@c This is part of the Emacs manual.
@c Copyright (C) 1985--1987, 1993--1995, 1997, 1999--2021 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Files
@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.
* Auto Revert::         Keeping buffers automatically up-to-date.
* Auto Save::           Auto Save periodically protects against loss of data.
* File Aliases::        Handling multiple names for one file.
* Directories::         Creating, deleting, and listing file directories.
* Comparing Files::     Finding where two files differ.
* Diff Mode::           Mode for editing file differences.
* Copying and Naming::  Copying, naming and renaming files.
* 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 machines.
* 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.
* Image Mode::          Viewing image files.
* Filesets::            Handling sets of files.
@end menu

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

@cindex default file name
  Many Emacs commands that operate on a file require you to specify
the file name, using the minibuffer (@pxref{Minibuffer File}).

  While in the minibuffer, you can use the usual completion and
history commands (@pxref{Minibuffer}).  Note that file name completion
ignores file names whose extensions appear in the variable
@code{completion-ignored-extensions} (@pxref{Completion Options}).
Note also that most commands use permissive completion with
confirmation for reading file names: you are allowed to submit a
nonexistent file name, but if you type @key{RET} immediately after
completing up to a nonexistent file name, Emacs prints
@samp{[Confirm]} and you must type a second @key{RET} to confirm.
@xref{Completion Exit}, for details.

Minibuffer history commands offer some special features for reading
file names, see @ref{Minibuffer History}.

@cindex default directory, of a buffer
@vindex default-directory
  Each buffer has a @dfn{default directory}, stored in the
buffer-local variable @code{default-directory}.  Whenever Emacs reads
a file name using the minibuffer, it usually inserts the default
directory into the minibuffer as the initial contents.  You can
inhibit this insertion by changing the variable
@code{insert-default-directory} to @code{nil} (@pxref{Minibuffer
File}).  Regardless, Emacs always assumes that any relative file name
is relative to the default directory, e.g., entering a file name
without a directory specifies a file in the default directory.

@findex cd
@findex pwd
  When you visit a file, Emacs sets @code{default-directory} in the
visiting buffer to the directory of its file.  When you create a new
buffer that is not visiting a file, via a command like @kbd{C-x b},
its default directory is usually copied from the buffer that was
current at the time (@pxref{Select Buffer}).  You can use the command
@kbd{M-x pwd} to see the value of @code{default-directory} in the
current buffer.  The command @kbd{M-x cd} prompts for a directory's
name, and sets the buffer's @code{default-directory} to that directory
(doing this does not change the buffer's file name, if any).

  As an example, when you visit the file @file{/u/rms/gnu/gnu.tasks},
the default directory is set to @file{/u/rms/gnu/}.  If you invoke a
command that reads a file name, entering just @samp{foo} in the
minibuffer, with a directory omitted, specifies the file
@file{/u/rms/gnu/foo}; entering @samp{../.login} specifies
@file{/u/rms/.login}; and entering @samp{new/foo} specifies
@file{/u/rms/gnu/new/foo}.

  When typing a file name into the minibuffer, you can make use of a
couple of shortcuts: a double slash ignores everything before the
second slash in the pair, and @samp{~/} is your home directory.
@xref{Minibuffer File}.

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

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

  You can include non-@acronym{ASCII} characters in file names.
@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.

@kindex C-x C-f
@findex find-file
  To visit a file, type @kbd{C-x C-f} (@code{find-file}) and use the
minibuffer to enter the name of the desired file.  While in the
minibuffer, you can abort the command by typing @kbd{C-g}.  @xref{File
Names}, for details about entering file names into minibuffers.

  If the specified file exists but the system does not allow you to
read it, an error message is displayed in the echo area (on GNU and
Unix systems you might be able to visit such a file using the
@samp{su} or @samp{sudo} methods; @pxref{Remote Files}).  Otherwise,
you can tell that @kbd{C-x C-f} has completed successfully by the
appearance of new text on the screen, and by the buffer name shown in
the mode line (@pxref{Mode Line}).  Emacs normally constructs the
buffer name from the file name, omitting the directory name.  For
example, a file named @file{/usr/rms/emacs.tex} is visited in a buffer
named @samp{emacs.tex}.  If there is already a buffer with that name,
Emacs constructs a unique name; the normal method is to add a suffix
based on the directory name (e.g., @samp{<rms>}, @samp{<tmp>},
and so on), but you can select other methods.  @xref{Uniquify}.

@cindex creating files
  To create a new file, just visit it using the same command, @kbd{C-x
C-f}.  Emacs displays @samp{(New file)} in the echo area, but in other
respects behaves as if you had visited an existing empty file.

@cindex modified (buffer)
  After visiting a file, the changes you make with editing commands are
made in the Emacs buffer.  They do not take effect in the visited
file, until you @dfn{save} the buffer (@pxref{Saving}).  If a buffer
contains changes that have not been saved, we say the buffer is
@dfn{modified}.  This implies that some changes will be lost if the
buffer is not saved.  The mode line displays two stars near the left
margin to indicate that the buffer is modified.

  If you visit a file that is already in Emacs, @kbd{C-x C-f} switches
to the existing buffer instead of making another copy.  Before doing
so, it checks whether the file has changed since you last visited or
saved it.  If the file has changed, Emacs offers to reread it.

@vindex large-file-warning-threshold
@cindex file, warning when size is large
@cindex size of file, warning when visiting
@cindex maximum buffer size exceeded, error message
  If you try to visit a file larger than
@code{large-file-warning-threshold} (the default is 10000000, which is
about 10 megabytes), Emacs asks you for confirmation first.  You can
answer @kbd{y} to proceed with visiting the file or @kbd{l} to visit
the file literally (see below).  Visiting large files literally speeds
up navigation and editing of such files, because various
potentially-expensive features are turned off.  Note, however, that
Emacs cannot visit files that are larger than the maximum Emacs buffer
size, which is limited by the amount of memory Emacs can allocate and
by the integers that Emacs can represent (@pxref{Buffers}).  If you
try, Emacs displays an error message saying that the maximum buffer
size has been exceeded.

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

@vindex query-about-changed-file
  If you're asking to visit a file that's already visited in a buffer,
but the file has changed externally, Emacs normally asks you whether
you want to re-read the file from disk.  But if you set
@code{query-about-changed-file} to @code{nil}, Emacs won't query you,
but will instead just display the buffer's contents before the
changes, and show an echo-area message telling you how to revert the
buffer from the file.

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

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

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

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

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

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

@kindex C-x 5 f
@findex find-file-other-frame
  @kbd{C-x 5 f} (@code{find-file-other-frame}) is similar, but opens a
new frame, or selects any existing frame showing the specified file.
@xref{Frames}.

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

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

  On text-mode terminals and on graphical displays when Emacs was
built without a GUI toolkit, you can visit files via the menu-bar
@samp{File} menu, which has the @samp{Visit New File} and the
@samp{Open File} items.

  Each time you visit a file, Emacs automatically scans its contents
to detect what character encoding and end-of-line convention it uses,
and converts these to Emacs's internal encoding and end-of-line
convention within the buffer.  When you save the buffer, Emacs
performs the inverse conversion, writing the file to disk with its
original encoding and end-of-line convention.  @xref{Coding Systems}.

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

@vindex find-file-hook
@vindex find-file-not-found-functions
  Two special hook variables allow extensions to modify the operation
of visiting files.  Visiting a file that does not exist runs the
functions in @code{find-file-not-found-functions}; this variable holds
a list of functions, which 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 @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 to its file (@code{save-buffer}).
@item C-x s
Save any or all buffers to their 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 current buffer is not modified (no changes have been made in it
since the buffer was created or last saved), saving is not really
done, because it would have no effect.  Instead, @kbd{C-x C-s}
displays a message like this in the echo area:

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

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

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

@table @kbd
@item y
@item @key{SPC}
Save this buffer and ask about the rest of the buffers.
@item n
@item @key{DEL}
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.
@item q
@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 C-f
Exit @code{save-some-buffers} and visit the buffer that you are
currently being asked about.
@item d
Diff the buffer against its corresponding file, so you can see what
changes you would be saving.  This calls the command
@code{diff-buffer-with-file} (@pxref{Comparing Files}).
@item C-h
Display a help message about these options.
@end table

@noindent
@vindex save-some-buffers-default-predicate
You can customize the value of
@code{save-some-buffers-default-predicate} to control which buffers
Emacs will ask about.

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

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

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

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

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

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

@node Backup
@subsection Backup Files
@cindex backup file
@vindex make-backup-files

  On most operating systems, rewriting a file automatically destroys all
record of what the file used to contain.  Thus, saving a file from Emacs
throws away the old contents of the file---or it would, except that
Emacs carefully copies the old contents to another file, called the
@dfn{backup} file, before actually saving.

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

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

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

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

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

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

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

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

@node Backup Names
@subsubsection Single or Numbered Backups
@cindex backup file names
@cindex names of backup files

@cindex @file{~}, in names of backup files
@cindex tilde (@file{~}) at end of backup file name
  When Emacs makes a backup file, its name is normally constructed by
appending @samp{~} to the file name being edited; thus, the backup
file for @file{eval.c} would be @file{eval.c~}.

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

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

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

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

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

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

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

@node Backup Deletion
@subsubsection Automatic Deletion of Backups

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

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

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

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

@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).

@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 made as follows:

@itemize
@item
If the variable @code{backup-by-copying} is non-@code{nil} (the
default is @code{nil}), use copying.

@item
Otherwise, if the variable @code{backup-by-copying-when-linked} is
non-@code{nil} (the default is @code{nil}), and the file has multiple
names, use copying.

@item
Otherwise, if the variable @code{backup-by-copying-when-mismatch} is
non-@code{nil} (the default is @code{t}), and renaming would change
the file's owner or group, use copying.

If you change @code{backup-by-copying-when-mismatch} to @code{nil},
Emacs checks the numeric user-id of the file's owner and the numeric
group-id of the file's group.  If either is
no greater than @code{backup-by-copying-when-privileged-mismatch}, then it
behaves as though @code{backup-by-copying-when-mismatch} is
non-@code{nil} anyway.

@item
Otherwise, renaming is the default choice.
@end itemize

  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 @dfn{committing} (a.k.a.@: @dfn{checking in},
@pxref{VCS Concepts}) new versions of files is 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}, Emacs adds such
newlines both on visiting and on saving.  If the value is @code{nil},
Emacs leaves the end of the file unchanged; any other non-@code{nil}
value means Emacs asks you whether to add a newline.  The default is
@code{nil}.

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

@vindex file-preserve-symlinks-on-save
If this option is non-@code{nil} and you're visiting a file via a
symbolic link, Emacs will break the symbolic link upon saving the
buffer, and will write the buffer to a file with the same name as the
symbolic link, if the value of @code{file-precious-flag} is
non-@code{nil} (@pxref{Saving Buffers, file-precious-flag,, elisp, The
Emacs Lisp Reference Manual}).  If you want Emacs to save the buffer
to the file the symbolic link points to (thereby preserving the link)
in these cases, customize the variable
@code{file-preserve-symlinks-on-save} to @code{t}.

@vindex write-region-inhibit-fsync
  Normally, when a program writes a file, the operating system briefly
caches the file's data in main memory before committing the data to
disk.  This can greatly improve performance; for example, when running
on laptops, it can avoid a disk spin-up each time a file is written.
However, it risks data loss if the operating system crashes before
committing the cache to disk.

  To lessen this risk, Emacs can invoke the @code{fsync} system call
after saving a file.  Using @code{fsync} does not eliminate the risk
of data loss, partly because many systems do not implement
@code{fsync} properly, and partly because Emacs's file-saving
procedure typically relies also on directory updates that might not
survive a crash even if @code{fsync} works properly.

  The @code{write-region-inhibit-fsync} variable controls whether
Emacs invokes @code{fsync} after saving a file.  The variable's
default value is @code{nil} when Emacs is interactive, and @code{t}
when Emacs runs in batch mode (@pxref{Initial Options, Batch Mode}).

  Emacs never uses @code{fsync} when writing auto-save files, as these
files might lose data anyway.

@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 is informed that
this is happening, whichever user saves first would later find that
their 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
@cindex .#, lock file names
@cindex file locking
  When you make the first modification in an Emacs buffer that is
visiting a file, Emacs records that the file is @dfn{locked} by you.
(It does this by creating a specially-named symbolic link@footnote{If
your file system does not support symbolic links, a regular file is
used.} with special contents in the same directory.  @xref{File
Locks,,, elisp}, for more details.)  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.

@vindex create-lockfiles
  You can prevent the creation of lock files by setting the variable
@code{create-lockfiles} to @code{nil}.  @strong{Caution:} by
doing so you will lose the benefits that this feature provides.  You
can also control where lock files are written by using the
@code{lock-file-name-transforms} variable.

@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

  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.

  Note that locking works on the basis of a file name; if a file has
multiple names, Emacs does not prevent two users from editing it
simultaneously under different names.

  A lock file cannot be written in some circumstances, e.g., if Emacs
lacks the system permissions or cannot create lock files for some
other reason.  In these cases, Emacs can still detect the collision
when you try to save a file, by checking the file's last-modification
date.  If the file has changed since the last time Emacs visited or
saved it, that implies that changes have been made in some other way,
and will be lost if Emacs proceeds with saving.  Emacs then displays a
warning message and asks for confirmation before saving; answer
@kbd{yes} to save, and @kbd{no} or @kbd{C-g} cancel the save.

  If you are notified that simultaneous editing has already taken
place, one way to compare the buffer to its file is the @kbd{M-x
diff-buffer-with-file} command.  @xref{Comparing Files}.

@vindex remote-file-name-inhibit-locks
  You can prevent the creation of remote lock files by setting the
variable @code{remote-file-name-inhibit-locks} to @code{t}.

@cindex lock-file-mode
  The minor mode @code{lock-file-mode}, called interactively, toggles
the local value of @code{create-lockfiles} in the current buffer.

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

  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 @w{@kbd{M-x
shadow-copy-files}}.

@cindex shadow cluster
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 @w{@kbd{M-x
shadow-define-cluster}}.

@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

To set up a shadow file group, use @w{@kbd{M-x
shadow-define-literal-group}} or @w{@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
@w{@kbd{M-x shadow-cancel}} to eliminate or change the shadow file group.

File Shadowing is not available on MS Windows.

@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 is updated
automatically each time you edit and save the file.  The time stamp
must 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 function @code{time-stamp} to the hook
@code{before-save-hook} (@pxref{Hooks}).  When you save the file, this
function then automatically updates the time stamp with the current
date and time.  You can also use the command @kbd{M-x time-stamp} to
update the time stamp manually.  By default the time stamp is
formatted according to your locale setting (@pxref{Environment}) and
time zone (@pxref{Time of Day,,, elisp, The Emacs Lisp Reference
Manual}).  For customizations, see the Custom group @code{time-stamp}.

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

  If you have made extensive changes to a file-visiting buffer and
then change your mind, you can @dfn{revert} the changes and go back to
the saved version of the file.  To do this, type @kbd{C-x x g}.  Since
reverting unintentionally could lose a lot of work, Emacs asks for
confirmation first if the buffer is modified.

  The @code{revert-buffer} command tries to position point in such a
way that, if the file was edited only slightly, you will be at
approximately the same part of the text as before.  But if you have
made major changes, point may end up in a totally different location.

  Reverting marks the buffer as not modified.  However, it adds the
reverted changes as a single modification to the buffer's undo history
(@pxref{Undo}).  Thus, after reverting, you can type @kbd{C-/} or its
aliases to bring the reverted changes back, if you happen to change
your mind.

@vindex revert-buffer-with-fine-grain-max-seconds
  To revert a buffer more conservatively, you can use the command
@code{revert-buffer-with-fine-grain}.  This command acts like
@code{revert-buffer}, but it tries to be as non-destructive as
possible, making an effort to preserve all markers, properties and
overlays in the buffer.  Since reverting this way can be very slow
when you have made a large number of changes, you can modify the
variable @code{revert-buffer-with-fine-grain-max-seconds} to
specify a maximum amount of seconds that replacing the buffer
contents this way should take.  Note that it is not ensured that the
whole execution of @code{revert-buffer-with-fine-grain} won't take
longer than this.

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

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

@vindex revert-buffer-quick-short-answers
@findex revert-buffer-quick
  The @kbd{C-x x g} keystroke is bound to the
@code{revert-buffer-quick} command.  This is like the
@code{revert-buffer} command, but prompts less.  Unlike
@code{revert-buffer}, it will not prompt if the current buffer visits
a file, and the buffer is not modified.  It also respects the
@code{revert-buffer-quick-short-answers} user option.  If this option
is non-@code{nil}, use a shorter @kbd{y/n} query instead of a longer
@kbd{yes/no} query.

  You can also tell Emacs to revert buffers automatically when their
visited files change on disk; @pxref{Auto Revert}.

@node Auto Revert
@section Auto Revert: Keeping buffers automatically up-to-date
@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

  A buffer can get out of sync with respect to its visited file on
disk if that file is changed by another program.  To keep it up to
date, you can enable Auto Revert mode by typing @kbd{M-x auto-revert-mode}.
This automatically reverts the buffer when its visited file changes on
disk.  To do the same for all file buffers, type
@kbd{M-x global-auto-revert-mode} to enable Global Auto Revert mode.

  Auto Revert will not revert a buffer if it has unsaved changes, or if
its file on disk is deleted or renamed.

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

@vindex auto-revert-verbose
  When a buffer is auto-reverted, a message is generated.  This can be
suppressed by setting @code{auto-revert-verbose} to @code{nil}.

@vindex auto-revert-remote-files
  The Auto Revert modes do not check or revert remote files, because
that is usually too slow.  This behavior can be changed by setting the
variable @code{auto-revert-remote-files} to non-@code{nil}.

@cindex file notifications
@vindex auto-revert-use-notify
@vindex auto-revert-interval
  By default, Auto Revert mode works using @dfn{file notifications},
whereby changes in the filesystem are reported to Emacs by the OS@.
You can disable use of file notifications by customizing the variable
@code{auto-revert-use-notify} to a @code{nil} value, then Emacs will
check for file changes by polling every five seconds.  You can change
the polling interval through the variable @code{auto-revert-interval}.

  Not all systems support file notifications; where they are not
supported, @code{auto-revert-use-notify} will be @code{nil} by
default.

@vindex auto-revert-avoid-polling
@vindex auto-revert-notify-exclude-dir-regexp
  By default, Auto Revert mode will poll files for changes
periodically even when file notifications are used.  Polling is
unnecessary in many cases, and turning it off may save power by
relying on notifications only.  To do so, set the variable
@code{auto-revert-avoid-polling} to non-@code{nil}.  However,
notification is ineffective on certain file systems; mainly network
file system on Unix-like machines, where files can be altered from
other machines.  For such file systems, polling may be necessary.
To force polling when
@code{auto-revert-avoid-polling} is non-@code{nil}, set
@code{auto-revert-notify-exclude-dir-regexp} to match files that
should be excluded from using notification.

  In Dired buffers (@pxref{Dired}), Auto Revert mode refreshes the
buffer when a file is created or deleted in the buffer's directory.

  @xref{VC Undo}, for commands to revert to earlier versions of files
under version control.  @xref{VC Mode Line}, for Auto Revert
peculiarities when visiting files under version control.

@ifnottex
@menu
* Non-File Buffers::    Auto Reverting Non-File Buffers.
@end menu
@include arevert-xtra.texi
@end ifnottex

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

  From time to time, Emacs automatically saves each visited file in a
separate file, without altering the file you actually use.  This is
called @dfn{auto-saving}.  It prevents you from losing more than a
limited amount of work if the system crashes.

@vindex auto-save-no-message
  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.  When
the @code{auto-save-no-message} variable is set to @code{nil} (the
default), the message @samp{Auto-saving...} is displayed in the echo
area during auto-saving, if any files are actually auto-saved; to
disable these messages, customize the variable to a non-@code{nil}
value.  Errors occurring during auto-saving are caught so that they do
not interfere with the execution of commands you have been typing.

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

@node Auto Save Files
@subsection Auto-Save Files

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

@cindex @file{#}, in auto-save file names
  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 @file{*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-mode
  If you want auto-saving to be done in the visited file rather than
in a separate auto-save file, enable the global minor mode
@code{auto-save-visited-mode}.  In this mode, auto-saving is identical
to explicit saving.  Note that this mode is orthogonal to the
@code{auto-save} mode described above; you can enable both at the same
time.  However, if @code{auto-save} mode is active in some buffer and
the obsolete @code{auto-save-visited-file-name} variable is set to a
non-@code{nil} value, that buffer won't be affected by
@code{auto-save-visited-mode}.

  You can use the variable @code{auto-save-visited-interval} to
customize the interval between auto-save operations in
@code{auto-save-visited-mode}; by default it's five seconds.
@code{auto-save-interval} and @code{auto-save-timeout} have no effect
on @code{auto-save-visited-mode}.  @xref{Auto Save Control}, for
details on these variables.

@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.

@vindex kill-buffer-delete-auto-save-files
  Killing a buffer, by default, doesn't remove the buffer's auto-save
file.  If @code{kill-buffer-delete-auto-save-files} is non-@code{nil},
killing a buffer that has an auto-save file will make Emacs prompt the
user for whether the auto-save file should be deleted.  (This is
inhibited if @code{delete-auto-save-files} is @code{nil}.)

@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{Initial Options}).  The default for this
variable is @code{t}, so auto-saving is the usual practice for
file-visiting buffers.  To toggle auto-saving in the current buffer,
type @kbd{M-x auto-save-mode}.  Auto Save mode acts as a buffer-local
minor mode (@pxref{Minor Modes}).

@vindex auto-save-interval
  Emacs auto-saves periodically based on how many characters you have
typed since the last auto-save.  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.  By
default, it does this after 30 seconds of idleness (at this time,
Emacs may also perform garbage collection; @pxref{Garbage
Collection,,, elisp, The Emacs Lisp Reference Manual}).  To change
this interval, customize the variable @code{auto-save-timeout}.  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.

@vindex auto-save-visited-interval
  When @code{auto-save-visited-mode} is enabled, Emacs will auto-save
file-visiting buffers after five seconds of idle time.  You can
customize the variable @code{auto-save-visited-interval} to change the
idle time interval.

  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 perform 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:

@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 in files named
@file{.saves-@var{pid}-@var{hostname}~} in the directory
@file{~/.emacs.d/auto-save-list/}.  This directory is determined by
the variable @code{auto-save-list-file-prefix}.  If you set
@code{auto-save-list-file-prefix} to @code{nil}, 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}.

@cindex directory name abbreviation
@vindex directory-abbrev-alist
  Sometimes, a directory is ordinarily accessed through a symbolic
link, and you may want Emacs to preferentially show its linked
name.  To do this, customize @code{directory-abbrev-alist}.  Each
element in this list should have the form @code{(@var{from}
. @var{to})}, which means to replace @var{from} with @var{to} whenever
@var{from} appears in a directory name.  The @var{from} string is a
regular expression (@pxref{Regexps}).  It is matched against directory
names anchored at the first character, and should start with @samp{\`}
(to support directory names with embedded newlines, which would defeat
@samp{^}).  The @var{to} string should be an ordinary absolute
directory name pointing to the same directory.  Do not use @samp{~} to
stand for a home directory in the @var{to} string; Emacs performs
these substitutions separately.  Here's an example, from a system on
which @file{/home/fsf} is normally accessed through a symbolic link
named @file{/fsf}:

@example
(("\\`/home/fsf" . "/fsf"))
@end example

@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 other attributes included).  Emacs also includes a
directory browser feature called Dired, which you can invoke with
@kbd{C-x d}; 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}.  If it isn't empty,
you will be asked whether you want to delete it recursively.
@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).

  In verbose directory listings, Emacs adds information about the
amount of free space on the disk that contains the directory.

  The command @kbd{M-x delete-directory} prompts for a directory's name
using the minibuffer, and deletes the directory if it is empty.  If
the directory is not empty, you will be asked whether you want to
delete it recursively.  On systems that have a ``Trash'' (or ``Recycle
Bin'') feature, you can make this command move the specified directory
to the Trash instead of deleting it outright, by changing the variable
@code{delete-by-moving-to-trash} to @code{t}.  @xref{Misc File Ops},
for more information about using the Trash.

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

@findex diff
@vindex diff-switches
  The command @kbd{M-x diff} prompts for two file names, using the
minibuffer, and displays the differences between the two files in a
buffer named @file{*diff*}.  This works by running the @command{diff}
program, using options taken from the variable @code{diff-switches}.
The value of @code{diff-switches} should be a string; the default is
@code{"-u"} to specify a unified context diff.
@c Note that the actual name of the info file is diffutils.info,
@c but it adds a dir entry for diff too.
@c On older systems, only "info diff" works, not "info diffutils".
@xref{Top,, Diff, diffutils, Comparing and Merging Files}, for more
information about the @command{diff} program.

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

  A (much more sophisticated) alternative is @kbd{M-x ediff}
(@pxref{Top, Ediff, Ediff, ediff, The Ediff Manual}).

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

@findex diff-buffer-with-file
  The command @kbd{M-x diff-buffer-with-file} compares a specified
buffer with its corresponding file.  This shows you what changes you
would make to the file if you save the buffer.

@findex diff-buffers
  The command @kbd{M-x diff-buffers} compares the contents of two
specified buffers.

@findex compare-windows
  The command @kbd{M-x compare-windows} compares the text in the
current window with that in the window that was the selected window
before you selected the current one.  (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
(@pxref{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 (@pxref{Repeating}), 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} by default ignores changes in whitespace, but a
prefix argument turns that off for that single invocation of the
command.

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

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

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

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

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

  The first hunk in a patch is preceded by a file header, which shows
the names of the new and the old versions of the file, and their time
stamps.  If a patch shows changes for more than one file, each file
has such a header before the first hunk of that file's changes.

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

  Diff mode arranges for hunks to be treated as compiler error
messages by @kbd{M-g M-n} and other commands that handle error messages
(@pxref{Compilation Mode}).  Thus, you can use the compilation-mode
commands to visit the corresponding source locations.

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

@table @kbd
@item M-n
@findex diff-hunk-next
Move to the next hunk-start (@code{diff-hunk-next}).  With prefix
argument @var{n}, move forward to the @var{n}th next hunk.

@vindex diff-refine
By default, Diff mode @dfn{refines} hunks as Emacs displays them,
highlighting their changes with better granularity.  Alternatively, if
you set @code{diff-refine} to the symbol @code{navigation}, Diff mode
only refines the hunk you move to with this command or with
@code{diff-hunk-prev}.

@item M-p
@findex diff-hunk-prev
Move to the previous hunk-start (@code{diff-hunk-prev}).  With prefix
argument @var{n}, move back to the @var{n}th previous hunk.  Like
@kbd{M-n}, this command refines the hunk you move to if you set
@code{diff-refine} to the symbol @code{navigation}.

@item M-@}
@findex diff-file-next
Move to the next file-start, in a multi-file patch
(@code{diff-file-next}).  With prefix argument @var{n}, move forward
to the start of the @var{n}th next file.

@item M-@{
@findex diff-file-prev
Move to the previous file-start, in a multi-file patch
(@code{diff-file-prev}).  With prefix argument @var{n}, move back to
the start of the @var{n}th previous file.

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

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

@item C-c C-a
@findex diff-apply-hunk
@cindex patches, applying
Apply this hunk to its target file (@code{diff-apply-hunk}).  With a
prefix argument of @kbd{C-u}, revert this hunk, i.e.@: apply the
reverse of the hunk, which changes the ``new'' version into the ``old''
version.  If @code{diff-jump-to-old-file} is non-@code{nil}, apply the
hunk to the ``old'' version of the file instead.

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

@vindex diff-refine
By default, Diff mode refines hunks as Emacs displays them, so you may
find this command useful if you customize @code{diff-refine} to a
non-default value.

@item C-c C-c
@findex diff-goto-source
@vindex diff-jump-to-old-file
Go to the source file and line corresponding to this hunk
(@code{diff-goto-source}).  By default, this jumps to the ``new''
version of the file, the one shown first on the file header.
With a prefix argument, jump to the ``old'' version instead.  If
@code{diff-jump-to-old-file} is non-@code{nil}, this command by
default jumps to the ``old'' file, and the meaning of the prefix
argument is reversed.  If the prefix argument is a number greater than
8 (e.g., if you type @kbd{C-u C-u C-c C-c}), then this command also
sets @code{diff-jump-to-old-file} for the next invocation.
If the source file is under version control (@pxref{Version Control}),
this jumps to the work file by default.  With a prefix argument, jump
to the ``old'' revision of the file (@pxref{Old Revisions}), when
point is on the old line, or otherwise jump to the ``new'' revision.

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

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

@item C-c C-r
@findex diff-reverse-direction
Reverse the direction of comparison for the entire buffer
(@code{diff-reverse-direction}).  With a prefix argument, reverse the
direction only inside the current region (@pxref{Mark}).  Reversing
the direction means changing the hunks and the file-start headers to
produce a patch that would change the ``new'' version into the ``old''
one.

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

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

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

@item C-c C-l
@findex diff-refresh-hunk
Re-generate the current hunk (@code{diff-refresh-hunk}).

@item C-c C-w
@findex diff-ignore-whitespace-hunk
Re-generate the current hunk, disregarding changes in whitespace
(@code{diff-ignore-whitespace-hunk}).

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

@c Trailing whitespace is NOT shown by default.
@c Emacs's dir-locals file enables this (for some reason).
@cindex trailing whitespace, in patches
@findex diff-delete-trailing-whitespace
  Patches sometimes include trailing whitespace on modified lines, as
an unintentional and undesired change.  There are two ways to deal
with this problem.  Firstly, if you enable Whitespace mode in a Diff
buffer (@pxref{Useless Whitespace}), it automatically highlights
trailing whitespace in modified lines.  Secondly, you can use the
command @kbd{M-x diff-delete-trailing-whitespace}, which searches for
trailing whitespace in the lines modified by the patch, and removes
that whitespace in both the patch and the patched source file(s).
This command does not save the modifications that it makes, so you can
decide whether to save the changes (the list of modified files is
displayed in the echo area).  With a prefix argument, it tries to
modify the original (``old'') source files rather than the patched
(``new'') source files.

@vindex diff-font-lock-syntax
  If @code{diff-font-lock-syntax} is non-@code{nil}, fragments of
source in hunks are highlighted according to the appropriate major
mode.

@node Copying and Naming
@section Copying, Naming and Renaming Files

  Emacs has several commands for copying, naming, and renaming files.
All of them read two file names, @var{old} (or @var{target}) and
@var{new}, using the minibuffer, and then copy or adjust a file's name
accordingly; they do not accept wildcard file names.

In all these commands, if the argument @var{new} is just a directory
name (@pxref{Directory Names,,, elisp, the Emacs Lisp Reference
Manual}), the real new name is in that directory, with the same
non-directory component as @var{old}.  For example, the command
@w{@kbd{M-x rename-file @key{RET} ~/foo @key{RET} /tmp/ @key{RET}}}
renames @file{~/foo} to @file{/tmp/foo}.  On GNU and other POSIX-like
systems, directory names end in @samp{/}.

All these commands ask for confirmation when the new file name already
exists.

@findex copy-file
@cindex copying files
  @kbd{M-x copy-file} copies the contents of the file @var{old} to the
file @var{new}.

@vindex copy-directory-create-symlink
@findex copy-directory
  @kbd{M-x copy-directory} copies directories, similar to the
@command{cp -r} shell command.  If @var{new} is a directory name, it
creates a copy of the @var{old} directory and puts it in @var{new}.
Otherwise it copies all the contents of @var{old} into a new directory
named @var{new}.  If @code{copy-directory-create-symlink} is
non-@code{nil} and @var{old} is a symbolic link, this command will
copy the symbolic link.  If @code{nil}, this command will follow the
link and copy the contents instead.  (This is the default.)

@cindex renaming files
@findex rename-file
  @kbd{M-x rename-file} 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.

@ifnottex
  If a file is under version control (@pxref{Version Control}), you
should rename it using @w{@kbd{M-x vc-rename-file}} instead of
@w{@kbd{M-x rename-file}}.  @xref{VC Delete/Rename}.
@end ifnottex

@findex add-name-to-file
@cindex hard links (creation)
  @kbd{M-x add-name-to-file} adds an additional name to an existing
file without removing the 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, and some remote
system types, it works by copying the file.

@findex make-symbolic-link
@cindex symbolic links (creation)
  @kbd{M-x make-symbolic-link} creates a symbolic link named
@var{new}, which points at @var{target}.  The effect is that future
attempts to open file @var{new} 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.  However, this command
does expand leading @samp{~} in @var{target} so that you can easily
specify home directories, and strips leading @samp{/:} so that you can
specify relative names beginning with literal @samp{~} or @samp{/:}.
@xref{Quoted File Names}.  On MS-Windows, this command works only on
MS Windows Vista and later.  When @var{new} is remote,
it works depending on the system type.

@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 delete-file
@cindex deletion (of files)
  @kbd{M-x delete-file} prompts for a file and deletes it.  If you are
deleting many files in one directory, it may be more convenient to use
Dired rather than @code{delete-file}.  @xref{Dired Deletion}.

@cindex trash
@cindex recycle bin
@findex move-file-to-trash
  @kbd{M-x move-file-to-trash} moves a file into the system
@dfn{Trash} (or @dfn{Recycle Bin}).  This is a facility available on
most operating systems; files that are moved into the Trash can be
brought back later if you change your mind.  (The way to restore
trashed files is system-dependent.)

@vindex delete-by-moving-to-trash
  By default, Emacs deletion commands do @emph{not} use the Trash.  To
use the Trash (when it is available) for common deletion commands,
change the variable @code{delete-by-moving-to-trash} to @code{t}.
This affects the commands @kbd{M-x delete-file} and @kbd{M-x
delete-directory} (@pxref{Directories}), as well as the deletion
commands in Dired (@pxref{Dired Deletion}).  Supplying a prefix
argument to @kbd{M-x delete-file} or @kbd{M-x delete-directory} makes
them delete outright, instead of using the Trash, regardless of
@code{delete-by-moving-to-trash}.

@ifnottex
  If a file is under version control (@pxref{Version Control}), you
should delete it using @kbd{M-x vc-delete-file} instead of @kbd{M-x
delete-file}.  @xref{VC Delete/Rename}.
@end ifnottex

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

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

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

@findex set-file-modes
@cindex file modes
@cindex file permissions
  @kbd{M-x set-file-modes} reads a file name followed by a @dfn{file
mode}, and applies that file mode to the specified file.  File modes,
also called @dfn{file permissions}, determine whether a file can be
read, written to, or executed, and by whom.  This command reads file
modes using the same symbolic or octal format accepted by the
@command{chmod} command; for instance, @samp{u+x} means to add
execution permission for the user who owns the file.  It has no effect
on operating systems that do not support file modes.  @code{chmod} is a
convenience alias for this function.

@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.
Clicking with the mouse on the file name in the Tar buffer does
likewise.  @kbd{v} extracts a file into a buffer in View mode
(@pxref{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.

  The @kbd{I} key adds a new (regular) file to the archive.  The file
is initially empty, but can readily be edited using the commands
above.  The command inserts the new file before the current one, so
that using it on the topmost line of the Tar buffer makes the new file
the first one in the archive, and using it at the end of the buffer
makes it the last one.

  @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.

  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
@pindex arc
@pindex jar
@pindex zip
@pindex rar
@pindex lzh
@pindex zoo
@pindex 7z
@cindex Java class archives
@cindex unzip archives
  A separate but similar Archive mode is used for @code{arc},
@code{jar}, @code{lzh}, @code{zip}, @code{rar}, @code{7z}, and
@code{zoo} archives, as well as @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 programs to unpack
and repack archives.  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.  Details of the program names and their
options can be set in the @samp{Archive} Customize group
(@pxref{Customization Groups}).

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

@noindent
To carry out this request, Emacs uses a remote-login program such as
@command{ssh}.
You must always specify in the file name which method to use---for
example, @file{/ssh:@var{user}@@@var{host}:@var{filename}} uses
@command{ssh}.  When you specify the pseudo method @samp{-} in the
file name, Emacs chooses the method as follows:

@enumerate
@item
If the host name starts with @samp{ftp.} (with dot), Emacs uses FTP.
@item
If the user name is @samp{ftp} or @samp{anonymous}, Emacs uses FTP.
@item
If the variable @code{tramp-default-method} is set to @samp{ftp},
Emacs uses FTP.
@item
If @command{ssh-agent} is running, Emacs uses @command{scp}.
@item
Otherwise, Emacs uses @command{ssh}.
@end enumerate

@cindex disabling remote files
@noindent
You can entirely turn off the remote file name feature by setting the
variable @code{tramp-mode} to @code{nil}.  You can turn off the
feature in individual cases by quoting the file name with @samp{/:}
(@pxref{Quoted File Names}).

@cindex @code{ange-ftp}
  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}.

@vindex ange-ftp-default-user
@cindex user name for remote file access
  When the Ange-FTP package is used, Emacs logs in through FTP using
the name @var{user}, if that is specified in the remote file name.  If
@var{user} is unspecified, Emacs logs in using your user name on the
local system; but if you set the variable @code{ange-ftp-default-user}
to a string, that string is used instead.  When logging in, Emacs may
also ask for a password.

@cindex backups for remote files
@vindex ange-ftp-make-backup-files
  For performance reasons, Emacs does not make backup files for files
accessed via FTP by default.  To make it do so, change the variable
@code{ange-ftp-make-backup-files} to a non-@code{nil} value.

  By default, auto-save files for remote files are made in the
temporary file directory on the local machine, as specified by the
variable @code{auto-save-file-name-transforms}.  @xref{Auto Save
Files}.

@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 (@pxref{Passwords}).

@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}}.

@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}.

  If you want to quote only special characters in the local part of a
remote file name, you can quote just the local part.
@samp{/ssh:baz:/:/foo:/bar} refers to the file @file{bar} of directory
@file{/foo:} on the host @file{baz}.

  @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-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
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 whose value
is a list of directories, like @code{load-path}.
@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.

@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{recentf-list} to a file, and @kbd{M-x recentf-edit-list} edits
it.

@c FIXME partial-completion-mode (complete.el) is obsolete.
  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}.

@node Image Mode
@section Viewing Image Files

@findex image-mode
@findex image-toggle-display
@cindex images, viewing
  Visiting image files automatically selects Image mode.  In this
major mode, you can type @kbd{C-c C-c} (@code{image-toggle-display})
to toggle between displaying the file as an image in the Emacs buffer,
and displaying its underlying text (or raw byte) representation.
Additionally you can type @kbd{C-c C-x} (@code{image-toggle-hex-display})
to toggle between displaying the file as an image in the Emacs buffer,
and displaying it in hex representation.  Displaying the file as an
image works only if Emacs is compiled with support for displaying
such images.

@vindex image-auto-resize
@vindex image-auto-resize-on-window-resize
If the displayed image is wider or taller than the window in which it
is displayed, the usual point motion keys (@kbd{C-f}, @kbd{C-p}, and
so forth) cause different parts of the image to be displayed.
However, by default images are resized automatically to fit the
window, so this is only necessary if you customize the default
behavior by using the options @code{image-auto-resize} and
@code{image-auto-resize-on-window-resize}.

@findex image-transform-fit-to-window
@findex image-transform-set-scale
@findex image-transform-reset
To resize the image manually you can use the command
@code{image-transform-fit-to-window} bound to @kbd{s w}
that fits the image to both the window height and width.
To scale the image specifying a scale factor, use the command
@code{image-transform-set-scale} bound to @kbd{s s}.
To reset all transformations to the initial state, use
@code{image-transform-reset} bound to @kbd{s 0}.

@findex image-next-file
@findex image-previous-file
You can press @kbd{n} (@code{image-next-file}) and @kbd{p}
(@code{image-previous-file}) to visit the next image file and the
previous image file in the same directory, respectively.  These
commands will consult the ``parent'' dired buffer to determine what
the next/previous image file is.  These commands also work when
opening a file from archive files (like zip or tar files), and will
then instead consult the archive mode buffer.  If neither an archive
nor a dired ``parent'' buffer can be found, a dired buffer is opened.

@findex image-mode-mark-file
@findex image-mode-unmark-file
@findex image-mode-copy-file-name-as-kill
  When looking through images, it's sometimes convenient to be able to
mark the files for later processing (for instance, if you want to
select a group of images to copy somewhere else).  The @kbd{m}
(@code{image-mode-mark-file}) command will mark the current file in
any Dired buffer(s) that display the current file's directory.  If no
such buffer is open, the directory is opened in a new buffer.  To
unmark files, use the @kbd{u} (@code{image-mode-mark-file}) command.
Finally, if you just want to copy the current buffers file name to the
kill ring, you can use the @kbd{w}
(@code{image-mode-copy-file-name-as-kill}) command.

@findex image-toggle-animation
@findex image-next-frame
@findex image-previous-frame
@findex image-goto-frame
@findex image-increase-speed
@findex image-decrease-speed
@findex image-reset-speed
@findex image-reverse-speed
@vindex image-animate-loop
@cindex image animation
@cindex animated images
  If the image can be animated, the command @key{RET}
(@code{image-toggle-animation}) starts or stops the animation.
Animation plays once, unless the option @code{image-animate-loop} is
non-@code{nil}.  With @kbd{f} (@code{image-next-frame}) and @kbd{b}
(@code{image-previous-frame}) you can step through the individual
frames.  Both commands accept a numeric prefix to step through several
frames at once.  You can go to a specific frame with @kbd{F}
(@code{image-goto-frame}).  Frames are indexed from 1.  Typing @kbd{a
+} (@code{image-increase-speed}) increases the speed of the animation,
@kbd{a -} (@code{image-decrease-speed}) decreases it, and @kbd{a r}
(@code{image-reverse-speed}) reverses it.  The command @kbd{a 0}
(@code{image-reset-speed}) resets the speed to the original value.

@cindex ImageMagick support
@vindex imagemagick-enabled-types
@vindex imagemagick-types-inhibit
  If Emacs was compiled with support for the ImageMagick library, it
can use ImageMagick to render a wide variety of images.  The variable
@code{imagemagick-enabled-types} lists the image types that Emacs may
render using ImageMagick; each element in the list should be an
internal ImageMagick name for an image type, as a symbol or an
equivalent string (e.g., @code{BMP} for @file{.bmp} images).  To
enable ImageMagick for all possible image types, change
@code{imagemagick-enabled-types} to @code{t}.  The variable
@code{imagemagick-types-inhibit} lists the image types which should
never be rendered using ImageMagick, regardless of the value of
@code{imagemagick-enabled-types} (the default list includes types like
@code{C} and @code{HTML}, which ImageMagick can render as an image
but Emacs should not).  To disable ImageMagick entirely, change
@code{imagemagick-types-inhibit} to @code{t}.

@vindex image-use-external-converter
  If Emacs doesn't have native support for the image format in
question, and @code{image-use-external-converter} is non-@code{nil},
Emacs will try to determine whether there are external utilities that
can be used to transform the image in question to @acronym{PNG} before
displaying.  GraphicsMagick, ImageMagick and @command{ffmpeg} are
currently supported for image conversions.

@findex thumbs-mode
@cindex mode, Thumbs
  The Image-Dired package can also be used to view images as
thumbnails.  @xref{Image-Dired}.

@node Filesets
@section Filesets
@cindex filesets
@cindex sets of files

@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 init file (@pxref{Init File}).  This
adds a @samp{Filesets} sub-menu to the menu bar's @samp{File} menu.

@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 @key{RET} @var{name} @key{RET}}.  If
there is no fileset @var{name}, this creates a new one, which
initially contains 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}).  Normally, a fileset is a simple list of
files, but you can also define a fileset as a 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.

   @xref{Version Control}, for a different concept of filesets:
groups of files bundled together for version control operations.
Filesets of that type are unnamed, and do not persist across Emacs
sessions.
