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@c This is part of the Emacs manual.
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@c Copyright (C) 1985-1987, 1993-1995, 1997, 1999-2011
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@c   Free Software Foundation, Inc.
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@c See file emacs.texi for copying conditions.
@node Files, Buffers, Keyboard Macros, Top
@chapter File Handling
@cindex files

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

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

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

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

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

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@cindex default file name
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  Many Emacs commands that operate on a file require you to specify
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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
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@code{completion-ignored-extensions} (@pxref{Completion Options}).
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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.
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@cindex default directory
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@vindex default-directory
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@vindex insert-default-directory
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  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.
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@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
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
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@file{/u/rms/.login}; and entering @samp{new/foo} specifies
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@file{/u/rms/gnu/new/foo}.

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  When typing a file name into the minibuffer, you can make use of a
couple of shortcuts: a double slash is interpreted as ``ignore
everything before the second slash in the pair,'' and @samp{~/} is
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interpreted as your home directory.  @xref{Minibuffer File}.
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@cindex environment variables in file names
@cindex expansion of environment variables
@cindex @code{$} in file names
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  @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{$} stands
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for itself.  Note that environment variables affect Emacs only if they
are applied before Emacs is started.
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  To access a file with @samp{$} in its name, if the @samp{$} causes
expansion, type @samp{$$}.  This pair is converted to a single
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@samp{$} at the same time that variable substitution is performed for
a single @samp{$}.  Alternatively, quote the whole file name with
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@samp{/:} (@pxref{Quoted File Names}).  File names which begin with a
literal @samp{~} should also be quoted with @samp{/:}.

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

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@kindex C-x C-f
@findex find-file
  To visit a file, type @kbd{C-x C-f} (@code{find-file}) and use the
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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.
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  If the specified file exists but the system does not allow you to
read it, an error message is displayed in the echo area.  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 append
@samp{<2>}, @samp{<3>}, and so on, but you can select other methods.
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@xref{Uniquify}.

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@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.
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@cindex modified (buffer)
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  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.
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@vindex large-file-warning-threshold
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@cindex file, warning when size is large
@cindex size of file, warning when visiting
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@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
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about 10 megabytes), Emacs asks you for confirmation first.  You can
answer @kbd{y} to proceed with visiting the file.  Note, however, that
Emacs cannot visit files that are larger than the maximum Emacs buffer
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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.
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@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}.

@kindex C-x C-v
@findex find-alternate-file
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  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.
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@vindex find-file-run-dired
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  If you ``visit'' a file that is actually a directory, Emacs invokes
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Dired, the Emacs directory browser.  @xref{Dired}.  You can disable
this behavior by setting the variable @code{find-file-run-dired} to
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@code{nil}; in that case, it is an error to try to visit a directory.
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  Files which are actually collections of other files, or @dfn{file
archives}, are visited in special modes which invoke a Dired-like
environment to allow operations on archive members.  @xref{File
Archives}, for more about these features.

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

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

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

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

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@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}.

  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' 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}.
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@findex find-file-literally
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  If you wish to edit a file as a sequence of @acronym{ASCII}
characters with no special encoding or conversion, use the @kbd{M-x
find-file-literally} command.  This visits a file, like @kbd{C-x C-f},
but does not do format conversion (@pxref{Formatted Text}), character
code conversion (@pxref{Coding Systems}), or automatic uncompression
(@pxref{Compressed Files}), and does not add a final newline because
of @code{require-final-newline} (@pxref{Customize Save}).  If you have
already visited the same file in the usual (non-literal) manner, this
command asks you whether to visit it literally instead.
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@vindex find-file-hook
@vindex find-file-not-found-functions
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  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.
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  Successful visiting of any file, whether existing or not, calls the
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functions in @code{find-file-hook}, with no arguments.  This variable
is a normal hook.  In the case of a nonexistent file, the
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@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
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Save the current buffer to its file (@code{save-buffer}).
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@item C-x s
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Save any or all buffers to their files (@code{save-some-buffers}).
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@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
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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:
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@example
(No changes need to be saved)
@end example

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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}.

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

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

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

@kindex M-~
@findex not-modified
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  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.)
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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
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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.)
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@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
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  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.
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@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
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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}).
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  If the new file name implies a major mode, then @kbd{C-x C-w} switches
to that major mode, in most cases.  The command
@code{set-visited-file-name} also does this.  @xref{Choosing Modes}.

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

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

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

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  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.

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  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.
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@xref{Backup Names}.
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@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}.

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  You can explicitly tell Emacs to make another backup file from a
buffer, even though that buffer has been saved before.  If you save
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the buffer with @kbd{C-u C-x C-s}, the version thus saved will be made
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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.
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@menu
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* 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.
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@end menu

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@node Backup Names
@subsubsection Single or Numbered Backups

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

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

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

@noindent
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The usual way to set this variable is globally, through your init file
or the customization buffer.  However, you can set
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@code{version-control} locally in an individual buffer to control the
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making of backups for that buffer's file (@pxref{Locals}).  You can
have Emacs set @code{version-control} locally whenever you visit a
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given file (@pxref{File Variables}).  Some modes, such as Rmail mode,
set this variable.
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@cindex @env{VERSION_CONTROL} environment variable
  If you set the environment variable @env{VERSION_CONTROL}, to tell
various GNU utilities what to do with backup files, Emacs also obeys the
environment variable by setting the Lisp variable @code{version-control}
accordingly at startup.  If the environment variable's value is @samp{t}
or @samp{numbered}, then @code{version-control} becomes @code{t}; if the
value is @samp{nil} or @samp{existing}, then @code{version-control}
becomes @code{nil}; if it is @samp{never} or @samp{simple}, then
@code{version-control} becomes @code{never}.

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

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

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

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

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

@node Backup Copying
@subsubsection Copying vs.@: Renaming

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

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

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

@node Customize Save
@subsection Customizing Saving of Files

@vindex require-final-newline
  If the value of the variable @code{require-final-newline} is
@code{t}, saving or writing a file silently puts a newline at the end
if there isn't already one there.  If the value is @code{visit}, Emacs
adds a newline at the end of any file that doesn't have one, just
after it visits the file.  (This marks the buffer as modified, and you
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can undo it.)  If the value is @code{visit-save}, Emacs adds such
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newlines both on visiting and on saving.  If the value is @code{nil},
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Emacs leaves the end of the file unchanged; any other non-@code{nil}
value means to asks you whether to add a newline.  The default is
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@code{nil}.

@vindex mode-require-final-newline
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  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.
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@vindex write-region-inhibit-fsync
  When Emacs saves a file, it invokes the @code{fsync} system call to
force the data immediately out to disk.  This is important for safety
if the system crashes or in case of power outage.  However, it can be
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disruptive on laptops using power saving, as it may force a disk
spin-up each time you save a file.  If you accept an increased risk of
data loss, you can set @code{write-region-inhibit-fsync} to a
non-@code{nil} value to disable the synchronization.
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@node Interlocking
@subsection Protection against Simultaneous Editing

@cindex file dates
@cindex simultaneous editing
  Simultaneous editing occurs when two users visit the same file, both
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make changes, and then both save them.  If nobody is informed that
this is happening, whichever user saves first would later find that
his changes were lost.
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  On some systems, Emacs notices immediately when the second user starts
to change the file, and issues an immediate warning.  On all systems,
Emacs checks when you save the file, and warns if you are about to
overwrite another user's changes.  You can prevent loss of the other
user's work by taking the proper corrective action instead of saving the
file.

@findex ask-user-about-lock
@cindex locking files
  When you make the first modification in an Emacs buffer that is
visiting a file, Emacs records that the file is @dfn{locked} by you.
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(It does this by creating a specially-named symbolic link in the same
directory.)  Emacs removes the lock when you save the changes.  The
idea is that the file is locked whenever an Emacs buffer visiting it
has unsaved changes.
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@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
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spurious collisions.  When you determine that the collision is
spurious, just use @kbd{p} to tell Emacs to go ahead anyway.
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  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 the system does not support symbolic
links.  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}.
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@node File Shadowing
@subsection Shadowing Files
@cindex shadow files
@cindex file shadows
@findex shadow-initialize

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

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

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

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

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

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

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You can arrange to put a time stamp in a file, so that it is updated
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automatically each time you edit and save the file.  The time stamp
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must be in the first eight lines of the file, and you should insert it
like this:
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@example
Time-stamp: <>
@end example

@noindent
or like this:

@example
Time-stamp: " "
@end example

@findex time-stamp
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  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.  For other customizations, see the
Custom group @code{time-stamp}.  Note that the time stamp is formatted
according to your locale setting (@pxref{Environment}).
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@node Reverting
@section Reverting a Buffer
@findex revert-buffer
@cindex drastic changes
@cindex reread a file

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  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{M-x
revert-buffer}.  Since reverting unintentionally could lose a lot of
work, Emacs asks for confirmation first.
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  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.
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  Reverting marks the buffer as ``not modified''.  It also clears the
buffer's undo history (@pxref{Undo}).  Thus, the reversion cannot be
undone---if you change your mind yet again, you can't use the undo
commands to bring the reverted changes back.
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  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.
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@vindex revert-without-query
  When you edit a file that changes automatically and frequently---for
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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
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regular expressions, @code{find-file} and @code{revert-buffer} will
revert it automatically if it has changed---provided the buffer itself
is not modified.  (If you have edited the text, it would be wrong to
discard your changes.)

@cindex Global Auto-Revert mode
@cindex mode, Global Auto-Revert
@cindex Auto-Revert mode
@cindex mode, Auto-Revert
@findex global-auto-revert-mode
@findex auto-revert-mode
@findex auto-revert-tail-mode
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@vindex auto-revert-interval
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  You can also tell Emacs to revert buffers periodically.  To do this
for a specific buffer, enable the minor mode Auto-Revert mode by
typing @kbd{M-x auto-revert-mode}.  This automatically reverts the
current buffer every five seconds; you can change the interval through
the variable @code{auto-revert-interval}.  To do the same for all file
buffers, type @kbd{M-x global-auto-revert-mode} to enable Global
Auto-Revert mode.  These minor modes do not check or revert remote
files, because that is usually too slow.
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  One use of Auto-Revert mode is to ``tail'' a file such as a system
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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.
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Auto-Revert Tail mode works also for remote files.
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  @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.
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@ifnottex
@include arevert-xtra.texi
@end ifnottex

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

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

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

@node Auto Save Files
@subsection Auto-Save Files

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  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}).
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  Normally, the auto-save file name is made by appending @samp{#} to the
front and rear of the visited file name.  Thus, a buffer visiting file
@file{foo.c} is auto-saved in a file @file{#foo.c#}.  Most buffers that
are not visiting files are auto-saved only if you request it explicitly;
when they are auto-saved, the auto-save file name is made by appending
@samp{#} to the front and rear of buffer name, then
adding digits and letters at the end for uniqueness.  For
example, the @samp{*mail*} buffer in which you compose messages to be
sent might be auto-saved in a file named @file{#*mail*#704juu}.  Auto-save file
names are made this way unless you reprogram parts of Emacs to do
something different (the functions @code{make-auto-save-file-name} and
@code{auto-save-file-name-p}).  The file name to be used for auto-saving
in a buffer is calculated when auto-saving is turned on in that buffer.

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

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

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

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

@node Auto Save Control
@subsection Controlling Auto-Saving

@vindex auto-save-default
@findex auto-save-mode
  Each time you visit a file, auto-saving is turned on for that file's
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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}).
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@vindex auto-save-interval
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  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.
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@vindex auto-save-timeout
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  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.
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  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
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  You can perform an auto-save explicitly with the command @kbd{M-x
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do-auto-save}.

@node Recover
@subsection Recovering Data from Auto-Saves

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

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

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

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

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

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

@vindex auto-save-list-file-prefix
  Emacs records information about interrupted sessions for later
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recovery 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.
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@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}.

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@cindex directory name abbreviation
@vindex directory-abbrev-alist
  Sometimes, a directory is ordinarily accessed through a symbolic
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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}:
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@example
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(("\\`/home/fsf" . "/fsf"))
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@end example

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@node Directories
@section File Directories

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

@table @kbd
@item C-x C-d @var{dir-or-pattern} @key{RET}
Display a brief directory listing (@code{list-directory}).
@item C-u C-x C-d @var{dir-or-pattern} @key{RET}
Display a verbose directory listing.
@item M-x make-directory @key{RET} @var{dirname} @key{RET}
Create a new directory named @var{dirname}.
@item M-x delete-directory @key{RET} @var{dirname} @key{RET}
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Delete the directory named @var{dirname}.  If it isn't empty,
you will be asked whether you want to delete it recursively.
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@end table

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

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

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

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

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

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

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

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  The command @kbd{M-x delete-directory} prompts for a directory name
using the minibuffer, and deletes the directory if it is empty.  If
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the directory is not empty, you will be asked whether you want to
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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.
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@node Comparing Files
@section Comparing Files
@cindex comparing files

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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@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.

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@item C-c C-c
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@findex diff-goto-source
Go to the source file and line corresponding to this hunk
(@code{diff-goto-source}).
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@item C-c C-e
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@findex diff-ediff-patch
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Start an Ediff session with the patch (@code{diff-ediff-patch}).
@xref{Top, Ediff, Ediff, ediff, The Ediff Manual}.

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

@item C-c C-s
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@findex diff-split-hunk
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Split the hunk at point (@code{diff-split-hunk}).  This is for
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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}
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(@code{diff-unified->context}).  With a prefix argument, convert only
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the text within the region.
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@item C-c C-u
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@findex diff-context->unified
Convert the entire buffer to unified diff format
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(@code{diff-context->unified}).  With a prefix argument, convert
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unified format to context format.  When the mark is active, convert
only the text within the region.
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@item C-c C-w
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@findex diff-refine-hunk
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Refine the current hunk so that it disregards changes in whitespace
(@code{diff-refine-hunk}).
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@item C-x 4 A
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@findex add-change-log-entry-other-window@r{, in Diff mode}
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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.
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@item M-x diff-show-trailing-whitespaces RET
@findex diff-show-trailing-whitespaces
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Highlight trailing whitespace characters, except for those used by the
patch syntax (@pxref{Useless Whitespace}).
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@end table


@node Misc File Ops
@section Miscellaneous File Operations

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

@findex delete-file
@cindex deletion (of files)
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  @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
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Dired rather than @code{delete-file}.  @xref{Dired Deletion}.
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@cindex trash
@cindex recycle bin
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  @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.
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@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
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@code{delete-by-moving-to-trash}.

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

@findex copy-directory
  @kbd{M-x copy-directory} copies directories, similar to the
@command{cp -r} shell command.  It prompts for a directory @var{old}
and a destination @var{new}.  If @var{new} is an existing directory,
it creates a copy of the @var{old} directory and puts it in @var{new}.
If @var{new} is not an existing directory, it copies all the contents
of @var{old} into a new directory named @var{new}.
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@findex rename-file
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  @kbd{M-x rename-file} reads two file names @var{old} and @var{new}
using the minibuffer, then renames file @var{old} as @var{new}.  If
the file name @var{new} already exists, you must confirm with
@kbd{yes} or renaming is not done; this is because renaming causes the
old meaning of the name @var{new} to be lost.  If @var{old} and
@var{new} are on different file systems, the file @var{old} is copied
and deleted.  If the argument @var{new} is just a directory name, the
real new name is in that directory, with the same non-directory
component as @var{old}.  For example, @kbd{M-x rename-file RET ~/foo
RET /tmp RET} renames @file{~/foo} to @file{/tmp/foo}.  The same rule
applies to all the remaining commands in this section.  All of them
ask for confirmation when the new file name already exists, too.
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@findex add-name-to-file
@cindex hard links (creation)
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  @kbd{M-x add-name-to-file} adds an additional name to an existing
file without removing its old name.  The new name is created as a
``hard link'' to the existing file.  The new name must belong on the
same file system that the file is on.  On MS-Windows, this command
works only if the file resides in an NTFS file system.  On MS-DOS, it
works by copying the file.
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@findex make-symbolic-link
@cindex symbolic links (creation)
  @kbd{M-x make-symbolic-link} reads two file names @var{target} and
@var{linkname}, then creates a symbolic link named @var{linkname},
which points at @var{target}.  The effect is that future attempts to
open file @var{linkname} will refer to whatever file is named
@var{target} at the time the opening is done, or will get an error if
the name @var{target} is nonexistent at that time.  This command does
not expand the argument @var{target}, so that it allows you to specify
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a relative name as the target of the link.  Not all systems support
symbolic links; on systems that don't support them, this command is
not defined.

@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}).
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@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}.
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@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
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on operating systems that do not support file modes.  @code{chmod} is a
convenience alias for this function.
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