based and distributed versioning, with atomic fileset
commits and file moving/renaming.  VC supports most basic editing
operations under Bazaar.
@end itemize

  Previous versions of VC supported a version control system known as
Meta-CVS.  This support has been dropped because of limited interest
from users and developers.

@node VCS Concepts
@subsubsection Concepts of Version Control

@cindex repository
@cindex registered file
   When a file is under version control, we say that it is
@dfn{registered} in the version control system.  The system has a
@dfn{repository} which stores both the file's present state and its
change history---enough to reconstruct the current version or any
earlier version.  The repository also contains other information, such
as @dfn{log entries} that describe the changes made to each file.

@cindex work file
@cindex checking out files
  A file @dfn{checked out} of a repository is called the @dfn{work
file}.  You edit the work file and make changes in it, as you would
with an ordinary file.  After you are done with a set of changes, you
@dfn{check in} or @dfn{commit} the file; this records the changes in
the repository, along with a log entry for those changes.

@cindex revision
@cindex revision ID
  A copy of a file stored in a repository is called a @dfn{revision}.
The history of a file is a sequence of revisions.  Each revisions is
named by a @dfn{revision ID}.  The format of the revision ID depends
on the version control system; in the simplest case, it is just an
integer.

  To go beyond these basic concepts, you will need to understand three
ways in which version control systems can differ from each other.
They can be locking-based or merging-based; they can be file-based or
changeset-based; and they can be centralized or decentralized.  VC
handles all these choices, but they lead to differing behaviors which
you will need to understand as you use it.

@cindex locking versus merging
  A version control system typically has some mechanism to coordinate
between users who want to change the same file.  There are two ways to
do this: merging and locking.

  In a version control system that uses merging, each user may check
out and modify a work file at any time.  The system lets you
@dfn{merge} your work file, which may contain changes that have not
been checked in, with the latest changes that others have checked into
the repository.

  Older version control systems use a @dfn{locking} scheme instead.
Here, work files are normally read-only.  To edit a file, you ask the
version control system to make it writable for you by @dfn{locking}
it; only one user can lock a given file at any given time.  This
procedure is analogous to, but different from, the locking that Emacs
uses to detect simultaneous editing of ordinary files
(@pxref{Interlocking}).  When you check in your changes, that unlocks
the file, and the work file becomes read-only again.  Other users may
then lock the file to make their own changes.

  Both locking and merging systems can have problems when multiple
users try to modify the same file at the same time.  Locking systems
have @dfn{lock conflicts}; a user may try to check a file out and be
unable to because it is locked.  In merging systems, @dfn{merge
conflicts} happen when you check in a change to a file that conflicts
with a change checked in by someone else after your checkout.  Both
kinds of conflict have to be resolved by human judgment and
communication.

  SCCS always uses locking.  RCS is lock-based by default but can be
told to operate in a merging style.  CVS and Subversion are
merge-based by default but can be told to operate in a locking mode.
Distributed version control systems, such as GNU Arch, git, and
Mercurial, are exclusively merging-based.  Experience has shown that
merging is superior to locking, both in convenience to developers and
in minimizing the number and severity of conflicts that actually
occur.  Sometimes, however, newer version control systems may have
locks retrofitted onto them for reasons having nothing to do with
technology@footnote{Usually the control-freak instincts of managers.}.

  VC mode supports both locking and merging version control and tries
to hide the differences between them as much as possible.

@cindex files versus changesets.
  On SCCS, RCS, CVS, and other early version control systems, version
control operations are @dfn{file-based}: each file has its own comment
and revision history separate from that of all other files in the
system.  Later systems, beginning with Subversion, are
@dfn{changeset-based}: a checkin may include changes to several files,
and the entire set of changes is treated as a unit by the system.  Any
comment associated with the change does not belong to a single file,
but to the changeset itself.

  Changeset-based version control is more flexible and powerful than
file-based version control; usually, when a change to multiple files
has to be reversed, it's good to be able to easily identify and remove
all of it.  But it took some years for designers to figure that out,
and while file-based systems are passing out of use, there are lots of
legacy repositories still to be dealt with as of this writing (2008).

  Prior to Emacs 23, VC supported only file-based systems, leading to
unhappy results when it was used to drive changeset-based
systems---the Subversion support, for example, used to break up
changesets into multiple per-file commits.  This has been fixed, but
it has left a mark in VC's terminology.  The terms ``checkin'' and
``checkout'' are associated with file-based and locking-based systems
and a bit archaic; nowadays those operations are usually called
``commit'' and ``update''.

@cindex centralized vs. decentralized version control
  Early version control systems were designed around a
@dfn{centralized} model in which each project has only one repository
used by all developers.  SCCS, RCS, CVS, and Subversion share this
kind of model.  One problem with this approach is that a single
repository is a single point of failure---if the repository server is
down, all work stops.

  Newer version control systems like GNU Arch, git, Mercurial, and
Bazaar are @dfn{decentralized}.  A project may have several different
repositories, and these systems support a sort of super-merge between
repositories that tries to reconcile their change histories.  At the
limit, each developer has his/her own repository, and repository
merges replace checkin/commit operations.

  VC's job is to help you manage the traffic between your personal
workfiles and a repository.  Whether that repository is a single
master or one of a network of peer repositories is not something VC
has to care about.  Thus, the difference between a centralized and a
decentralized version control system is invisible to VC mode.

@node Types of Log File
@subsubsection Types of Log File
@cindex types of log file
@cindex log File, types of
@cindex version control log

  Projects that use a version control system can have two types of log
for changes.  One is the log maintained by the version control system:
each time you check in a change, you fill out a @dfn{log entry} for
the change (@pxref{Log Buffer}).  This is called the @dfn{version
control log}.

  The other kind of log is the file @file{ChangeLog} (@pxref{Change
Log}).  It provides a chronological record of all changes to a large
portion of a program---typically one directory and its subdirectories.
A small program would use one @file{ChangeLog} file; a large program
may have a @file{ChangeLog} file in each major directory.
@xref{Change Log}.

  Actually, the fact that both kinds of log exist is partly a legacy
from file-based version control.  Changelogs are a GNU convention,
later more widely adopted, that help developers to get a
changeset-based view of a project even when its version control system
has that information split up in multiple file-based logs.

  Changeset-based version systems, on the other hand, often maintain a
changeset-based modification log for the entire system that makes
ChangeLogs somewhat redundant.  One advantage that ChangeLogs retain
is that it is sometimes useful to be able to view the transaction
history of a single directory separately from those of other
directories.

  A project maintained with version control can use just the version
control log, or it can use both kinds of logs.  It can handle some
files one way and some files the other way.  Each project has its
policy, which you should follow.

  When the policy is to use both, you typically want to write an entry
for each change just once, then put it into both logs.  You can write
the entry in @file{ChangeLog}, then copy it to the log buffer with
@kbd{C-c C-a} when checking in the change (@pxref{Log Buffer}).  Or
you can write the entry in the log buffer while checking in the
change, and later use the @kbd{C-x v a} command to copy it to
@file{ChangeLog}
@iftex
(@pxref{Change Logs and VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Change Logs and VC}).
@end ifnottex

@node VC Mode Line
@subsection Version Control and the Mode Line

  When you visit a file that is under version control, Emacs indicates
this on the mode line.  For example, @samp{RCS-1.3} says that RCS is
used for that file, and the current version is 1.3.

  The character between the back-end name and the revision ID
indicates the version control status of the file.  @samp{-} means that
the work file is not locked (if locking is in use), or not modified (if
locking is not in use).  @samp{:} indicates that the file is locked, or
that it is modified.  If the file is locked by some other user (for
instance, @samp{jim}), that is displayed as @samp{RCS:jim:1.3}.

  On a graphical display, you can move the mouse over this mode line
indicator to pop up a ``tool-tip'', which displays a more verbose
description of the version control status.  Pressing @key{Mouse-1}
over the indicator pops up a menu of VC commands.  This menu is
identical to the @samp{Version Control} menu item, which can be found
in the @samp{Tools} menu on the menu bar.

@vindex auto-revert-check-vc-info
  When Auto Revert mode (@pxref{Reverting}) reverts a buffer that is
under version control, it updates the version control information in
the mode line.  However, Auto Revert mode may not properly update this
information if the version control status changes without changes to
the work file, from outside the current Emacs session.  If you set
@code{auto-revert-check-vc-info} to @code{t}, Auto Revert mode updates
the version control status information every
@code{auto-revert-interval} seconds, even if the work file itself is
unchanged.  The resulting CPU usage depends on the version control
system, but is usually not excessive.

@node Basic VC Editing
@subsection Basic Editing under Version Control

@cindex filesets
   Most VC commands operate on @dfn{VC filesets}.  A VC fileset is a
group of one or more files that are treated as a unit, for the
purposes of version control.

   If you are visiting a version-controlled file in the current
buffer, the VC fileset is simply that one file.  If you are visiting a
VC directory buffer, and some files in it are marked, the VC fileset
consists of the marked files (@pxref{VC Directory Mode}).

  The principal VC command is an all-purpose command, @kbd{C-x v v}
(@code{vc-next-action}), that performs either locking, merging or a
check-in on the current VC fileset, depending on the situation.  You
can call @kbd{C-x v v} from a version-controlled file, or from the VC
Directory buffer.

@table @kbd
@itemx C-x v v
Perform the next logical version control operation on the VC fileset.
@end table

@findex vc-next-action
@kindex C-x v v
  The precise action of @kbd{C-x v v} depends on the state of the VC
fileset, and whether the version control system uses locking or
merging.  This is described in detail in the subsequent sections.

  VC filesets are the way that VC mode bridges the gap between
file-based and changeset-based version control systems.  They are,
essentially, a way to pass multiple file arguments as a group to
version control commands.  For example, on Subversion, a checkin with
a multi-file VC fileset becomes a joint commit, as though you had
typed @command{svn commit} with those file arguments at the shell
command line.  All files in a VC fileset must be under the same
version control system; if they are not, Emacs signals an error when
you attempt to execute a command on the fileset.

  If you are accustomed to previous versions of VC, most of the
changes to VC in Emacs 23 are found in VC directory mode (@pxref{VC
Directory Mode}).  When multiple files are marked in the VC directory
buffer, they are treated as a VC fileset; typing @kbd{C-x v v} in the
VC directory buffer passes them to the version control backends as a
single unit.  Other commands in VC directory mode now act on the VC
fileset, rather than the file on the current line.  These changes
allow VC to interoperate correctly with changeset-based version
control systems.

  VC filesets are distinct from the ``named filesets'' used for
viewing and visiting files in functional groups (@pxref{Filesets}).
Unlike named filesets, VC filesets are not named and don't persist
across sessions.

@menu
* VC With A Merging VCS::  Without locking: default mode for CVS.
* VC With A Locking VCS::  RCS in its default mode, SCCS, and optionally CVS.
* Advanced C-x v v::       Advanced features available with a prefix argument.
* Log Buffer::             Features available in log entry buffers.
@end menu

@node VC With A Merging VCS
@subsubsection Basic Version Control with Merging

  When your version control system is merging-based (the default for
CVS and all newer version control systems), work files are always
writable; you need not do anything special to begin editing a file.
The status indicator on the mode line is @samp{-} if the file is
unmodified; it flips to @samp{:} as soon as you save any changes
(@pxref{VC Mode Line}).

  Here is what @kbd{C-x v v} does when using a merging-based system:

@itemize @bullet
@item
If the work file is the same as in the repository, it does nothing.

@item
If you have not changed the work file, but some other user has checked
in changes to the repository, @kbd{C-x v v} merges those changes into
the work file.

@item
If you have made modifications to the work file, @kbd{C-x v v}
attempts to check in your changes.  To do this, Emacs first reads the
log entry for the new revision (@pxref{Log Buffer}).  If some other
user has checked in changes to the repository since you last checked
it out, the checkin fails.  In that case, type @kbd{C-x v v} again to
merge those changes into your own work file; this puts the work file
into a ``conflicted'' state.  Type @kbd{C-x v v} to clear the
``conflicted'' state; VC then regards the file as up-to-date and
modified, and you can try to check it in again.

To pick up any recent changes from the repository @emph{without}
trying to commit your own changes, type @kbd{C-x v m @key{RET}}.
@xref{Merging}.
@end itemize

  These rules also apply when you use RCS in its ``non-locking'' mode,
except that changes will not be automatically merged from the
repository.  Nothing informs you if another user has checked in
changes in the same file since you began editing it; when you check in
your revision, his changes are removed (however, they remain in the
repository and are thus not irrevocably lost).  Therefore, you must
verify that the current revision is unchanged before checking in your
changes.  In addition, locking is possible with RCS even in this mode:
@kbd{C-x v v} with an unmodified file locks the file, just as it does
with RCS in its normal locking mode (@pxref{VC With A Locking VCS}).

@node VC With A Locking VCS
@subsubsection Basic Version Control with Locking

  Under a locking-based version control system (such as SCCS, and RCS
in its default mode), @kbd{C-x v v} does the following:

 @itemize @bullet
@item
If the file is not locked, @kbd{C-x v v} locks it, and makes it
writable so that you can change it.

@item
If the file is locked by you, and contains changes, @kbd{C-x v v}
checks in the changes.  In order to do this, it first reads the log
entry for the new revision.  @xref{Log Buffer}.

@item
If the file is locked by you, but you have not changed it since you
locked it, @kbd{C-x v v} releases the lock and makes the file
read-only again.

@item
If the file is locked by some other user, @kbd{C-x v v} asks you whether
you want to ``steal the lock'' from that user.  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.
@end itemize

  These rules also apply when you use CVS in locking mode, except
that there is no such thing as stealing a lock.

@node Advanced C-x v v
@subsubsection Advanced Control in @kbd{C-x v v}

@cindex revision ID to check in/out
  When you give a prefix argument to @code{vc-next-action} (@kbd{C-u
C-x v v}), it still performs the next logical version control
operation, but accepts additional arguments to specify precisely how
to do the operation.

@itemize @bullet
@item
If the file is modified (or locked), you can specify the revision ID
to use for the new version that you check in.  This is one way
to create a new branch (@pxref{Branches}).

@item
If the file is not modified (and unlocked), you can specify the
revision to select; this lets you start working from an older
revision, or on another branch.  If you do not enter any revision,
that takes you to the highest (``head'') revision on the current
branch; therefore @kbd{C-u C-x v v @key{RET}} is a convenient way to
get the latest version of a file from the repository.

@item
@cindex specific version control system
Instead of the revision ID, you can also specify the name of a
version control system.  This is useful when one file is being managed
with two version control systems at the same time
@iftex
(@pxref{Local Version Control,,,emacs-xtra, Specialized Emacs
Features}).
@end iftex
@ifnottex
(@pxref{Local Version Control}).
@end ifnottex

@end itemize

@node Log Buffer
@subsubsection Features of the Log Entry Buffer

  When you check in changes, Emacs pops up a buffer called
@samp{*VC-Log*} for you to enter a log entry.

  After you have finished editing the log message, type @kbd{C-c C-c}
to exit the buffer and commit the change.

@findex log-edit-show-files
@findex log-edit-show-diff
  In the @samp{*VC-Log*} buffer, typing @kbd{C-c C-f}
(@code{log-edit-show-files}) displays a list of files in the VC
fileset you are committing.  If you called @kbd{C-x v v} directly from
a work file, the VC fileset consists of that single file, so this
command is not very useful.  If you called @kbd{C-x v v} from a VC
directory buffer, the VC fileset may consist of multiple files
(@pxref{VC Directory Mode}).

@findex log-edit-insert-changelog
  Type @kbd{C-c C-d} (@code{log-edit-show-diff}) to show a ``diff'' of
the changes you have made (i.e., the differences between the work file
and the repository revision from which you started editing the file).
The diff is displayed in a special buffer in another window.
@xref{Comparing Files}.

  If you have written an entry in the @file{ChangeLog} (@pxref{Change
Log}), type @kbd{C-c C-a} (@code{log-edit-insert-changelog}) to pull
it into the @samp{*VC-Log*} buffer.  If the topmost item in the
@file{ChangeLog} was made under your user name on the current date,
this command searches that item for entries that match the file(s) to
be committed; if found, these entries are inserted.
@iftex
@xref{Change Logs and VC,,,emacs-xtra, Specialized Emacs Features},
@end iftex
@ifnottex
@xref{Change Logs and VC},
@end ifnottex
for the opposite way of working---generating ChangeLog entries from
the revision control log.

  To abort a check-in, just @strong{don't} type @kbd{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 the @samp{*VC-Log*} 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.  (This
is the normal way to do things on a changeset-oriented system, where
comments are attached to changesets rather than the history of
individual files.)  The most convenient way to do this is to mark all
the files in VC Directory Mode and check in from there; the log buffer
will carry the fileset information with it and do a group commit when
you type @kbd{C-c C-c}.

  You can also browse the history of previous log entries to duplicate
a checkin comment. This can be useful when you want several files to
have checkin comments that vary only slightly from each other. The
commands @kbd{M-n}, @kbd{M-p}, @kbd{M-s} and @kbd{M-r} for doing this
work just like the minibuffer history commands (except that these
versions are used outside the minibuffer).

@vindex vc-log-mode-hook
  Each time you check in a change, the log entry buffer is put into VC
Log Edit mode, which involves running two hooks: @code{text-mode-hook}
and @code{vc-log-mode-hook}.  @xref{Hooks}.

@node Old Revisions
@subsection Examining And Comparing Old Revisions

  One of the convenient features of version control is the ability
to examine any revision of a file, or compare two revisions.

@table @kbd
@item C-x v ~ @var{revision} @key{RET}
Examine revision @var{revision} of the visited file, in a buffer of its
own.

@item C-x v =
Compare the buffer contents associated with the current
fileset with the working revision(s) from which you started editing.

@item C-u C-x v = @key{RET} @var{oldvers} @key{RET} @var{newvers} @key{RET}
Compare the specified two repository revisions of the current fileset.

@item C-x v g
Display an annotated version of the file: for each line, show the
latest revision in which it was modified.
@end table

@findex vc-revision-other-window
@kindex C-x v ~
  To examine an old revision, visit the work file and type @kbd{C-x v
~ @var{revision} @key{RET}} (@code{vc-revision-other-window}).  Here,
@var{revision} is either the desired revision ID (@pxref{VCS
Concepts}), or the name of a tag or branch
@iftex
(@pxref{Tags,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Tags}).
@end ifnottex
This command puts the text of the old revision in a file named
@file{@var{filename}.~@var{revision}~}, and visits it in its own
buffer in a separate window.

@findex vc-diff
@kindex C-x v =
  @kbd{C-x v =} (@code{vc-diff}) compares the current buffer contents
of each file in the current VC fileset (saving them if necessary) with
the repository revision from which you started editing.  Note that the
latter may or may not be the latest revision of the file(s).  The diff
is displayed in a special buffer in another window.  @xref{Comparing
Files}.

@findex vc-diff
@kindex C-u C-x v =
  To compare two arbitrary revisions of the current VC fileset, call
@code{vc-diff} with a prefix argument: @kbd{C-u C-x v =}.  This
prompts for two revision IDs, using the minibuffer, and displays the
diff in a special buffer in another window.  Instead of providing a
revision ID, you can give an empty input, which specifies the current
contents of the work file; or a tag or branch name
@iftex
(@pxref{Tags,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Tags}).
@end ifnottex
If your version control system is file-based (e.g. CVS) rather than
changeset-based (Subversion, GNU Arch, git, Mercurial), supplying a
revision ID for a multi-file fileset (as opposed to a symbolic tag
name) is unlikely to return diffs that are connected in any meaningful
way.

  If you invoke @kbd{C-x v =} or @kbd{C-u C-x v =} from a buffer that
is neither visiting a version-controlled file nor a VC directory
buffer, these commands generate a diff of all registered files in the
current directory and its subdirectories.

@vindex vc-diff-switches
@vindex vc-rcs-diff-switches
  @kbd{C-x v =} works by running a variant of the @code{diff} utility
designed to work with the version control system in use.  When you
invoke @code{diff} this way, in addition to the options specified by
@code{diff-switches} (@pxref{Comparing Files}), it receives those
specified by @code{vc-diff-switches}, plus those specified for the
specific back end by @code{vc-@var{backend}-diff-switches}.  For
instance, when the version control back end is CVS, @code{diff} uses
the options in @code{vc-cvs-diff-switches}.  The
@samp{vc@dots{}diff-switches} variables are @code{nil} by default.

  The buffer produced by @kbd{C-x v =} supports the commands of
Compilation mode (@pxref{Compilation Mode}), such as @kbd{C-x `} and
@kbd{C-c C-c}, in both the ``old'' and ``new'' text, and they always
find the corresponding locations in the current work file.  (Older
revisions are not, in general, present as files on your disk.)

@findex vc-annotate
@kindex C-x v g
  For some back ends, you can display the file @dfn{annotated} with
per-line revision information, by typing @kbd{C-x v g}
(@code{vc-annotate}).  This creates a new buffer (the ``annotate
buffer'') displaying the file's text, with each part colored to show
how old it is.  Text colored red is new, blue means old, and
intermediate colors indicate intermediate ages.  By default, the color
is scaled over the full range of ages, such that the oldest changes
are blue, and the newest changes are red.

  When you give a prefix argument to this command, Emacs reads two
arguments using the minibuffer: the ID of which revision to display and
annotate (instead of the current file contents), and the time span in
days the color range should cover.  

  From the annotate buffer, these and other color scaling options are
available from the @samp{VC-Annotate} menu.  In this buffer, you can
also use the following keys to browse the annotations of past revisions,
view diffs, or view log entries:

@table @kbd
@item p
Annotate the previous revision, that is to say, the revision before
the one currently annotated.  A numeric prefix argument is a repeat
count, so @kbd{C-u 10 p} would take you back 10 revisions.

@item n
Annotate the next revision---the one after the revision currently
annotated.  A numeric prefix argument is a repeat count.

@item j
Annotate the revision indicated by the current line.

@item a
Annotate the revision before the one indicated by the current line.
This is useful to see the state the file was in before the change on
the current line was made.

@item f
Show in a buffer the file revision indicated by the current line.

@item d
Display the diff between the current line's revision and the previous
revision.  This is useful to see what the current line's revision
actually changed in the file.

@item D
Display the diff between the current line's revision and the previous
revision for all files in the changeset (for VC systems that support
changesets).  This is useful to see what the current line's revision
actually changed in the tree.

@item l
Show the log of the current line's revision.  This is useful to see
the author's description of the changes in the revision on the current
line.

@item w
Annotate the working revision--the one you are editing.  If you used
@kbd{p} and @kbd{n} to browse to other revisions, use this key to
return to your working revision.

@item v
Toggle the annotation visibility.  This is useful for looking just at
the file contents without distraction from the annotations.
@end table

@node Secondary VC Commands
@subsection The Secondary Commands of VC

  This section explains the secondary commands of VC.

@menu
* Registering::         Putting a file under version control.
* VC Status::           Viewing the VC status of files.
* VC Undo::             Canceling changes before or after check-in.
@end menu

@node Registering
@subsubsection Registering a File for Version Control

@kindex C-x v i
@findex vc-register
  You can put any file under version control by simply visiting it, and
then typing @w{@kbd{C-x v i}} (@code{vc-register}).

@table @kbd
@item C-x v i
Register the visited file for version control.
@end table

  To register the file, Emacs must choose which version control system
to use for it.  If the file's directory already contains files
registered in a version control system, Emacs uses that system.  If
there is more than one system in use for a directory, Emacs uses the
one that appears first in @code{vc-handled-backends}
@iftex
(@pxref{Customizing VC,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Customizing VC}).
@end ifnottex
On the other hand, if there are no files already registered, Emacs uses
the first system from @code{vc-handled-backends} that could register
the file (for example, you cannot register a file under CVS if its
directory is not already part of a CVS tree); with the default value
of @code{vc-handled-backends}, this means that Emacs uses RCS in this
situation.

  If locking is in use, @kbd{C-x v i} leaves the file unlocked and
read-only.  Type @kbd{C-x v v} if you wish to start editing it.  After
registering a file with CVS, you must subsequently commit the initial
revision by typing @kbd{C-x v v}.  Until you do that, the revision ID
appears as @samp{@@@@} in the mode line.

@vindex vc-default-init-revision
@cindex initial revision ID to register
  The default initial revision ID for a newly registered file
varies by what VCS you are using; normally it will be 1.1 on VCSes
that use dot-pair revision IDs and 1 on VCSes that use monotonic IDs.
You can specify a different default by setting the variable
@code{vc-default-init-revision}, or you can give @kbd{C-x v i} a
numeric argument; then it reads the initial revision ID for this
particular file using the minibuffer.

@vindex vc-initial-comment
  If @code{vc-initial-comment} is non-@code{nil}, @kbd{C-x v i} reads an
initial comment to describe the purpose of this source file.  Reading
the initial comment works like reading a log entry (@pxref{Log Buffer}).

@node VC Status
@subsubsection VC Status Commands

@table @kbd
@item C-x v l
Display revision control state and change history.
@end table

@kindex C-x v l
@findex vc-print-log
  To view the detailed revision control status and history of a file,
type @kbd{C-x v l} (@code{vc-print-log}).  This pops up a special
buffer named @samp{*vc-change-log*}, in a new window, that displays
the history of changes to the current file, including the text of the
log entries.  The point is centered at the revision of the file that
is currently being visited.

  In the @samp{*vc-change-log*} buffer, you can use the following keys
to move between the logs of revisions and of files, to view past
revisions, to modify change comments, to view annotations and to view
diffs:

@table @kbd
@item p
Move to the previous revision-item in the buffer.  (Revision entries in the log
buffer are usually in reverse-chronological order, so the previous
revision-item usually corresponds to a newer revision.)  A numeric
prefix argument is a repeat count.

@item n
Move to the next revision-item (which most often corresponds to the
previous revision of the file).  A numeric prefix argument is a repeat
count.

@item P
Move to the log of the previous file, when the logs of multiple files
are in the log buffer (@pxref{VC Directory Mode}).  Otherwise, just
move to the beginning of the log.  A numeric prefix argument is a
repeat count, so @kbd{C-u 10 P} would move backward 10 files.

@item N
Move to the log of the next file, when the logs of multiple files are
in the log buffer (@pxref{VC Directory Mode}).  It also takes a
numeric prefix argument as a repeat count.

@item a
Annotate the revision indicated by the current line.

@item e
Modify the change comment displayed at point.  Note that not all VC
systems support modifying change comments.

@item f
Visit the revision indicated at the current line, like typing @kbd{C-x
v ~} and specifying this revision's ID (@pxref{Old Revisions}).

@item d
Display the diff (@pxref{Comparing Files}) between the revision
indicated at the current line and the next earlier revision.  This is
useful to see what actually changed in the file when the revision
indicated on the current line was committed.

@item D
Display the changeset diff (@pxref{Comparing Files}) between the
revision indicated at the current line and the next earlier revision.
This is useful to see all the changes to all files that the revision
indicated on the current line did when it was committed.
@end table

@node VC Undo
@subsubsection Undoing Version Control Actions

@table @kbd
@item C-x v u
Revert the buffer and the file to the working revision from which you started
editing the file.

@item C-x v c
Remove the last-entered change from the master for the visited file.
This undoes your last check-in.
@end table

@kindex C-x v u
@findex vc-revert-buffer
  If you want to discard your current set of changes and revert to the
working revision from which you started editing the file, use @kbd{C-x
v u} (@code{vc-revert-buffer}).  If the version control system is
locking-based, this leaves the file unlocked, and you must lock it
again before making new changes.  @kbd{C-x v u} requires confirmation,
unless it sees that you haven't made any changes with respect to the
master copy of the working revision.

  @kbd{C-x v u} is also the command to unlock a file if you lock it and
then decide not to change it.

@kindex C-x v c
@findex vc-rollback
  To cancel a change that you already checked in, use @kbd{C-x v c}
(@code{vc-rollback}).  This command discards all record of the most
recent checked-in revision, but only if your work file corresponds to
that revision---you cannot use @kbd{C-x v c} to cancel a revision that
is not the latest on its branch.  Note that many version control
systems do not support rollback at all; this command is something of a
historical relic.

@node VC Directory Mode
@subsection VC Directory Mode

@kindex C-x v d
@findex vc-dir
  When you are working on a large program, it is often useful to find
out which files have changed within an entire directory tree, or to
view the status of all files under version control at once, and to
perform version control operations on collections of files.  You can
use the command @kbd{C-x v d} (@code{vc-dir}) to make a directory
listing that includes only files relevant for version control.  This
creates a @dfn{VC Directory buffer} and displays it in a separate
window.

@cindex PCL-CVS
@pindex cvs
@cindex CVS directory mode
  The VC Directory buffer described here works with all the version
control systems that VC supports.  Another more powerful facility,
designed specifically for CVS, is called PCL-CVS.  @xref{Top, , About
PCL-CVS, pcl-cvs, PCL-CVS --- The Emacs Front-End to CVS}.

  The VC Directory buffer contains a list of version-controlled files
in the current directory and its subdirectories.  Files which are
up-to-date (have no local differences from the repository copy) are
omitted; if all files in a directory are up-to-date, the directory is
omitted as well.  (However, the directory in which @code{vc-dir} was
run will always be shown as @file{./}.)  There is an exception to this
rule: if VC mode detects that a file has changed to an up-to-date
state since you last looked at it, that file and its state are shown.

  If a directory uses more that one version control system, you can
select which system to use for the @code{vc-dir} command by invoking
@code{vc-dir} with a prefix argument: @kbd{C-u C-x v d}.

  The line for an individual file shows the version control state of
the file.  Under RCS and SCCS, the name of the user locking the file
is shown; under CVS, an abbreviated version of the @samp{cvs status}
output is used.  Here is an example using CVS:

@smallexample
@group
                       ./
    modified           file1.c
    needs-update       file2.c
    needs-merge        file3.c
@end group
@end smallexample

@noindent
In this example, @samp{file1.c} is modified with respect to the
repository, and @samp{file2.c} is not.  @samp{file3.c} is modified,
but other changes have also been checked in to the repository---you
need to merge them with the work file before you can check it in.

@vindex vc-stay-local
@vindex vc-cvs-stay-local
  In the above, if the repository were on a remote machine, VC only
contacts it when the variable @code{vc-stay-local} (or
@code{vc-cvs-stay-local}) is nil (@pxref{CVS Options}).  This is
because access to the repository may be slow, or you may be working
offline and not have access to the repository at all.  As a
consequence, VC would not be able to tell you that @samp{file3.c} is
in the ``merge'' state; you would learn that only when you try to
check-in your modified copy of the file, or use a command such as
@kbd{C-x v m}.

  In practice, this is not a problem because CVS handles this case
consistently whenever it arises.  In VC, you'll simply get prompted to
merge the remote changes into your work file first.  The benefits of
less network communication usually outweigh the disadvantage of not
seeing remote changes immediately.

@vindex vc-directory-exclusion-list
  When a VC directory displays subdirectories it omits some that
should never contain any files under version control.  By default,
this includes Version Control subdirectories such as @samp{RCS} and
@samp{CVS}; you can customize this by setting the variable
@code{vc-directory-exclusion-list}.

@menu
* VC Directory Commands:: Commands to use in a VC directory buffer.
@end menu

@node VC Directory Commands
@subsubsection VC Directory Commands

  VC Directory mode has a full set of navigation and marking commands
for picking out filesets.  Some of these are also available in a
context menu invoked by the @kbd{mouse-2} button.

  Up- and down-arrow keys move in the buffer; @kbd{n} and @kbd{p}  also
move vertically as in other list-browsing modes.  @key{SPC} and
@key{TAB} behave like down-arrow, and @key{BackTab} behaves like
up-arrow.

  Both @kbd{C-m} and @kbd{f} visit the file on the current
line.  @kbd{o} visits that file in another window.  @kbd{q} dismisses
the directory buffer.

  @kbd{x} toggles hiding of up-to-date files.
  
  @kbd{m} marks the file or directory on the current line.  If the
region is active, @kbd{m} marks all the files in the region.  There
are some restrictions when marking: a file cannot be marked if any of
its parent directories are marked, and a directory cannot be marked if
any files in it or in its child directories are marked.

  @kbd{M} marks all the files with the same VC state as the current
file if the cursor is on a file.  If the cursor is on a directory, it
marks all child files.  With a prefix argument: marks all files and
directories.

  @kbd{u} unmarks the file or directory on the current line.  If the
region is active, it unmarks all the files in the region.

  @kbd{U} marks all the files with the same VC state as the current file
if the cursor is on a file.  If the cursor is on a directory, it
unmarks all child files.  With a prefix argument: unmarks all marked
files and directories.

  It is possible to do search, search and replace, incremental search,
and incremental regexp search on multiple files.  These commands will
work on all the marked files or the current file if nothing is marked.
If a directory is marked, the files in that directory shown in the VC
directory buffer will be used.

  @kbd{S} searches the marked files.

  @kbd{Q} does a query replace on the marked files.

  @kbd{M-s a C-s} does an incremental search on the marked files.

  @kbd{M-s a C-M-s} does an incremental search on the marked files.

  Commands are also accessible from the VC-dir menu.  Note that some VC
backends use the VC-dir menu to make available extra backend specific
commands.

  Normal VC commands with the @kbd{C-x v} prefix work in VC directory
buffers.  Some single-key shortcuts are available as well; @kbd{=},
@kbd{+}, @kbd{l}, @kbd{i}, and @kbd{v} behave as through prefixed with
@kbd{C-x v}.

  The command @kbd{C-x v v} (@code{vc-next-action}) operates on all the
marked files, so that you can check in several files at once.
If the underlying VC supports atomic commits of multiple-file
changesets, @kbd{C-x v v} with a selected set of modified but not
committed files will commit all of them at once as a single changeset.

  When @kbd{C-x v v} (@code{vc-next-action}) operates on a set of files,
it requires that all of those files must be either in the same state or
in compatible states; otherwise it will throw an error (added,
modified and removed states are considered compatible).  Note that this
differs from the behavior of older versions of VC, which did not have
fileset operations and simply did @code{vc-next-action} on each file
individually.

  If any files are in a state that calls for commit, @kbd{C-x v v} reads a
single log entry and uses it for the changeset as a whole.  If the
underling VCS is file- rather than changeset-oriented, the log entry
will be replicated into the history of each file.

@node Branches
@subsection Multiple Branches of a File
@cindex branch (version control)
@cindex trunk (version control)

  One use of version control is to maintain multiple ``current''
revisions of a file.  For example, you might have different revisions of a
program in which you are gradually adding various unfinished new
features.  Each such independent line of development is called a
@dfn{branch}.  VC allows you to create branches, switch between
different branches, and merge changes from one branch to another.
Please note, however, that branches are not supported for SCCS.

  A file's main line of development is usually called the @dfn{trunk}.
You can create multiple branches from the trunk.  How the difference
between trunk and branch is made visible is dependent on whether the
VCS uses dot-pair or monotonic version IDs.

  In VCSes with dot-pair revision IDs, the revisions on the trunk are
normally IDed 1.1, 1.2, 1.3, etc.  At any such revision, you can
start an independent branch.  A branch starting at revision 1.2 would
have revision ID 1.2.1.1, and consecutive revisions on this branch
would have IDs 1.2.1.2, 1.2.1.3, 1.2.1.4, and so on.  If there is
a second branch also starting at revision 1.2, it would consist of
revisions 1.2.2.1, 1.2.2.2, 1.2.2.3, etc.

   In VCSes with monotonic revision IDs, trunk revisions are IDed as
1, 2, 3, etc.  A branch from (say) revision 2 might start with 2.1 and
continue through 2.2, 2.3, etc.  But naming conventions for branches
and subbranches vary widely on these systems, and some (like
Mercurial) never depart from the monotonic integer sequence at all.
Consult the documentation of the VCS you are using.

@cindex head revision
  If you omit the final component of a dot-pair revision ID, that is called a
@dfn{branch ID}.  It refers to the highest existing revision on that
branch---the @dfn{head revision} of that branch.  The branches in the
dot-pair example above have branch IDs 1.2.1 and 1.2.2.

@menu
* Switching Branches::    How to get to another existing branch.
* Creating Branches::     How to start a new branch.
* Merging::               Transferring changes between branches.
* Multi-User Branching::  Multiple users working at multiple branches
                            in parallel.
@end menu

@node Switching Branches
@subsubsection Switching between Branches

  To switch between branches, type @kbd{C-u C-x v v} and specify the
revision ID you want to select.  On a locking-based system, this
version is then visited @emph{unlocked} (write-protected), so you can
examine it before locking it.  Switching branches in this way is allowed
only when the file is not locked.

  On a VCS with dot-pair IDs, you can omit the minor part, thus giving
only the branch ID; this takes you to the head version on the
chosen branch.  If you only type @key{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.

@node Creating Branches
@subsubsection Creating New Branches

  To create a new branch from a head revision (one that is the latest in
the branch that contains it), first select that revision if necessary,
lock it with @kbd{C-x v v}, and make whatever changes you want.  Then,
when you check in the changes, use @kbd{C-u C-x v v}.  This lets you
specify the revision ID for the new revision.  You should specify a
suitable branch ID for a branch starting at the current revision.
For example, if the current revision is 2.5, the branch ID 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 revision (one that is no longer the
head of a branch), first select that revision (@pxref{Switching
Branches}).  Your procedure will then differ depending on whether you
are using a locking or merging-based VCS.

  On a locking VCS, you will need to lock the old revision branch with
@kbd{C-x v v}.  You'll be asked to confirm, when you lock the old
revision, that you really mean to create a new branch---if you say no,
you'll be offered a chance to lock the latest revision instead.  On
a merging-based VCS you will skip this step.

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

  After the branch is created, you ``stay'' on it.  That means that
subsequent check-ins create new revisions on that branch.  To leave the
branch, you must explicitly select a different revision with @kbd{C-u C-x
v v}.  To transfer changes from one branch to another, use the merge
command, described in the next section.

@node Merging
@subsubsection Merging Branches

@cindex merging changes
  When you have finished the changes on a certain branch, you will
often want to incorporate them into the file's main line of development
(the trunk).  This is not a trivial operation, because development might
also have proceeded on the trunk, so that you must @dfn{merge} the
changes into a file that has already been changed otherwise.  VC allows
you to do this (and other things) with the @code{vc-merge} command.

@table @kbd
@item C-x v m (vc-merge)
Merge changes into the work file.
@end table

@kindex C-x v m
@findex vc-merge
  @kbd{C-x v m} (@code{vc-merge}) takes a set of changes and merges it
into the current version of the work file.  It firsts asks you in the
minibuffer where the changes should come from.  If you just type
@key{RET}, Emacs merges any changes that were made on the same branch
since you checked the file out (we call this @dfn{merging the news}).
This is the common way to pick up recent changes from the repository,
regardless of whether you have already changed the file yourself.

  You can also enter a branch ID or a pair of revision IDs in
the minibuffer.  Then @kbd{C-x v m} finds the changes from that
branch, or the differences between the two revisions you specified, and
merges them into the current revision of the current file.

  As an example, suppose that you have finished a certain feature on
branch 1.3.1.  In the meantime, development on the trunk has proceeded
to revision 1.5.  To merge the changes from the branch to the trunk,
first go to the head revision of the trunk, by typing @kbd{C-u C-x v v
@key{RET}}.  Revision 1.5 is now current.  If locking is used for the file,
type @kbd{C-x v v} to lock revision 1.5 so that you can change it.  Next,
type @kbd{C-x v m 1.3.1 @key{RET}}.  This takes the entire set of changes on
branch 1.3.1 (relative to revision 1.3, where the branch started, up to
the last revision on the branch) and merges it into the current revision
of the work file.  You can now check in the changed file, thus creating
revision 1.6 containing the changes from the branch.

  It is possible to do further editing after merging the branch, before
the next check-in.  But it is usually wiser to check in the merged
revision, then lock it and make the further changes.  This will keep
a better record of the history of changes.

@cindex conflicts
@cindex resolving conflicts
  When you merge changes into a file that has itself been modified, the
changes might overlap.  We call this situation a @dfn{conflict}, and
reconciling the conflicting changes is called @dfn{resolving a
conflict}.

  Whenever conflicts occur during merging, VC detects them, tells you
about them in the echo area, and asks whether you want help in merging.
If you say yes, it starts an Ediff session (@pxref{Top,
Ediff, Ediff, ediff, The Ediff Manual}).

  If you say no, the conflicting changes are both inserted into the
file, surrounded by @dfn{conflict markers}.  The example below shows how
a conflict region looks; the file is called @samp{name} and the current
master file revision with user B's changes in it is 1.11.

@c @w here is so CVS won't think this is a conflict.
@smallexample
@group
@w{<}<<<<<< name
  @var{User A's version}
=======
  @var{User B's version}
@w{>}>>>>>> 1.11
@end group
@end smallexample

@cindex vc-resolve-conflicts
  Then you can resolve the conflicts by editing the file manually.  Or
you can type @code{M-x vc-resolve-conflicts} after visiting the file.
This starts an Ediff session, as described above.  Don't forget to
check in the merged version afterwards.

@node Multi-User Branching
@subsubsection Multi-User Branching

  It is often useful for multiple developers to work simultaneously on
different branches of a file.  CVS and later systems allow this by
default; for RCS, it is possible if you create multiple source
directories.  Each source directory should have a link named
@file{RCS} which points to a common directory of RCS master files.
Then each source directory can have its own choice of selected
revisions, but all share the same common RCS records.

  This technique works reliably and automatically, provided that the
source files contain RCS version headers
@iftex
(@pxref{Version Headers,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Version Headers}).
@end ifnottex
The headers enable Emacs to be sure, at all times, which revision
ID is present in the work file.

  If the files do not have version headers, you must instead tell Emacs
explicitly in each session which branch you are working on.  To do this,
first find the file, then type @kbd{C-u C-x v v} and specify the correct
branch ID.  This ensures that Emacs knows which branch it is using
during this particular editing session.

@ifnottex
@include vc1-xtra.texi
@end ifnottex

@node Directories
@section File Directories

@cindex file directory
@cindex directory listing
  The file system groups files into @dfn{directories}.  A @dfn{directory
listing} is a list of all the files in a directory.  Emacs provides
commands to create and delete directories, and to make directory
listings in brief format (file names only) and verbose format (sizes,
dates, and authors included).  Emacs also includes a directory browser
feature called Dired; see @ref{Dired}.

@table @kbd
@item C-x C-d @var{dir-or-pattern} @key{RET}
Display a brief directory listing (@code{list-directory}).
@item C-u C-x C-d @var{dir-or-pattern} @key{RET}
Display a verbose directory listing.
@item M-x make-directory @key{RET} @var{dirname} @key{RET}
Create a new directory named @var{dirname}.
@item M-x delete-directory @key{RET} @var{dirname} @key{RET}
Delete the directory named @var{dirname}.  It must be empty,
or you get an error.
@end table

@findex list-directory
@kindex C-x C-d
  The command to display a directory listing is @kbd{C-x C-d}
(@code{list-directory}).  It reads using the minibuffer a file name
which is either a directory to be listed or a wildcard-containing
pattern for the files to be listed.  For example,

@example
C-x C-d /u2/emacs/etc @key{RET}
@end example

@noindent
lists all the files in directory @file{/u2/emacs/etc}.  Here is an
example of specifying a file name pattern:

@example
C-x C-d /u2/emacs/src/*.c @key{RET}
@end example

  Normally, @kbd{C-x C-d} displays a brief directory listing containing
just file names.  A numeric argument (regardless of value) tells it to
make a verbose listing including sizes, dates, and owners (like
@samp{ls -l}).

@vindex list-directory-brief-switches
@vindex list-directory-verbose-switches
  The text of a directory listing is mostly obtained by running
@code{ls} in an inferior process.  Two Emacs variables control the
switches passed to @code{ls}: @code{list-directory-brief-switches} is
a string giving the switches to use in brief listings (@code{"-CF"} by
default), and @code{list-directory-verbose-switches} is a string
giving the switches to use in a verbose listing (@code{"-l"} by
default).

@vindex directory-free-space-program
@vindex directory-free-space-args
  In verbose directory listings, Emacs adds information about the
amount of free space on the disk that contains the directory.  To do
this, it runs the program specified by
@code{directory-free-space-program} with arguments
@code{directory-free-space-args}.

@node Comparing Files
@section Comparing Files
@cindex comparing files

@findex diff
@vindex diff-switches
  The command @kbd{M-x diff} prompts for two file names, using the
minibuffer, and displays the differences between the two files in a
buffer named @samp{*diff*}.  This works by running the @command{diff}
program, using options taken from the variable @code{diff-switches}.
The value of @code{diff-switches} should be a string; the default is
@code{"-c"} to specify a context diff.  @xref{Top,, Diff, diff,
Comparing and Merging Files}, for more information about
@command{diff} output formats.

  The output of the @code{diff} command is shown using a major mode
called Diff mode.  @xref{Diff Mode}.

@findex diff-backup
  The command @kbd{M-x diff-backup} compares a specified file with its
most recent backup.  If you specify the name of a backup file,
@code{diff-backup} compares it with the source file that it is a
backup of.  In all other respects, this behaves like @kbd{M-x diff}.

@findex compare-windows
  The command @kbd{M-x compare-windows} compares the text in the
current window with that in the next window.  (For more information
about windows in Emacs, @ref{Windows}.)  Comparison starts at point in
each window, after pushing each initial point value on the mark ring
in its respective buffer.  Then it moves point forward in each window,
one character at a time, until it reaches characters that don't match.
Then the command exits.

  If point in the two windows is followed by non-matching text when
the command starts, @kbd{M-x compare-windows} tries heuristically to
advance up to matching text in the two windows, and then exits.  So if
you use @kbd{M-x compare-windows} repeatedly, each time it either
skips one matching range or finds the start of another.

@vindex compare-ignore-case
@vindex compare-ignore-whitespace
  With a numeric argument, @code{compare-windows} ignores changes in
whitespace.  If the variable @code{compare-ignore-case} is
non-@code{nil}, the comparison ignores differences in case as well.
If the variable @code{compare-ignore-whitespace} is non-@code{nil},
@code{compare-windows} normally ignores changes in whitespace, and a
prefix argument turns that off.

@cindex Smerge mode
@findex smerge-mode
@cindex failed merges
@cindex merges, failed
@cindex comparing 3 files (@code{diff3})
  You can use @kbd{M-x smerge-mode} to turn on Smerge mode, a minor
mode for editing output from the @command{diff3} program.  This is
typically the result of a failed merge from a version control system
``update'' outside VC, due to conflicting changes to a file.  Smerge
mode provides commands to resolve conflicts by selecting specific
changes.

@iftex
@xref{Emerge,,, emacs-xtra, Specialized Emacs Features},
@end iftex
@ifnottex
@xref{Emerge},
@end ifnottex
for the Emerge facility, which provides a powerful interface for
merging files.

@node Diff Mode
@section Diff Mode
@cindex Diff mode
@findex diff-mode
@cindex patches, editing

  Diff mode is a major mode used for the output of @kbd{M-x diff} and
other similar commands, as well as the output of the @command{diff}
program.  This kind of output is called a @dfn{patch}, because it can
be passed to the @command{patch} command to automatically apply the
specified changes.  To select Diff mode manually, type @kbd{M-x
diff-mode}.

@cindex hunk, diff
  The changes specified in a patch are grouped into @dfn{hunks}, which
are contiguous chunks of text that contain one or more changed lines.
Hunks can also include unchanged lines to provide context for the
changes.  Each hunk is preceded by a @dfn{hunk header}, which
specifies the old and new line numbers at which the hunk occurs.  Diff
mode highlights each hunk header, to distinguish it from the actual
contents of the hunk.

@vindex diff-update-on-the-fly
  You can edit a Diff mode buffer like any other buffer.  (If it is
read-only, you need to make it writable first.  @xref{Misc Buffer}.)
Whenever you change a hunk, Diff mode attempts to automatically
correct the line numbers in the hunk headers, to ensure that the diff
remains ``correct''.  To disable automatic line number correction,
change the variable @code{diff-update-on-the-fly} to @code{nil}.

  Diff mode treats each hunk as an ``error message,'' similar to
Compilation mode.  Thus, you can use commands such as @kbd{C-x '} to
visit the corresponding source locations.  @xref{Compilation Mode}.

  In addition, Diff mode provides the following commands to navigate,
manipulate and apply parts of patches:

@table @kbd
@item M-n
@findex diff-hunk-next
Move to the next hunk-start (@code{diff-hunk-next}).

@item M-p
@findex diff-hunk-prev
Move to the previous hunk-start (@code{diff-hunk-prev}).

@item M-@}
@findex diff-file-next
Move to the next file-start, in a multi-file patch
(@code{diff-file-next}).

@item M-@{
@findex diff-file-prev
Move to the previous file-start, in a multi-file patch
(@code{diff-file-prev}).

@item M-k
@findex diff-hunk-kill
Kill the hunk at point (@code{diff-hunk-kill}).

@item M-K
@findex diff-file-kill
In a multi-file patch, kill the current file part.
(@code{diff-file-kill}).

@item C-c C-a
@findex diff-apply-hunk
Apply this hunk to its target file (@code{diff-apply-hunk}).  With a
prefix argument of @kbd{C-u}, revert this hunk.

@item C-c C-b
@findex diff-refine-hunk
Highlight the changes of the hunk at point with a finer granularity
(@code{diff-refine-hunk}).  This allows you to see exactly which parts
of each changed line were actually changed.

@item C-c C-c
@findex diff-goto-source
Go to the source file and line corresponding to this hunk
(@code{diff-goto-source}).

@item C-c C-e
@findex diff-ediff-patch
Start an Ediff session with the patch (@code{diff-ediff-patch}).
@xref{Top, Ediff, Ediff, ediff, The Ediff Manual}.

@item C-c C-n
@findex diff-restrict-view
Restrict the view to the current hunk (@code{diff-restrict-view}).
@xref{Narrowing}.  With a prefix argument of @kbd{C-u}, restrict the
view to the current file of a multiple-file patch.  To widen again,
use @kbd{C-x n w} (@code{widen}).

@item C-c C-r
@findex diff-reverse-direction
Reverse the direction of comparison for the entire buffer
(@code{diff-reverse-direction}).

@item C-c C-s
@findex diff-split-hunk
Split the hunk at point (@code{diff-split-hunk}).  This is for
manually editing patches, and only works with the @dfn{unified diff
format} produced by the @option{-u} or @option{--unified} options to
the @command{diff} program.  If you need to split a hunk in the
@dfn{context diff format} produced by the @option{-c} or
@option{--context} options to @command{diff}, first convert the buffer
to the unified diff format with @kbd{C-c C-u}.

@item C-c C-d
@findex diff-unified->context
Convert the entire buffer to the @dfn{context diff format}
(@code{diff-unified->context}).  With a prefix argument, convert only
the text within the region.

@item C-c C-u
@findex diff-context->unified
Convert the entire buffer to unified diff format
(@code{diff-context->unified}).  With a prefix argument, convert
unified format to context format.  When the mark is active, convert
only the text within the region.

@item C-c C-w
@findex diff-refine-hunk
Refine the current hunk so that it disregards changes in whitespace
(@code{diff-refine-hunk}).

@item C-x 4 A
@findex diff-add-change-log-entries-other-window
@findex add-change-log-entry-other-window@r{, in Diff mode}
Generate a ChangeLog entry, like @kbd{C-x 4 a} does (@pxref{Change
Log}), for each one of the hunks
(@code{diff-add-change-log-entries-other-window}).  This creates a
skeleton of the log of changes that you can later fill with the actual
descriptions of the changes.  @kbd{C-x 4 a} itself in Diff mode
operates on behalf of the current hunk's file, but gets the function
name from the patch itself.  This is useful for making log entries for
functions that are deleted by the patch.

@item M-x diff-show-trailing-whitespaces RET
@findex diff-show-trailing-whitespaces
Highlight syntactically problematic trailing whitespace characters
(usually errors introduced when editing a patch).
@end table


@node Misc File Ops
@section Miscellaneous File Operations

  Emacs has commands for performing many other operations on files.
All operate on one file; they do not accept wildcard file names.

@findex view-file
@cindex viewing
@cindex View mode
@cindex mode, View
  @kbd{M-x view-file} allows you to scan or read a file by sequential
screenfuls.  It reads a file name argument using the minibuffer.  After
reading the file into an Emacs buffer, @code{view-file} displays the
beginning.  You can then type @key{SPC} to scroll forward one windowful,
or @key{DEL} to scroll backward.  Various other commands are provided
for moving around in the file, but none for changing it; type @kbd{?}
while viewing for a list of them.  They are mostly the same as normal
Emacs cursor motion commands.  To exit from viewing, type @kbd{q}.
The commands for viewing are defined by a special minor mode called View
mode.

  A related command, @kbd{M-x view-buffer}, views a buffer already present
in Emacs.  @xref{Misc Buffer}.

@kindex C-x i
@findex insert-file
  @kbd{M-x insert-file} (also @kbd{C-x i}) inserts a copy of the
contents of the specified file into the current buffer at point,
leaving point unchanged before the contents.  The position after the
inserted contents is added to the mark ring, without activating the
mark (@pxref{Mark Ring}).

@findex insert-file-literally
  @kbd{M-x insert-file-literally} is like @kbd{M-x insert-file},
except the file is inserted ``literally'': it is treated as a sequence
of @acronym{ASCII} characters with no special encoding or conversion,
similar to the @kbd{M-x find-file-literally} command
(@pxref{Visiting}).

@findex write-region
  @kbd{M-x write-region} is the inverse of @kbd{M-x insert-file}; it
copies the contents of the region into the specified file.  @kbd{M-x
append-to-file} adds the text of the region to the end of the
specified file.  @xref{Accumulating Text}.  The variable
@code{write-region-inhibit-fsync} applies to these commands, as well
as saving files; see @ref{Customize Save}.

@findex delete-file
@cindex deletion (of files)
  @kbd{M-x delete-file} deletes the specified file, like the @code{rm}
command in the shell.  If you are deleting many files in one directory, it
may be more convenient to use Dired (@pxref{Dired}).

@findex rename-file
  @kbd{M-x rename-file} reads two file names @var{old} and @var{new} using
the minibuffer, then renames file @var{old} as @var{new}.  If the file name
@var{new} already exists, you must confirm with @kbd{yes} or renaming is not
done; this is because renaming causes the old meaning of the name @var{new}
to be lost.  If @var{old} and @var{new} are on different file systems, the
file @var{old} is copied and deleted.

  If the argument @var{new} is just a directory name, the real new
name is in that directory, with the same non-directory component as
@var{old}.  For example, @kbd{M-x rename-file RET ~/foo RET /tmp RET}
renames @file{~/foo} to @file{/tmp/foo}.  The same rule applies to all
the remaining commands in this section.  All of them ask for
confirmation when the new file name already exists, too.

@findex add-name-to-file
@cindex hard links (creation)
  The similar command @kbd{M-x add-name-to-file} is used to add an
additional name to an existing file without removing its old name.
The new name is created as a ``hard link'' to the existing file.
The new name must belong on the same file system that the file is on.
On MS-Windows, this command works only if the file resides in an NTFS
file system.  On MS-DOS, it works by copying the file.

@findex copy-file
@cindex copying files
  @kbd{M-x copy-file} reads the file @var{old} and writes a new file
named @var{new} with the same contents.

@findex make-symbolic-link
@cindex symbolic links (creation)
  @kbd{M-x make-symbolic-link} reads two file names @var{target} and
@var{linkname}, then creates a symbolic link named @var{linkname},
which points at @var{target}.  The effect is that future attempts to
open file @var{linkname} will refer to whatever file is named
@var{target} at the time the opening is done, or will get an error if
the name @var{target} is nonexistent at that time.  This command does
not expand the argument @var{target}, so that it allows you to specify
a relative name as the target of the link.

  Not all systems support symbolic links; on systems that don't
support them, this command is not defined.

@node Compressed Files
@section Accessing Compressed Files
@cindex compression
@cindex uncompression
@cindex Auto Compression mode
@cindex mode, Auto Compression
@pindex gzip

  Emacs automatically uncompresses compressed files when you visit
them, and automatically recompresses them if you alter them and save
them.  Emacs recognizes compressed files by their file names.  File
names ending in @samp{.gz} indicate a file compressed with
@code{gzip}.  Other endings indicate other compression programs.

  Automatic uncompression and compression apply to all the operations in
which Emacs uses the contents of a file.  This includes visiting it,
saving it, inserting its contents into a buffer, loading it, and byte
compiling it.

@findex auto-compression-mode
@vindex auto-compression-mode
  To disable this feature, type the command @kbd{M-x
auto-compression-mode}.  You can disable it permanently by
customizing the variable @code{auto-compression-mode}.

@node File Archives
@section File Archives
@cindex mode, tar
@cindex Tar mode
@cindex file archives

  A file whose name ends in @samp{.tar} is normally an @dfn{archive}
made by the @code{tar} program.  Emacs views these files in a special
mode called Tar mode which provides a Dired-like list of the contents
(@pxref{Dired}).  You can move around through the list just as you
would in Dired, and visit the subfiles contained in the archive.
However, not all Dired commands are available in Tar mode.

  If Auto Compression mode is enabled (@pxref{Compressed Files}), then
Tar mode is used also for compressed archives---files with extensions
@samp{.tgz}, @code{.tar.Z} and @code{.tar.gz}.

  The keys @kbd{e}, @kbd{f} and @key{RET} all extract a component file
into its own buffer.  You can edit it there, and if you save the
buffer, the edited version will replace the version in the Tar buffer.
@kbd{v} extracts a file into a buffer in View mode.  @kbd{o} extracts
the file and displays it in another window, so you could edit the file
and operate on the archive simultaneously.  @kbd{d} marks a file for
deletion when you later use @kbd{x}, and @kbd{u} unmarks a file, as in
Dired.  @kbd{C} copies a file from the archive to disk and @kbd{R}
renames a file within the archive.  @kbd{g} reverts the buffer from
the archive on disk.

  The keys @kbd{M}, @kbd{G}, and @kbd{O} change the file's permission
bits, group, and owner, respectively.

  If your display supports colors and the mouse, moving the mouse
pointer across a file name highlights that file name, indicating that
you can click on it.  Clicking @kbd{Mouse-2} on the highlighted file
name extracts the file into a buffer and displays that buffer.

  Saving the Tar buffer writes a new version of the archive to disk with
the changes you made to the components.

  You don't need the @code{tar} program to use Tar mode---Emacs reads
the archives directly.  However, accessing compressed archives
requires the appropriate uncompression program.

@cindex Archive mode
@cindex mode, archive
@cindex @code{arc}
@cindex @code{jar}
@cindex @code{rar}
@cindex @code{zip}
@cindex @code{lzh}
@cindex @code{zoo}
@pindex arc
@pindex jar
@pindex zip
@pindex rar
@pindex lzh
@pindex zoo
@cindex Java class archives
@cindex unzip archives
  A separate but similar Archive mode is used for archives produced by
the programs @code{arc}, @code{jar}, @code{lzh}, @code{zip},
@code{rar}, and @code{zoo}, which have extensions corresponding to the
program names.  Archive mode also works for those @code{exe} files
that are self-extracting executables.

  The key bindings of Archive mode are similar to those in Tar mode,
with the addition of the @kbd{m} key which marks a file for subsequent
operations, and @kbd{M-@key{DEL}} which unmarks all the marked files.
Also, the @kbd{a} key toggles the display of detailed file
information, for those archive types where it won't fit in a single
line.  Operations such as renaming a subfile, or changing its mode or
owner, are supported only for some of the archive formats.

  Unlike Tar mode, Archive mode runs the archiving program to unpack
and repack archives.  Details of the program names and their options
can be set in the @samp{Archive} Customize group.  However, you don't
need these programs to look at the archive table of contents, only to
extract or manipulate the subfiles in the archive.

@node Remote Files
@section Remote Files

@cindex Tramp
@cindex FTP
@cindex remote file access
  You can refer to files on other machines using a special file name
syntax:

@example
@group
/@var{host}:@var{filename}
/@var{user}@@@var{host}:@var{filename}
/@var{user}@@@var{host}#@var{port}:@var{filename}
/@var{method}:@var{user}@@@var{host}:@var{filename}
/@var{method}:@var{user}@@@var{host}#@var{port}:@var{filename}
@end group
@end example

@noindent
To carry out this request, Emacs uses a remote-login program such as
@command{ftp}, @command{ssh}, @command{rlogin}, or @command{telnet}.
You can always specify in the file name which method to use---for
example, @file{/ftp:@var{user}@@@var{host}:@var{filename}} uses FTP,
whereas @file{/ssh:@var{user}@@@var{host}:@var{filename}} uses
@command{ssh}.  When you don't specify a method in the file name,
Emacs chooses the method as follows:

@enumerate
@item
If the host name starts with @samp{ftp.} (with dot), then Emacs uses
FTP.
@item
If the user name is @samp{ftp} or @samp{anonymous}, then Emacs uses
FTP.
@item
Otherwise, Emacs uses @command{ssh}.
@end enumerate

@noindent
Remote file access through FTP is handled by the Ange-FTP package, which
is documented in the following.  Remote file access through the other
methods is handled by the Tramp package, which has its own manual.
@xref{Top, The Tramp Manual,, tramp, The Tramp Manual}.

When the Ange-FTP package is used, Emacs logs in through FTP using your
user name or the name @var{user}.  It may ask you for a password from
time to time; this is used for logging in on @var{host}.  The form using
@var{port} allows you to access servers running on a non-default TCP
port.

@cindex backups for remote files
@vindex ange-ftp-make-backup-files
  If you want to disable backups for remote files, set the variable
@code{ange-ftp-make-backup-files} to @code{nil}.

  By default, the auto-save files (@pxref{Auto Save Files}) for remote
files are made in the temporary file directory on the local machine.
This is achieved using the variable @code{auto-save-file-name-transforms}.

@cindex ange-ftp
@vindex ange-ftp-default-user
@cindex user name for remote file access
  Normally, if you do not specify a user name in a remote file name,
that means to use your own user name.  But if you set the variable
@code{ange-ftp-default-user} to a string, that string is used instead.

@cindex anonymous FTP
@vindex ange-ftp-generate-anonymous-password
  To visit files accessible by anonymous FTP, you use special user
names @samp{anonymous} or @samp{ftp}.  Passwords for these user names
are handled specially.  The variable
@code{ange-ftp-generate-anonymous-password} controls what happens: if
the value of this variable is a string, then that string is used as
the password; if non-@code{nil} (the default), then the value of
@code{user-mail-address} is used; if @code{nil}, then Emacs prompts
you for a password as usual.

@cindex firewall, and accessing remote files
@cindex gateway, and remote file access with @code{ange-ftp}
@vindex ange-ftp-smart-gateway
@vindex ange-ftp-gateway-host
  Sometimes you may be unable to access files on a remote machine
because a @dfn{firewall} in between blocks the connection for security
reasons.  If you can log in on a @dfn{gateway} machine from which the
target files @emph{are} accessible, and whose FTP server supports
gatewaying features, you can still use remote file names; all you have
to do is specify the name of the gateway machine by setting the
variable @code{ange-ftp-gateway-host}, and set
@code{ange-ftp-smart-gateway} to @code{t}.  Otherwise you may be able
to make remote file names work, but the procedure is complex.  You can
read the instructions by typing @kbd{M-x finder-commentary @key{RET}
ange-ftp @key{RET}}.

@vindex file-name-handler-alist
@cindex disabling remote files
  You can entirely turn off the FTP file name feature by removing the
entries @code{ange-ftp-completion-hook-function} and
@code{ange-ftp-hook-function} from the variable
@code{file-name-handler-alist}.  You can turn off the feature in
individual cases by quoting the file name with @samp{/:} (@pxref{Quoted
File Names}).

@node Quoted File Names
@section Quoted File Names

@cindex quoting file names
@cindex file names, quote special characters
  You can @dfn{quote} an absolute file name to prevent special
characters and syntax in it from having their special effects.
The way to do this is to add @samp{/:} at the beginning.

  For example, you can quote a local file name which appears remote, to
prevent it from being treated as a remote file name.  Thus, if you have
a directory named @file{/foo:} and a file named @file{bar} in it, you
can refer to that file in Emacs as @samp{/:/foo:/bar}.

  @samp{/:} can also prevent @samp{~} from being treated as a special
character for a user's home directory.  For example, @file{/:/tmp/~hack}
refers to a file whose name is @file{~hack} in directory @file{/tmp}.

  Quoting with @samp{/:} is also a way to enter in the minibuffer a
file name that contains @samp{$}.  In order for this to work, the
@samp{/:} must be at the beginning of the minibuffer contents.  (You
can also double each @samp{$}; see @ref{File Names with $}.)

  You can also quote wildcard characters with @samp{/:}, for visiting.
For example, @file{/:/tmp/foo*bar} visits the file
@file{/tmp/foo*bar}.

  Another method of getting the same result is to enter
@file{/tmp/foo[*]bar}, which is a wildcard specification that matches
only @file{/tmp/foo*bar}.  However, in many cases there is no need to
quote the wildcard characters because even unquoted they give the
right result.  For example, if the only file name in @file{/tmp} that
starts with @samp{foo} and ends with @samp{bar} is @file{foo*bar},
then specifying @file{/tmp/foo*bar} will visit only
@file{/tmp/foo*bar}.

@node File Name Cache
@section File Name Cache

@cindex file name caching
@cindex cache of file names
@pindex find
@kindex C-@key{TAB}
@findex file-cache-minibuffer-complete
  You can use the @dfn{file name cache} to make it easy to locate a
file by name, without having to remember exactly where it is located.
When typing a file name in the minibuffer, @kbd{C-@key{tab}}
(@code{file-cache-minibuffer-complete}) completes it using the file
name cache.  If you repeat @kbd{C-@key{tab}}, that cycles through the
possible completions of what you had originally typed.  (However, note
that the @kbd{C-@key{tab}} character cannot be typed on most text-only
terminals.)

  The file name cache does not fill up automatically.  Instead, you
load file names into the cache using these commands:

@findex file-cache-add-directory
@table @kbd
@item M-x file-cache-add-directory @key{RET} @var{directory} @key{RET}
Add each file name in @var{directory} to the file name cache.
@item M-x file-cache-add-directory-using-find @key{RET} @var{directory} @key{RET}
Add each file name in @var{directory} and all of its nested
subdirectories to the file name cache.
@item M-x file-cache-add-directory-using-locate @key{RET} @var{directory} @key{RET}
Add each file name in @var{directory} and all of its nested
subdirectories to the file name cache, using @command{locate} to find
them all.
@item M-x file-cache-add-directory-list @key{RET} @var{variable} @key{RET}
Add each file name in each directory listed in @var{variable}
to the file name cache.  @var{variable} should be a Lisp variable
such as @code{load-path} or @code{exec-path}, whose value is a list
of directory names.
@item M-x file-cache-clear-cache @key{RET}
Clear the cache; that is, remove all file names from it.
@end table

  The file name cache is not persistent: it is kept and maintained
only for the duration of the Emacs session.  You can view the contents
of the cache with the @code{file-cache-display} command.

@node File Conveniences
@section Convenience Features for Finding Files

  In this section, we introduce some convenient facilities for finding
recently-opened files, reading file names from a buffer, and viewing
image files.

@findex recentf-mode
@vindex recentf-mode
@findex recentf-save-list
@findex recentf-edit-list
  If you enable Recentf mode, with @kbd{M-x recentf-mode}, the
@samp{File} menu includes a submenu containing a list of recently
opened files.  @kbd{M-x recentf-save-list} saves the current
@code{recent-file-list} to a file, and @kbd{M-x recentf-edit-list}
edits it.

  The @kbd{M-x ffap} command generalizes @code{find-file} with more
powerful heuristic defaults (@pxref{FFAP}), often based on the text at
point.  Partial Completion mode offers other features extending
@code{find-file}, which can be used with @code{ffap}.
@xref{Completion Options}.

@findex image-mode
@findex image-toggle-display
@cindex images, viewing
  Visiting image files automatically selects Image mode.  This major
mode allows you to toggle between displaying the file as an image in
the Emacs buffer, and displaying its underlying text representation,
using the command @kbd{C-c C-c} (@code{image-toggle-display}).  This
works only when Emacs can display the specific image type.  If the
displayed image is wider or taller than the frame, the usual point
motion keys (@kbd{C-f}, @kbd{C-p}, and so forth) cause different parts
of the image to be displayed.

@findex thumbs-mode
@findex mode, thumbs
  See also the Image-Dired package (@pxref{Image-Dired}) for viewing
images as thumbnails.

@node Filesets
@section Filesets
@cindex filesets

@findex filesets-init
  If you regularly edit a certain group of files, you can define them
as a @dfn{fileset}.  This lets you perform certain operations, such as
visiting, @code{query-replace}, and shell commands on all the files
at once.  To make use of filesets, you must first add the expression
@code{(filesets-init)} to your @file{.emacs} file (@pxref{Init File}).
This adds a @samp{Filesets} menu to the menu bar.

@findex filesets-add-buffer
@findex filesets-remove-buffer
  The simplest way to define a fileset is by adding files to it one
at a time.  To add a file to fileset @var{name}, visit the file and
type @kbd{M-x filesets-add-buffer @kbd{RET} @var{name} @kbd{RET}}.  If
there is no fileset @var{name}, this creates a new one, which
initially creates only the current file.  The command @kbd{M-x
filesets-remove-buffer} removes the current file from a fileset.

  You can also edit the list of filesets directly, with @kbd{M-x
filesets-edit} (or by choosing @samp{Edit Filesets} from the
@samp{Filesets} menu).  The editing is performed in a Customize buffer
(@pxref{Easy Customization}).  Filesets need not be a simple list of
files---you can also define filesets using regular expression matching
file names.  Some examples of these more complicated filesets are
shown in the Customize buffer.  Remember to select @samp{Save for
future sessions} if you want to use the same filesets in future Emacs
sessions.

  You can use the command @kbd{M-x filesets-open} to visit all the
files in a fileset, and @kbd{M-x filesets-close} to close them.  Use
@kbd{M-x filesets-run-cmd} to run a shell command on all the files in
a fileset.  These commands are also available from the @samp{Filesets}
menu, where each existing fileset is represented by a submenu.

   Emacs uses the concept of a fileset elsewhere @pxref{Version
Control} to describe sets of files to be treated as a group for
purposes of version control operations.  Those filesets are unnamed
and do not persist across Emacs sessions.

@ignore
   arch-tag: 768d32cb-e15a-4cc1-b7bf-62c00ee12250
@end ignore