Improve the explanation of version control to include concepts

[emacs.git] / man / 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 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

  Most Emacs commands that operate on a file require you to specify the
file name.  (Saving and reverting are exceptions; the buffer knows which
file name to use for them.)  You enter the file name using the
minibuffer (@pxref{Minibuffer}).  @dfn{Completion} is available
(@pxref{Completion}) to make it easier to specify long file names.  When
completing file names, Emacs ignores those whose file-name extensions
appear in the variable @code{completion-ignored-extensions}; see
@ref{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;
this makes it easy to operate on that file with any of the Emacs file
commands.

@vindex default-directory
  Each buffer has a default directory which is normally the same as the
directory of the file visited in that buffer.  When you enter a file
name without a directory, the default directory is used.  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.  The
default directory is kept in the variable @code{default-directory},
which has a separate value in every buffer.

@findex cd
@findex pwd
  The command @kbd{M-x pwd} displays the current buffer's 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.

  For example, if the default file name is @file{/u/rms/gnu/gnu.tasks}
then the default directory is normally @file{/u/rms/gnu/}.  If you
type just @samp{foo}, which does not specify a directory, it is short
for @file{/u/rms/gnu/foo}.  @samp{../.login} would stand for
@file{/u/rms/.login}.  @samp{new/foo} would stand for the file name
@file{/u/rms/gnu/new/foo}.

@vindex insert-default-directory
  The default directory actually appears in the minibuffer when the
minibuffer becomes active to read a file name.  This serves two
purposes: it @emph{shows} you what the default is, so that you can type
a relative file name and know with certainty what it will mean, and it
allows you to @emph{edit} the default to specify a different directory.
This insertion of the default directory is inhibited if the variable
@code{insert-default-directory} is set to @code{nil}.

  Note that it is legitimate to type an absolute file name after you
enter the minibuffer, ignoring the presence of the default directory
name as part of the text.  The final minibuffer contents may look
invalid, but that is not so.  For example, if the minibuffer starts out
with @samp{/usr/tmp/} and you add @samp{/x1/rms/foo}, you get
@samp{/usr/tmp//x1/rms/foo}; but Emacs ignores everything through the
first slash in the double slash; the result is @samp{/x1/rms/foo}.
@xref{Minibuffer File}.

@cindex home directory shorthand
  You can use @file{~/} in a file name to mean your home directory,
or @file{~@var{user-id}/} to mean the home directory of a user whose
login name is @code{user-id}@footnote{
On MS-Windows and MS-DOS systems, where a user doesn't have a home
directory, Emacs replaces @file{~/} with the value of the
environment variable @code{HOME}; see @ref{General Variables}.  On
these systems, the @file{~@var{user-id}/} construct is supported only
for the current user, i.e., only if @var{user-id} is the current
user's login name.}.

@cindex environment variables in file names
@cindex expansion of environment variables
@cindex @code{$} in file names
  @anchor{File Names with $}@samp{$} in a file name is used to
substitute an environment variable.  The environment variable name
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
you can use @file{/u/$FOO/test.c} or @file{/u/$@{FOO@}/test.c} as an
abbreviation for @file{/u/rms/hacks/test.c}.  If the environment
variable is not defined, no substitution occurs: @file{/u/$notdefined}
stands for itself (assuming the environment variable @env{notdefined}
is not defined).

  Note that shell commands to set environment variables affect Emacs
only when done 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{$} at the same time as variable substitution is performed for a
single @samp{$}.  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{/:}.

@findex substitute-in-file-name
  The Lisp function that performs the @samp{$}-substitution is called
@code{substitute-in-file-name}.  The substitution is performed only on
file names read as such using the minibuffer.

  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.  We often say that this buffer ``is visiting'' that
file, or that the buffer's ``visited file'' is that file.  Emacs
constructs the buffer name from the file name by throwing away the
directory, keeping just the name proper.  For example, a file named
@file{/usr/rms/emacs.tex} would get 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 (@pxref{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.

  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.  Saving the buffer
means that Emacs writes the current contents of the buffer into its
visited file.  @xref{Saving}.

@cindex modified (buffer)
  If a buffer contains changes that have not been saved, we say the
buffer is @dfn{modified}.  This is important because it 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, use the command @kbd{C-x C-f} (@code{find-file}).  Follow
the command with the name of the file you wish to visit, terminated by a
@key{RET}.

  The file name is read using the minibuffer (@pxref{Minibuffer}), with
defaulting and completion in the standard manner (@pxref{File Names}).
While in the minibuffer, you can abort @kbd{C-x C-f} by typing
@kbd{C-g}.  File-name completion ignores certain file names; for more
about this, see @ref{Completion Options}.

  Your confirmation that @kbd{C-x C-f} has completed successfully is
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, then you get an error,
with an error message 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 will ask 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 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 Unix and
GNU/Linux platforms, Emacs does that 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.  However,
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
  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 any changes and
save them, the file is created.

  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.

@vindex find-file-run-dired
  If the file you specify is actually a directory, @kbd{C-x C-f} invokes
Dired, the Emacs directory browser, so that you can ``edit'' the contents
of the directory (@pxref{Dired}).  Dired is a convenient way to view, delete,
or operate on the files in the directory.  However, if the variable
@code{find-file-run-dired} is @code{nil}, then 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.

@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}.

  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 C-v
@findex find-alternate-file
  If you visit a nonexistent file unintentionally (because you typed the
wrong file name), use the @kbd{C-x C-v} command
(@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.

@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}.

@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.
It 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}.
If you already have 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

@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 you do not want 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.)  You could also use
@code{set-visited-file-name} (see below) to mark the buffer as visiting
a different file name, one which is not in use for anything important.
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.)  You can also kill the buffer.

@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}).  It 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.

  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.

@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}.

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

  You can also explicitly request making another backup file from a
buffer even though it has already been saved at least once.  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
* One or Many: Numbered Backups. Whether to make one backup file or many.
* 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 Numbered Backups
@subsubsection Numbered Backups

@vindex version-control
  The choice of single backup file or multiple numbered backup files
is controlled by the variable @code{version-control}.  Its possible
values are:

@table @code
@item t
Make numbered backups.
@item nil
Make numbered backups for files that have numbered backups already.
Otherwise, make single 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.  For example, Rmail mode
locally sets @code{version-control} to @code{never} to make sure that
there is only one backup for an Rmail file.  @xref{Locals}.

@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}.

@node Backup Names
@subsubsection Single or Numbered Backups

  When Emacs makes a single 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~}.

@vindex make-backup-file-name-function
@vindex backup-directory-alist
  You can change this behavior by defining the variable
@code{make-backup-file-name-function} to a suitable function.
Alternatively you can customize the variable
@code{backup-directory-alist} to specify that files matching certain
patterns should be backed up in specific directories.

  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, say, @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.

  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.

  If you choose to have a series of numbered backup files, 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.  The variable
@code{backup-directory-alist} applies to numbered backups just as
usual.

@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 symbolic link in the same directory with a
different name.)  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.

  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.

  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.

  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'' until another change is
made.

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

@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, whenever you
visit the file again with @kbd{C-x C-f}.

  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

  You may find it useful to have Emacs revert files automatically when
they change.  Three minor modes are available to do this.

  @kbd{M-x global-auto-revert-mode} enables Global Auto-Revert mode,
which periodically checks all file buffers and reverts when the
corresponding file has changed.  @kbd{M-x auto-revert-mode} enables a
local version, Auto-Revert mode, which applies only to the current
buffer.

  You can use Auto-Revert mode 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.

@vindex auto-revert-interval
  The variable @code{auto-revert-interval} controls how often to check
for a changed file.  Since checking a remote file is too slow, these
modes do not check or revert remote files.

  @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

  Emacs saves all the visited files from time to time (based on counting
your keystrokes) without being asked.  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 program that is in an inconsistent
state when you have made half of a planned change.  Instead, auto-saving
is done in a different file called the @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}}.  All
of this name except the @file{@var{pid}-@var{hostname}} part comes
from the value of @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

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

  The Emacs version control interface is called VC.  Its commands work
with different version control systems---currently, it supports CVS,
GNU Arch, RCS, Meta-CVS, Subversion, and SCCS.  Of these, the GNU
project distributes CVS, GNU Arch, and RCS; we recommend that you use
either CVS or GNU Arch for your projects, and RCS for individual
files.  We also have free software to replace SCCS, known as CSSC; if
you are using SCCS and don't want to make the incompatible change to
RCS or CVS, you can switch to CSSC.

  VC is enabled by default in Emacs.  To disable it, 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 Versions::        Examining and comparing old versions.
* Secondary VC Commands::    The commands used a little less frequently.
* Branches::            Multiple lines of development.
@ifnottex
* Remote Repositories:: Efficient access to remote CVS servers.
* Snapshots::           Sets of file versions treated as a unit.
* 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.  VC
provides a uniform interface to version control, so that regardless of
which version control system is in use, you can use it 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 Systems::  Supported version control back-end systems.
* VC Concepts::      Words and concepts related to version control.
* Types of Log File::    The per-file VC 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: 
@dfn{reversibility}. @dfn{concurrency}, and @dfn{history}.

  The most basic capability you get from a version-control system is
reversibility, the ability to back up to a saved, known-good state when
you discover that some modification you did was a mistake or a bad idea.

  Version-control systems also support concurrency, the ability to
have many people modifying the same collection of code or documents
knowing that conflicting modifications can be detected and resolved.

  Version-control systems give you the capability to attach a history
to your data, 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 
become a vitally important form of communication among developers.

@node Version Systems
@subsubsection Supported Version Control Systems

@cindex back end (version control)
  VC currently works with six different version control systems or
``back ends'': CVS, GNU Arch, RCS, Meta-CVS, Subversion, and SCCS.

@cindex CVS
  CVS is a free version control system that is used for the majority
of free software projects today.  It allows concurrent multi-user
development either locally or over the network.  Some of its
shortcomings, corrected by newer systems such as GNU Arch, are that it
lacks atomic commits or support for renaming files.  VC supports all
basic editing operations under CVS, but for some less common tasks you
still need to call CVS from the command line.  Note also that before
using CVS you must set up a repository, which is a subject too complex
to treat here.

@cindex GNU Arch
@cindex Arch
  GNU Arch is a new version control system that is designed for
distributed work.  It differs in many ways from old well-known
systems, such as CVS and RCS.  It supports different transports for
interoperating between users, offline operations, and it has good
branching and merging features.  It also supports atomic commits, and
history of file renaming and moving.  VC does not support all
operations provided by GNU Arch, so you must sometimes invoke it from
the command line, or use a specialized module.

@cindex RCS
  RCS is the free version control system around which VC was initially
built.  The VC commands are therefore conceptually closest to RCS.
Almost everything you can do with RCS can be done through VC.  You
cannot use RCS over the network though, and it only works at the level
of individual files, rather than projects.  You should use it if you
want a simple, yet reliable tool for handling individual files.

@cindex SVN
@cindex Subversion
  Subversion is a free version control system designed to be similar
to CVS but without CVS's problems.  Subversion supports atomic commits,
and versions directories, symbolic links, meta-data, renames, copies,
and deletes.  It can be used via http or via its own protocol.

@cindex MCVS
@cindex Meta-CVS
  Meta-CVS is another attempt to solve problems arising in CVS.  It
supports directory structure versioning, improved branching and
merging, and use of symbolic links and meta-data in repositories.

@cindex SCCS
  SCCS is a proprietary but widely used version control system.  In
terms of capabilities, it is the weakest of the six that VC supports.
VC compensates for certain features missing in SCCS (snapshots, for
example) by implementing them itself, but some other VC features, such
as multiple branches, are not available with SCCS.  Since SCCS is
non-free, not respecting its users freedom, you should not use it;
use its free replacement CSSC instead.  But you should use CSSC only
if for some reason you cannot use RCS, or one of the higher-level
systems such as CVS or GNU Arch.

In the following, we discuss mainly RCS, SCCS and CVS.  Nearly
everything said about CVS applies to GNU Arch, Subversion and Meta-CVS
as well.

@node VC Concepts
@subsubsection Concepts of Version Control

@cindex master file
@cindex registered file
   When a file is under version control, we also say that it is
@dfn{registered} in the version control system.  Each registered file
has a corresponding @dfn{master file} which represents the file's
present state plus its change history---enough to reconstruct the
current version or any earlier version.  Usually the master file also
records a @dfn{log entry} for each version, describing in words what was
changed in that version.

@cindex work file
@cindex checking out files
  The file that is maintained under version control is sometimes called
the @dfn{work file} corresponding to its master file.  You edit the work
file and make changes in it, as you would with an ordinary file.  (With
SCCS and RCS, you must @dfn{lock} the file before you start to edit it.)
After you are done with a set of changes, you @dfn{check the file in},
which records the changes in the master file, along with a log entry for
them.

  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 or merging; 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.  One method is
@dfn{locking} (analogous to the locking that Emacs uses to detect
simultaneous editing of a file, but distinct from it).  In a locking
system, such as SCCS, you must @dfn{lock} a file before you start to
edit it.  The other method is @dfn{merging}; the system tries to 
merge your changes with other people's changes when you check them in.

  With version control locking, work files are normally read-only so
that you cannot change them.  You ask the version control system to make
a work file writable for you by locking it; only one user can do
this at any given time.  When you check in your changes, that unlocks
the file, making the work file read-only again.  This allows other users
to lock the file to make further changes.

  By contrast, a merging system lets each user check out and modify a
work file at any time.  When you check in a a file, the system will
attempt to merge your changes with any others checked into the
repository since you checked out the file.

  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 is merge-based by default but can
be told to operate in a locking mode.  Most later version-control
systems, such as Subversion and GNU Arch, have been fundamentally
merging-based rather than locking-based.  This is because experience
has shown that the merging-based approach is generally superior to
the locking one, both in convenience to developers and in minimizing
the number and severity of conflicts that actually occur.

   While it is rather unlikely that anyone will ever again build a
fundamentally locking-based rather than merging-based version-control
system in the future, merging-based version-systems sometimes have locks
retrofitted onto them for reasons having nothing to do with technology.
@footnote{Usually the control-freak instincts of managers}  For this
reason, and to support older systems still in use, 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, checkins
and other operations are @dfn{file-based}; each file has its own
@dfn{master file} with 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 that change set is treated as a unit by the
system.  Any comment associated with the change doesn't belong to any
one file, but is attached to the changeset itself.

  Changeset-based version control is in general both more flexible and
more powerful than file-based version control; usually, when a change to
multiple files has to be backed out, 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 at time of
writing in 2007.

@cindex centralized vs. decentralized

  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.
It has two important problems. One is that a single repository is a
single point of failure--if the repository server is down all work
stops.  The other is that you need to be connected live to the server to
do checkins and checkouts; if you're offline, you can't work.

  Newer version-control systems like GNU Arch 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.

@iftex
(@pxref{CVS Options,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{CVS Options}).
@end ifnottex


@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 revision control system can have @emph{two}
types of log for changes.  One is the per-file log maintained by the
revision control system: each time you check in a change, you must
fill out a @dfn{log entry} for the change (@pxref{Log Buffer}).  This
kind of log is called the @dfn{version control log}, also the
@dfn{revision control log}, @dfn{RCS log}, or @dfn{CVS 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 well merit a @file{ChangeLog} file in each major directory.
@xref{Change Log}.

  A project maintained with version control can use just the per-file
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 when you
check in the change.  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 version number
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}.

@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

  The principal VC command is an all-purpose command that performs
either locking or check-in, depending on the situation.

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

@findex vc-next-action
@kindex C-x v v
  The precise action of this command depends on the state of the file,
and whether the version control system uses locking or not.  SCCS and
RCS normally use locking; CVS normally does not use locking.

@findex vc-toggle-read-only
@kindex C-x C-q @r{(Version Control)}
  As a special convenience that is particularly useful for files with
locking, you can let Emacs check a file in or out whenever you change
its read-only flag.  This means, for example, that you cannot
accidentally edit a file without properly checking it out first.  To
achieve this, bind the key @kbd{C-x C-q} to @kbd{vc-toggle-read-only}
in your @file{~/.emacs} file.  (@xref{Init Rebinding}.)

@menu
* VC with Locking::     RCS in its default mode, SCCS, and optionally CVS.
* Without Locking::     Without locking: default mode for 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 Locking
@subsubsection Basic Version Control with Locking

  If locking is used for the file (as with SCCS, and RCS in its default
mode), @kbd{C-x v v} can either lock a file or check it in:

@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 version.  @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 Without Locking
@subsubsection Basic Version Control without Locking

  When there is no locking---the default for CVS---work files are always
writable; you do not need to do anything before you begin to edit 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 in the
work file.

  Here is what @kbd{C-x v v} does when using CVS:

@itemize @bullet
@item
If some other user has checked in changes into the master file, Emacs
asks you whether you want to merge those changes into your own work
file.  You must do this before you can check in your own changes.  (To
pick up any recent changes from the master file @emph{without} trying
to commit your own changes, type @kbd{C-x v m @key{RET}}.)
@xref{Merging}.

@item
If there are no new changes in the master file, but you have made
modifications in your work file, @kbd{C-x v v} checks in your changes.
In order to do this, it first reads the log entry for the new version.
@xref{Log Buffer}.

@item
If the file is not modified, the @kbd{C-x v v} does nothing.
@end itemize

  These rules also apply when you use RCS in the mode that does not
require locking, except that automatic merging of changes from the
master file is not implemented.  Unfortunately, this means that nothing
informs you if another user has checked in changes in the same file
since you began editing it, and when this happens, his changes will be
effectively removed when you check in your version (though they will
remain in the master file, so they will not be entirely lost).  You must
therefore verify that the current version is unchanged, before you
check in your changes.  We hope to eliminate this risk and provide
automatic merging with RCS in a future Emacs version.

  In addition, locking is possible with RCS even in this mode, although
it is not required; @kbd{C-x v v} with an unmodified file locks the
file, just as it does with RCS in its normal (locking) mode.

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

@cindex version number 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 version
number 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
version to select; this lets you start working from an older version,
or on another branch.  If you do not enter any version, that takes you
to the highest version 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 version number, 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, @kbd{C-x v v} first reads a log entry.  It
pops up a buffer called @samp{*VC-Log*} for you to enter the log entry.

  Sometimes the @samp{*VC-Log*} buffer contains default text when you enter it,
typically the last log message entered.  If it does, mark and point
are set around the entire contents of the buffer so that it is easy to
kill the contents of the buffer with @kbd{C-w}.

@findex log-edit-insert-changelog
  If you work by writing entries in the @file{ChangeLog}
(@pxref{Change Log}) and then commit the change under revision
control, you can generate the Log Edit text from the ChangeLog using
@kbd{C-c C-a} (@kbd{log-edit-insert-changelog}).  This looks for
entries for the file(s) concerned in the top entry in the ChangeLog
and uses those paragraphs as the log text.  This text is only inserted
if the top entry was made under your user name on the current date.
@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.

  In the @samp{*VC-Log*} buffer, @kbd{C-c C-f} (@kbd{M-x
log-edit-show-files}) shows the list of files to be committed in case
you need to check that.  (This can be a list of more than one file if
you use VC Dired mode or PCL-CVS.
@iftex
@xref{VC Dired Mode,,,emacs-xtra, Specialized Emacs Features},
@end iftex
@ifnottex
@xref{VC Dired Mode},
@end ifnottex
and @ref{Top, , About PCL-CVS, pcl-cvs, PCL-CVS --- The Emacs
Front-End to CVS}.)

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

  To abort 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.  To do
this, use the history of previous log entries.  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 file, the log entry buffer is put into VC Log
mode, which involves running two hooks: @code{text-mode-hook} and
@code{vc-log-mode-hook}.  @xref{Hooks}.

@node Old Versions
@subsection Examining And Comparing Old Versions

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

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

@item C-x v =
Compare the current buffer contents with the master version from which
you started editing.

@item C-u C-x v = @var{file} @key{RET} @var{oldvers} @key{RET} @var{newvers} @key{RET}
Compare the specified two versions of @var{file}.

@item C-x v g
Display the file with per-line version information and using colors.
@end table

@findex vc-version-other-window
@kindex C-x v ~
  To examine an old version in its entirety, visit the file and then type
@kbd{C-x v ~ @var{version} @key{RET}} (@code{vc-version-other-window}).
This puts the text of version @var{version} in a file named
@file{@var{filename}.~@var{version}~}, and visits it in its own buffer
in a separate window.  (In RCS, you can also select an old version
and create a branch from it.  @xref{Branches}.)

@findex vc-diff
@kindex C-x v =
  It is usually more convenient to compare two versions of the file,
with the command @kbd{C-x v =} (@code{vc-diff}).  Plain @kbd{C-x v =}
compares the current buffer contents (saving them in the file if
necessary) with the master version from which you started editing the
file (this is not necessarily the latest version of the file).
@kbd{C-u C-x v =}, with a numeric argument, reads a file name and two
version numbers, then compares those versions of the specified file.
Both forms display the output in a special buffer in another window.

  You can specify a checked-in version by its number; an empty input
specifies the current contents of the work file (which may be different
from all the checked-in versions).  You can also specify a snapshot name
@iftex
(@pxref{Snapshots,,,emacs-xtra, Specialized Emacs Features})
@end iftex
@ifnottex
(@pxref{Snapshots})
@end ifnottex
instead of one or both version numbers.

  If you supply a directory name instead of the name of a registered
file, this command compares the two specified versions of all registered
files in that 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 RCS, @code{diff} uses
the options in @code{vc-rcs-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
versions 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 version information and using colors to enhance the visual
appearance, with the command @kbd{M-x vc-annotate}.  It 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, it uses the
minibuffer to read two arguments: which version number 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 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 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 workfile version--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 current version.
@end table

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

  This section explains the secondary commands of VC; those that you might
use once a day.

@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.
@ifnottex
* VC Dired Mode::       Listing files managed by version control.
* VC Dired Commands::   Commands to use in a VC Dired buffer.
@end ifnottex
@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
version by typing @kbd{C-x v v}.  Until you do that, the version
appears as @samp{@@@@} in the mode line.

@vindex vc-default-init-version
@cindex initial version number to register
  The initial version number for a newly registered file is 1.1, by
default.  You can specify a different default by setting the variable
@code{vc-default-init-version}, or you can give @kbd{C-x v i} a numeric
argument; then it reads the initial version number 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 version control state and change history.
@end table

@kindex C-x v l
@findex vc-print-log
  To view the detailed version control status and history of a file,
type @kbd{C-x v l} (@code{vc-print-log}).  It displays the history of
changes to the current file, including the text of the log entries.  The
output appears in a separate window.  The point is centered at the
revision of the file that is currently being visited.

  In the change log buffer, you can use the following keys to move
between the logs of revisions and of files, to view past revisions, 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
@iftex
(@pxref{VC Dired Mode,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{VC Dired Mode}).
@end ifnottex
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
@iftex
(@pxref{VC Dired Mode,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{VC Dired Mode}).
@end ifnottex
It also takes a numeric prefix argument as a repeat count.

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

@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 when the revision indicated on the
current line 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 version 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
version from which you started editing the file, use @kbd{C-x v u}
(@code{vc-revert-buffer}).  This leaves the file unlocked; if locking
is in use, you must first lock the file again before you change it
again.  @kbd{C-x v u} requires confirmation, unless it sees that you
haven't made any changes with respect to the master version.

  @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-cancel-version
  To cancel a change that you already checked in, use @kbd{C-x v c}
(@code{vc-cancel-version}).  This command discards all record of the
most recent checked-in version, but only if your work file corresponds
to that version---you cannot use @kbd{C-x v c} to cancel a version
that is not the latest on its branch.  @kbd{C-x v c} also offers to
revert your work file and buffer to the previous version (the one that
precedes the version that is deleted).

  If you answer @kbd{no}, VC keeps your changes in the buffer, and locks
the file.  The no-revert option is useful when you have checked in a
change and then discover a trivial error in it; you can cancel the
erroneous check-in, fix the error, and check the file in again.

  When @kbd{C-x v c} does not revert the buffer, it unexpands all
version control headers in the buffer instead
@iftex
(@pxref{Version Headers,,,emacs-xtra, Specialized Emacs Features}).
@end iftex
@ifnottex
(@pxref{Version Headers}).
@end ifnottex
This is because the buffer no longer corresponds to any existing
version.  If you check it in again, the check-in process will expand
the headers properly for the new version number.

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

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

@ifnottex
@c vc1-xtra.texi needs extra level of lowering.
@lowersections
@include vc1-xtra.texi
@raisesections
@end ifnottex

@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''
versions of a file.  For example, you might have different versions of a
program in which you are gradually adding various unfinished new
features.  Each such independent line of development is called a
@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}.
The versions on the trunk are normally numbered 1.1, 1.2, 1.3, etc.  At
any such version, you can start an independent branch.  A branch
starting at version 1.2 would have version number 1.2.1.1, and consecutive
versions on this branch would have numbers 1.2.1.2, 1.2.1.3, 1.2.1.4,
and so on.  If there is a second branch also starting at version 1.2, it
would consist of versions 1.2.2.1, 1.2.2.2, 1.2.2.3, etc.

@cindex head version
  If you omit the final component of a version number, that is called a
@dfn{branch number}.  It refers to the highest existing version on that
branch---the @dfn{head version} of that branch.  The branches in the
example above have branch numbers 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
version number you want to select.  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.

  You can omit the minor version number, thus giving only the branch
number; 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 version (one that is the latest in
the branch that contains it), first select that version 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 version number for the new version.  You should specify a
suitable branch number for a branch starting at the current version.
For example, if the current version is 2.5, the branch number should be
2.5.1, 2.5.2, and so on, depending on the number of existing branches at
that point.

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

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

  After the branch is created, you ``stay'' on it.  That means that
subsequent check-ins create new versions on that branch.  To leave the
branch, you must explicitly select a different version 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 number or a pair of version numbers in
the minibuffer.  Then @kbd{C-x v m} finds the changes from that
branch, or the differences between the two versions you specified, and
merges them into the current version 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 version 1.5.  To merge the changes from the branch to the trunk,
first go to the head version of the trunk, by typing @kbd{C-u C-x v v
@key{RET}}.  Version 1.5 is now current.  If locking is used for the file,
type @kbd{C-x v v} to lock version 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 version 1.3, where the branch started, up to
the last version on the branch) and merges it into the current version
of the work file.  You can now check in the changed file, thus creating
version 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
version, 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 version 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 allows 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 versions, 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 version
number 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 number.  This ensures that Emacs knows which branch it is using
during this particular editing session.

@ifnottex
@include vc2-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} compares two files, displaying the
differences in an Emacs buffer named @samp{*diff*}.  It works by
running the @code{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.

@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.

@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 used for the output of @kbd{M-x diff}; it is also
useful for editing patches and comparisons produced by the
@command{diff} program.  To select Diff mode manually, type @kbd{M-x
diff-mode}.

  One general feature of Diff mode is that manual edits to the patch
automatically correct line numbers, including those in the hunk
header, so that you can actually apply the edited patch.  Diff mode
treats each hunk location as an ``error message,'' so that you can use
commands such as @kbd{C-x '} to visit the corresponding source
locations.  It also provides the following commands to navigate,
manipulate and apply parts of patches:

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

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

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

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

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

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

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

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

@item C-c C-n
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 patch of a multiple file patch.  To widen again,
use @kbd{C-x n w}.

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

@item C-c C-s
Split the hunk at point (@code{diff-split-hunk}).  This is for
manually editing patches, and only works with the unified diff format.

@item C-c C-u
Convert the entire buffer to unified format
(@code{diff-context->unified}).  With a prefix argument, convert
unified format to context format.  In Transient Mark mode, when the
mark is active, this command operates only on the region.

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

  @kbd{C-x 4 a} in Diff mode operates on behalf of the target file,
but gets the function name from the patch itself.  @xref{Change Log}.
This is useful for making log entries for functions that are deleted
by the patch.

@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 and the mark after them.

@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{zip}
@cindex @code{lzh}
@cindex @code{zoo}
@pindex arc
@pindex jar
@pindex zip
@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}, 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 either the FTP program or a
remote-login program such as @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.

@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.

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