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File: emacs,  Node: Spelling,  Prev: Fixing Case,  Up: Fixit
Checking and Correcting Spelling
================================
   This section describes the commands to check the spelling of a single
word or of a portion of a buffer.
`M-$'
     Check and correct spelling of word at point (`ispell-word').
`M-x ispell-buffer'
     Check and correct spelling of each word in the buffer.
`M-x ispell-region'
     Check and correct spelling of each word in the region.
`M-x ispell-string RET WORD RET'
     Check spelling of WORD.
`M-x reload-ispell'
     Make the Ispell subprocess reread your private dictionary.
`M-x kill-ispell'
     Kill the Ispell subprocess.
   To check the spelling of the word around or next to point, and
optionally correct it as well, use the command `M-$' (`ispell-word').
If the word is not correct, the command offers you various alternatives
for what to do about it.
   To check the entire current buffer, use `M-x ispell-buffer'.  Use
`M-x ispell-region' to check just the current region.  Each time these
commands encounter an incorrect word, they ask you what to do.
   Whenever one of these commands finds an incorrect word, it displays a
list of alternatives, usually including several "near-misses"--words
that are close to the word being checked.  Here are the valid responses:
`SPC'
     Skip this word--continue to consider it incorrect, but don't
     change it here.
`r NEW RET'
     Replace the word (just this time) with NEW.
`DIGIT'
     Replace the word (just this time) with one of the displayed
     near-misses.  Each near-miss is listed with a digit; type that
     digit to select it.
     Accept the incorrect word--treat it as correct, but only in this
     editing session.
     Insert this word in your private dictionary file so that Ispell
     will consider it correct it from now on, even in future sessions.
`l REGEXP RET'
     Look in the dictionary for words that match REGEXP.  These words
     become the new list of "near-misses"; you can select one of them to
     replace with by typing a digit.
`C-g'
     Quit interactive spell checking.  You can restart it again
     afterward with `C-u M-$'.
   The first time you use any of the spell checking commands, it starts
an Ispell subprocess.  The first thing the subprocess does is read your
private dictionary, which is the file `~/ispell.words'.  Words that you
"insert" with the `i' command are added to that file, but not right
away--only at the end of the interactive replacement procedure.  Use
the `M-x reload-ispell' command to reload your private dictionary from
`~/ispell.words' if you edit the file outside of Ispell.
   Once started, the Ispell subprocess continues to run (waiting for
something to do), so that subsequent spell checking commands complete
more quickly.  If you want to get rid of the Ispell process, use `M-x
kill-ispell'.  This is not usually necessary, since the process uses no
time except when you do spelling correction.
File: emacs,  Node: Files,  Next: Buffers,  Prev: Fixit,  Up: Top
File Handling
*************
   The basic unit of stored data in Unix is the "file".  To edit a file,
you must tell Emacs to examine the file and prepare a buffer containing
a copy of the file's text.  This is called "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
"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 Save::        Auto Save periodically protects against loss of data.
* File Aliases::     Handling multiple names for one file.
* Version Control::  Version control systems (RCS and SCCS).
* ListDir::          Listing the contents of a file directory.
* Comparing Files::  Finding where two files differ.
* Misc File Ops::    Other things you can do on files.
File: emacs,  Node: File Names,  Next: Visiting,  Up: Files
File Names
==========
   Most Emacs commands that operate on a file require you to specify the
file name.  (Saving and reverting are exceptions; the buffer knows which
file name to use for them.)  You enter the file name using the
minibuffer (*note Minibuffer::.).  "Completion" is available, to make
it easier to specify long file names.  *Note Completion::.
   For most operations, there is a "default file name" which is used if
you type just RET to enter an empty argument.  Normally the default
file name is the name of the file visited in the current buffer; this
makes it easy to operate on that file with any of the Emacs file
commands.
   Each buffer has a default directory, normally the same as the
directory of the file visited in that buffer.  When you enter a file
name without a directory, the default directory is used.  If you specify
a directory in a relative fashion, with a name that does not start with
a slash, it is interpreted with respect to the default directory.  The
default directory is kept in the variable `default-directory', which
has a separate value in every buffer.
   For example, if the default file name is `/u/rms/gnu/gnu.tasks' then
the default directory is `/u/rms/gnu/'.  If you type just `foo', which
does not specify a directory, it is short for `/u/rms/gnu/foo'.
`../.login' would stand for `/u/rms/.login'.  `new/foo' would stand for
the file name `/u/rms/gnu/new/foo'.
   The command `M-x pwd' prints the current buffer's default directory,
and the command `M-x cd' sets it (to a value read using the
minibuffer).  A buffer's default directory changes only when the `cd'
command is used.  A file-visiting buffer's default directory is
initialized to the directory of the file that is visited there.  If you
create a buffer with `C-x b', its default directory is copied from that
of the buffer that was current at the time.
   The default directory actually appears in the minibuffer when the
minibuffer becomes active to read a file name.  This serves two
purposes: it *shows* you what the default is, so that you can type a
relative file name and know with certainty what it will mean, and it
allows you to *edit* the default to specify a different directory.
This insertion of the default directory is inhibited if the variable
`insert-default-directory' is set to `nil'.
   Note that it is legitimate to type an absolute file name after you
enter the minibuffer, ignoring the presence of the default directory
name as part of the text.  The final minibuffer contents may look
invalid, but that is not so.  For example, if the minibuffer starts out
with `/usr/tmp/' and you add `/x1/rms/foo', the double slash says to
ignore the default directory and use just the name that you typed.
*Note Minibuffer File::.
   You can refer to files on other machines using a special file name
syntax:
     /HOST:FILENAME
     /USER@HOST:FILENAME
   When you do this, Emacs uses the FTP program to read and write files
on the specified host.  It logs in through FTP using your user name or
the name USER.  It may ask you for a password from time to time; this
is used for logging in on HOST.
   `$' in a file name is used to substitute environment variables.  For
example, if you have used the shell command `setenv FOO rms/hacks' to
set up an environment variable named `FOO', then you can use
`/u/$FOO/test.c' or `/u/${FOO}/test.c' as an abbreviation for
`/u/rms/hacks/test.c'.  The environment variable name consists of all
the alphanumeric characters after the `$'; alternatively, it may be
enclosed in braces after the `$'.  Note that the `setenv' command
affects Emacs only if done before Emacs is started.
   To access a file with `$' in its name, type `$$'.  This pair is
converted to a single `$' at the same time as variable substitution is
performed for single `$'.  The Lisp function that performs the
substitution is called `substitute-in-file-name'.  The substitution is
performed only on file names read as such using the minibuffer.
File: emacs,  Node: Visiting,  Next: Saving,  Prev: File Names,  Up: Files
Visiting Files
==============
`C-x C-f'
     Visit a file (`find-file').
`C-x C-r'
     Visit a file for viewing, without allowing changes to it
     (`find-file-read-only').
`C-x C-v'
     Visit a different file instead of the one visited last
     (`find-alternate-file').
`C-x 4 C-f'
     Visit a file, in another window (`find-file-other-window').  Don't
     change the selected window.
`C-x 5 C-f'
     Visit a file, in a new frame (`find-file-other-frame').  Don't
     change the selected frame.
   "Visiting" a file means copying its contents into an Emacs buffer so
you can edit them.  Emacs makes a new buffer for each file that you
visit.  We say that this buffer is visiting the file that it was created
to hold.  Emacs constructs the buffer name from the file name by
throwing away the directory, keeping just the name proper.  For example,
a file named `/usr/rms/emacs.tex' would get a buffer named `emacs.tex'.
If there is already a buffer with that name, a unique name is
constructed by appending `<2>', `<3>', or so on, using the lowest
number that makes a name that is not already in use.
   Each window's mode line shows the name of the buffer that is being
displayed in that window, so you can always tell what buffer you are
editing.
   The changes you make with Emacs are made in the Emacs buffer.  They
do not take effect in the file that you visited, or any place
permanent, until you "save" the buffer.  Saving the buffer means that
Emacs writes the current contents of the buffer into its visited file.
*Note Saving::.
   If a buffer contains changes that have not been saved, we say the
buffer is "modified".  This is important because it implies that some
changes will be lost if the buffer is not saved.  The mode line
displays two stars near the left margin to indicate that the buffer is
modified.
   To visit a file, use the command `C-x C-f' (`find-file').  Follow
the command with the name of the file you wish to visit, terminated by a
   The file name is read using the minibuffer (*note Minibuffer::.),
with defaulting and completion in the standard manner (*note File
Names::.).  While in the minibuffer, you can abort `C-x C-f' by typing
`C-g'.
   Your confirmation that `C-x C-f' has completed successfully is the
appearance of new text on the screen and a new buffer name in the mode
line.  If the specified file does not exist and could not be created, or
cannot be read, then you get an error, with an error message displayed
in the echo area.
   If you visit a file that is already in Emacs, `C-x C-f' does not make
another copy.  It selects the existing buffer containing that file.
However, before doing so, it checks that the file itself has not changed
since you visited or saved it last.  If the file has changed, a warning
message is printed.  *Note Simultaneous Editing: Interlocking.
   What if you want to create a new file?  Just visit it.  Emacs prints
`(New File)' in the echo area, but in other respects behaves as if you
had visited an existing empty file.  If you make any changes and save
them, the file is created.
   If the file you specify is actually a directory, Dired is called on
that directory (*note Dired::.).  This can be inhibited by setting the
variable `find-file-run-dired' to `nil'; then it is an error to try to
visit a directory.
   If you visit a file that the operating system won't let you modify,
Emacs makes the buffer read-only, so that you won't go ahead and make
changes that you'll have trouble saving afterward.  You can make the
buffer writable with `C-x C-q' (`toggle-read-only').  *Note Misc
Buffer::.
   Occasionally you might want to visit a file as read-only in order to
protect yourself from entering changes accidentally; do so by visiting
the file with the command `C-x C-r' (`find-file-read-only').
   If you visit a nonexistent file unintentionally (because you typed
the wrong file name), use the `C-x C-v' command (`find-alternate-file')
to visit the file you really wanted.  `C-x C-v' is similar to `C-x
C-f', but it kills the current buffer (after first offering to save it
if it is modified).  When it 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.
   `C-x 4 f' (`find-file-other-window') is like `C-x C-f' except that
the buffer containing the specified file is selected in another window.
The window that was selected before `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.  *Note Windows::.
   `C-x 5 f' (`find-file-other-frame') is similar, but opens a new
frame.  This feature is available only when you are using a window
system.  *Note Frames::.
   Two special hook variables allow extensions to modify the operation
of visiting files.  Visiting a file that does not exist runs the
functions in the list `find-file-not-found-hooks'; this variable holds
a list of functions, and the functions are called one by one until one
of them returns non-`nil'.  Any visiting of a file, whether extant or
not, expects `find-file-hooks' to contain a list of functions and calls
them all, one by one.  In both cases the functions receive no
arguments.  Of these two variables, `find-file-not-found-hooks' takes
effect first.  These variables are *not* normal hooks, and their names
end in `-hooks' rather than `-hook' to indicate that fact.
   There are several ways to specify automatically the major mode for
editing the file (*note Choosing Modes::.), and to specify local
variables defined for that file (*note File Variables::.).
File: emacs,  Node: Saving,  Next: Reverting,  Prev: Visiting,  Up: Files
Saving Files
============
   "Saving" a buffer in Emacs means writing its contents back into the
file that was visited in the buffer.
`C-x C-s'
     Save the current buffer in its visited file (`save-buffer').
`C-x s'
     Save any or all buffers in their visited files
     (`save-some-buffers').
`M-~'
     Forget that the current buffer has been changed (`not-modified').
`C-x C-w'
     Save the current buffer in a specified file (`write-file').
`M-x set-visited-file-name'
     Change file the name under which the current buffer will be saved.
   When you wish to save the file and make your changes permanent, type
`C-x C-s' (`save-buffer').  After saving is finished, `C-x C-s' prints
a message such as
     Wrote /u/rms/gnu/gnu.tasks
If the selected buffer is not modified (no changes have been made in it
since the buffer was created or last saved), saving is not really done,
because it would have no effect.  Instead, `C-x C-s' prints a message
in the echo area saying
     (No changes need to be written)
   The command `C-x s' (`save-some-buffers') offers to save any or all
modified buffers.  It asks you what to do with each buffer.  The
options are analogous to those of `query-replace':
     Save this buffer and ask about the rest of the buffers.
     Don't save this buffer, but ask about the rest of the buffers.
     Save this buffer and all the rest with no more questions.
`ESC'
     Terminate `save-some-buffers' without any more saving.
     Save this buffer, then exit `save-some-buffers' without even asking
     about other buffers.
`C-r'
     View the buffer that you are currently being asked about.  When
     you exit View mode, you get back to `save-some-buffers', which
     asks the question again.
`C-h'
     Display a help message about these options.
   `C-x C-c', the key sequence to exit Emacs, invokes
`save-some-buffers' and therefore asks the same questions.
   If you have changed a buffer and do not want the changes to be saved,
you should take some action to prevent it.  Otherwise, each time you use
`C-x s' or `C-x C-c', you are liable to save it by mistake.  One thing
you can do is type `M-~' (`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.  (`~' is
often used as a mathematical symbol for `not'; thus `M-~' is `not',
metafied.)  You could also use `set-visited-file-name' (see below) to
mark the buffer as visiting a different file name, one which is not in
use for anything important.  Alternatively, you can cancel all the
changes made since the file was visited or saved, by reading the text
from the file again.  This is called "reverting".  *Note Reverting::.
You could also undo all the changes by repeating the undo command `C-x
u' until you have undone all the changes; but reverting is easier.
   `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 specifies the visited file name and changes the
buffer name correspondingly (as long as the new name is not in use).
`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 `C-x C-s' in
that buffer *will* save.
   If you wish to mark the buffer as visiting a different file and save
it right away, use `C-x C-w' (`write-file').  It is precisely
equivalent to `set-visited-file-name' followed by `C-x C-s'.  `C-x C-s'
used on a buffer that is not visiting with a file has the same effect
as `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.
   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.
*Note Simultaneous Editing: Interlocking.
   If the variable `require-final-newline' is non-`nil', Emacs puts a
newline at the end of any file that doesn't already end in one, every
time a file is saved or written.
   You can implement other ways to write files, and other things to be
done before writing them, using the variable `write-file-hooks'.  The
value of this variable should be a list of Lisp functions.  When a file
is to be written, the functions in the list are called, one by one,
with no arguments.  If one of them returns a non-`nil' value, Emacs
takes this to mean that the file has been written in some suitable
fashion; the rest of the functions are not called, and normal writing
is not done.  If this variable is local to a buffer, changing major
modes does not clear it.  However, changing the visited file name does
clear this variable to `nil'.
   The list `local-write-file-hooks' is used just like
`write-file-hooks'.  This list is for use as a local variable.  It is a
permanent local, so that switching major modes does not affect it.
   The variable `write-contents-hooks' holds another list of functions
to be called before writing out a buffer to a file.  These functions
are used just like the ones in `write-file-hooks'.  The difference
between this and `write-file-hooks' is that changing the major mode
does clear this variable, and setting the visited file name does not
clear it.
   The hook variables described here are not normal hooks, because the
values returned by the hook functions do matter.  This is why they have
names ending in `-hooks' rather than `-hook'.  *Note Hooks::.
* Menu:
* Backup::        How Emacs saves the old version of your file.
* Interlocking::  How Emacs protects against simultaneous editing
                    of one file by two users.
File: emacs,  Node: Backup,  Next: Interlocking,  Up: Saving
Backup Files
------------
   Because Unix does not provide version numbers in file names,
rewriting a file in Unix automatically destroys all record of what the
file used to contain.  Thus, saving a file from Emacs throws away the
old contents of the file--or it would, except that Emacs carefully
copies the old contents to another file, called the "backup" file,
before actually saving.  (This assumes that the variable
`make-backup-files' is non-`nil'.  Backup files are not written if this
variable is `nil'.)
   At your option, Emacs can keep either a single backup file or a
series of numbered backup files for each file that you edit.
   Emacs makes a backup for a file only the first time the file is saved
from one buffer.  No matter how many times you save a file, its backup
file continues to contain the contents from before the file was visited.
Normally this means that the backup file contains the contents from
before the current editing session; however, if you kill the buffer and
then visit the file again, a new backup file will be made by the next
save.
* Menu:
* Names: Backup Names.		How backup files are named;
				  choosing single or numbered backup files.
* Deletion: Backup Deletion.	Emacs deletes excess numbered backups.
* Copying: Backup Copying.	Backups can be made by copying or renaming.
File: emacs,  Node: Backup Names,  Next: Backup Deletion,  Up: Backup
Single or Numbered Backups
..........................
   If you choose to have a single backup file (this is the default),
the backup file's name is constructed by appending `~' to the file name
being edited; thus, the backup file for `eval.c' would be `eval.c~'.
   If you choose to have a series of numbered backup files, backup file
names are made by appending `.~', the number, and another `~' to the
original file name.  Thus, the backup files of `eval.c' would be called
`eval.c.~1~', `eval.c.~2~', and so on, through names like
`eval.c.~259~' and beyond.
   If protection stops you from writing backup files under the usual
names, the backup file is written as `%backup%~' in your home directory.
Only one such file can exist, so only the most recently made such
backup is available.
   The choice of single backup or numbered backups is controlled by the
variable `version-control'.  Its possible values are
     Make numbered backups.
`nil'
     Make numbered backups for files that have numbered backups already.
     Otherwise, make single backups.
`never'
     Do not in any case make numbered backups; always make single
     backups.
You can set `version-control' locally in an individual buffer to
control the making of backups for that buffer's file.  For example,
Rmail mode locally sets `version-control' to `never' to make sure that
there is only one backup for an Rmail file.  *Note Locals::.
   If you set the environment variable `VERSION_CONTROL', to tell
various GNU utilities what to do with backup files, Emacs also obeys the
environment variable by setting the Lisp variable `version-control'
accordingly at startup.  If the environment variable's value is `t' or
`numbered', then `version-control' becomes `t'; if the value is `nil'
or `existing', then `version-control' becomes `nil'; if it is `never'
or `simple', then `version-control' becomes `never'.
File: emacs,  Node: Backup Deletion,  Next: Backup Copying,  Prev: Backup Names,  Up: Backup
Automatic Deletion of Backups
.............................
   To prevent unlimited consumption of disk space, Emacs can delete
numbered backup versions automatically.  Generally Emacs keeps the
first few backups and the latest few backups, deleting any in between.
This happens every time a new backup is made.
   The two variables `kept-old-versions' and `kept-new-versions'
control this deletion.  Their values are, respectively the number of
oldest (lowest-numbered) backups to keep and the number of newest
(highest-numbered) ones to keep, each time a new backup is made.
Recall that these values are used just after a new backup version is
made; that newly made backup is included in the count in
`kept-new-versions'.  By default, both variables are 2.
   If `trim-versions-without-asking' is non-`nil', the excess middle
versions are deleted without a murmur.  If it is `nil', the default,
then you are asked whether the excess middle versions should really be
deleted.
   Dired's `.' (Period) command can also be used to delete old versions.
*Note Dired Deletion::.
File: emacs,  Node: Backup Copying,  Prev: Backup Deletion,  Up: Backup
Copying vs. Renaming
....................
   Backup files can be made by copying the old file or by renaming it.
This makes a difference when the old file has multiple names.  If the
old file is renamed into the backup file, then the alternate names
become names for the backup file.  If the old file is copied instead,
then the alternate names remain names for the file that you are
editing, and the contents accessed by those names will be the new
contents.
   The method of making a backup file may also affect the file's owner
and group.  If copying is used, these do not change.  If renaming is
used, you become the file's owner, and the file's group becomes the
default (different operating systems have different defaults for the
group).
   Having the owner change is usually a good idea, because then the
owner always shows who last edited the file.  Also, the owners of the
backups show who produced those versions.  Occasionally there is a file
whose owner should not change; it is a good idea for such files to
contain local variable lists to set `backup-by-copying-when-mismatch'
locally (*note File Variables::.).
   The choice of renaming or copying is controlled by three variables.
Renaming is the default choice.  If the variable `backup-by-copying' is
non-`nil', copying is used.  Otherwise, if the variable
`backup-by-copying-when-linked' is non-`nil', then copying is used for
files that have multiple names, but renaming may still used when the
file being edited has only one name.  If the variable
`backup-by-copying-when-mismatch' is non-`nil', then copying is used if
renaming would cause the file's owner or group to change.
File: emacs,  Node: Interlocking,  Prev: Backup,  Up: Saving
Protection against Simultaneous Editing
---------------------------------------
   Simultaneous editing occurs when two users visit the same file, both
make changes, and then both save them.  If nobody were informed that
this was happening, whichever user saved first would later find that his
changes were lost.  On some systems, Emacs notices immediately when the
second user starts to change the file, and issues an immediate warning.
   For the sake of systems where that is not possible, and in case
someone else proceeds to change the file despite the warning, Emacs also
checks when the file is saved, and issues a second warning if you are
about to overwrite a file containing another user's changes.  You can
prevent loss of the other user's work by taking the proper corrective
action at that time.
   When you make the first modification in an Emacs buffer that is
visiting a file, Emacs records that  the file is "locked" by you.  (It
does this by writing another file in a directory reserved for this
purpose.)  The lock is removed when you save the changes.  The idea is
that the file is locked whenever an Emacs buffer visiting it has
unsaved changes.
   If you begin to modify the buffer while the visited file is locked by
someone else, this constitutes a "collision".  When Emacs detects a
collision, it asks you what to do, by calling the Lisp function
`ask-user-about-lock'.  You can redefine this function for the sake of
customization.  The standard definition of this function asks you a
question and accepts three possible answers:
     Steal the lock.  Whoever was already changing the file loses the
     lock, and you gain the lock.
     Proceed.  Go ahead and edit the file despite its being locked by
     someone else.
     Quit.  This causes an error (`file-locked') and the modification
     you were trying to make in the buffer does not actually take place.
   Note that locking works on the basis of a file name; if a file has
multiple names, Emacs does not realize that the two names are the same
file and cannot prevent two users from editing it simultaneously under
different names.  However, basing locking on names means that Emacs can
interlock the editing of new files that will not really exist until
they are saved.
   Some systems are not configured to allow Emacs to make locks.  On
these systems, Emacs cannot detect trouble in advance, but it still can
detect the collision when you try to save a file and overwrite someone
else's changes.
   Every time Emacs saves a buffer, it first checks the
last-modification date of the existing file on disk to verify that it
has not changed since the file was last visited or saved.  If the date
does not match, it implies that changes were made in the file in some
other way, and these changes are about to be lost if Emacs actually
does save.  To prevent this, Emacs prints a warning message and asks
for confirmation before saving.  Occasionally you will know why the
file was changed and know that it does not matter; then you can answer
`yes' and proceed.  Otherwise, you should cancel the save with `C-g'
and investigate the situation.
   The first thing you should do when notified that simultaneous
editing has already taken place is to list the directory with `C-u C-x
C-d' (*note Directory Listing: ListDir.).  This  shows the file's
current author.  You should attempt to contact him to warn him not to
continue editing.  Often the next step is to save the contents of your
Emacs buffer under a different name, and use `diff' to compare the two
files.
   Simultaneous editing checks are also made when you visit with `C-x
C-f' a file that is already visited and when you start to modify a
file.  This is not strictly necessary, but it can cause you to find out
about the collision earlier, when perhaps correction takes less work.
File: emacs,  Node: Reverting,  Next: Auto Save,  Prev: Saving,  Up: Files
Reverting a Buffer
==================
   If you have made extensive changes to a file and then change your
mind about them, you can get rid of them by reading in the previous
version of the file.  To do this, use `M-x revert-buffer', which
operates on the current buffer.  Since this is a very dangerous thing
to do, you must confirm it with `yes'.
   `revert-buffer' keeps point at the same distance (measured in
characters) from the beginning of the file.  If the file was edited only
slightly, you will be at approximately the same piece of text after
reverting as before.  If you have made drastic changes, the same value
of point in the old file may address a totally different piece of text.
   Reverting marks the buffer as "not modified" until another change is
made.
   Some kinds of buffers whose contents reflect data bases other than
files, such as Dired buffers, can also be reverted.  For them,
reverting means recalculating their contents from the appropriate data
base.  Buffers created randomly with `C-x b' cannot be reverted;
`revert-buffer' reports an error when asked to do so.
File: emacs,  Node: Auto Save,  Next: File Aliases,  Prev: Reverting,  Up: Files
Auto-Saving: Protection Against Disasters
=========================================
   Emacs saves all the visited files from time to time (based on
counting your keystrokes) without being asked.  This is called
"auto-saving".  It prevents you from losing more than a limited amount
of work if the system crashes.
   When Emacs determines that it is time for auto-saving, each buffer is
considered, and is auto-saved if auto-saving is turned on for it and it
has been changed since the last time it was auto-saved.  If any
auto-saving is done, the message `Auto-saving...' is displayed in the
echo area until auto-saving is finished.  Errors occurring during
auto-saving are caught so that they do not interfere with the execution
of commands you have been typing.
* Menu:
* Files: Auto Save Files.       The file where auto-saved changes are
                                  actually made until you save the file.
* Control: Auto Save Control.   Controlling when and how often to auto-save.
* Recover::		        Recovering text from auto-save files.
File: emacs,  Node: Auto Save Files,  Next: Auto Save Control,  Up: Auto Save
Auto-Save Files
---------------
   Auto-saving does not normally save in the files that you visited,
because it can be very undesirable to save a program that is in an
inconsistent state when you have made half of a planned change.
Instead, auto-saving is done in a different file called the "auto-save
file", and the visited file is changed only when you request saving
explicitly (such as with `C-x C-s').
   Normally, the auto-save file name is made by appending `#' to the
front and rear of the visited file name.  Thus, a buffer visiting file
`foo.c' is auto-saved in a file `#foo.c#'.  Most buffers that are not
visiting files are auto-saved only if you request it explicitly; when
they are auto-saved, the auto-save file name is made by appending `#%'
to the front and `#' to the rear of buffer name.  For example, the
`*mail*' buffer in which you compose messages to be sent is auto-saved
in a file named `#%*mail*#'.  Auto-save file names are made this way
unless you reprogram parts of Emacs to do something different (the
functions `make-auto-save-file-name' and `auto-save-file-name-p').  The
file name to be used for auto-saving in a buffer is calculated when
auto-saving is turned on in that buffer.
   If you want auto-saving to be done in the visited file, set the
variable `auto-save-visited-file-name' to be non-`nil'.  In this mode,
there is really no difference between auto-saving and explicit saving.
   A buffer's auto-save file is deleted when you save the buffer in its
visited file.  To inhibit this, set the variable
`delete-auto-save-files' to `nil'.  Changing the visited file name with
`C-x C-w' or `set-visited-file-name' renames any auto-save file to go
with the new visited name.
   When you delete a large amount of a buffer's text, auto-saving turns
off 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 restart auto-saving in that buffer, use
`M-x auto-save' with a positive argument.
File: emacs,  Node: Auto Save Control,  Next: Recover,  Prev: Auto Save Files,  Up: Auto Save
Controlling Auto-Saving
-----------------------
   Each time you visit a file, auto-saving is turned on for that file's
buffer if the variable `auto-save-default' is non-`nil' (but not in
batch mode; *note Entering Emacs::.).  The default for this variable is
`t', so auto-saving is the usual practice for file-visiting buffers.
Auto-saving can be turned on or off for any existing buffer with the
command `M-x auto-save-mode'.  Like other minor mode commands, `M-x
auto-save-mode' turns auto-saving on with a positive argument, off with
a zero or negative argument; with no argument, it toggles.
   Emacs does auto-saving periodically based on counting how many
characters you have typed since the last time auto-saving was done.
The variable `auto-save-interval' specifies how many characters there
are between auto-saves.  By default, it is 300.
   Auto-saving also takes place when you stop typing for a while.  The
variable `auto-save-timeout' says how many seconds Emacs should wait
before it does an auto save (and perhaps also a garbage collection).
(The actual time period is longer if the current buffer is long; this
is a heuristic which aims to keep out of your way when you are editing
long buffers in which auto-save takes an appreciable amount of time.)
Auto-saving during idle periods accomplishes two things: first, it
makes sure all your work is saved if you go away from the terminal for
a while; second, it may avoid some auto-saving while you are actually
typing.
   Emacs also does auto-saving whenever it gets a fatal error.  This
includes killing the Emacs job with a shell command such as `kill
%emacs', or disconnecting a phone line or network connection.
   You can request an auto-save explicitly with the command `M-x
do-auto-save'.
File: emacs,  Node: Recover,  Prev: Auto Save Control,  Up: Auto Save
Recovering Data from Auto-Saves
-------------------------------
   The way to use the contents of an auto-save file to recover from a
loss of data is with the command `M-x recover-file RET FILE RET'.  This
visits FILE and then (after your confirmation) restores the contents
from from its auto-save file `#FILE#'.  You can then save with `C-x
C-s' to put the recovered text into FILE itself.  For example, to
recover file `foo.c' from its auto-save file `#foo.c#', do:
     M-x recover-file RET foo.c RET
     yes RET
     C-x C-s
   Before asking for confirmation, `M-x recover-file' displays a
directory listing describing the specified file and the auto-save file,
so you can compare their sizes and dates.  If the auto-save file is
older, `M-x recover-file' does not offer to read it.
File: emacs,  Node: File Aliases,  Next: Version Control,  Prev: Auto Save,  Up: Files
File Name Aliases
=================
   Symbolic links and hard links both make it possible for several file
names to refer to the same file.  For example, when `foo' is a symbolic
link to `bar', the file has two names, but `bar' is the real name, and
`foo' is just an alias.  More complex cases occur when symbolic links
point to directories.
   If you visit two names for the same file, normally Emacs makes two
different buffers, but it warns you about the situation.
   If you wish to avoid visiting the same file in two buffers under
different names, set the variable `find-file-existing-other-name' to a
non-`nil' value.  Then `find-file' uses the existing buffer visiting
the file, no matter which of the file's names you specify.
   If the variable `find-file-visit-truename' is non-`nil', then the
file name recorded for a buffer is the file's "truename" (made by
replacing all symbolic links with their target names), rather than the
name you specify.  Setting `find-file-visit-truename' also implies the
effect of `find-file-existing-other-name'.
File: emacs,  Node: Version Control,  Next: ListDir,  Prev: File Aliases,  Up: Files
Version Control
===============
   "Version control systems" are packages that can record multiple
versions of a source file, usually storing the unchanged parts of the
file just once.  Version control systems also record history information
such as the creation time of each version, who created it, and a
description of what was changed in that version.
   The GNU project recommends the version control system known as RCS,
which is free software and available from the Free Software Foundation.
Emacs supports use of either RCS or SCCS (a proprietary, but widely
used, version control system that is not quite as powerful as RCS)
through a facility called VC.  The same Emacs commands work with either
RCS or SCCS, so you hardly have to know which one of them you are using.
* Menu:
* Concepts of VC::              Basic version control information;
                                  checking files in and out.
* Editing with VC::             Commands for editing a file maintained
                                  with version control.
* Variables for Check-in/out::  Variables that affect the commands used
                                  to check files in or out.
* Log Entries::                 Logging your changes.
* Change Logs and VC::          Generating a change log file from log
                                  entries.
* Comparing Versions::          How to compare two versions of a file.
* VC Status::                   Commands to view the VC status of files and
                                  look at log entries.
* Renaming and VC::             A command to rename both the source and
                                  master file correctly.
* Snapshots::                   How to make and use snapshots, a set of
                                  file versions that can be treated as a unit.
* Version Headers::             Inserting version control headers into
                                  working files.
File: emacs,  Node: Concepts of VC,  Next: Editing with VC,  Up: Version Control
Concepts of Version Control
---------------------------
   When a file is under version control, we also say that it is
"registered" in the version control system.  Each registered file has a
corresponding "master file" which represents the file's present state
plus its change history, so that you can reconstruct from it either the
current version or any specified earlier version.  Usually the master
file also records a "log entry" for each version describing what was
changed in that version.
   The file that is maintained under version control is sometimes called
the "work file" corresponding to its master file.
   To examine a file, you "check it out".  This extracts a version of
the source file (typically, the most recent) from the master file.  If
you want to edit the file, you must check it out "locked".  Only one
user can do this at a time for any given source file.  (This kind of
locking is completely unrelated to the locking that Emacs uses to
detect simultaneous editing of a file.)
   When you are done with your editing, you must "check in" the new
version.  This records the new version in the master file, and unlocks
the source file so that other people can lock it and thus modify it.
   Checkin and checkout are the basic operations of version control.
You can do both of them with a single Emacs command: `C-x C-q'
(`vc-toggle-read-only').
File: emacs,  Node: Editing with VC,  Next: Variables for Check-in/out,  Prev: Concepts of VC,  Up: Version Control
Editing with Version Control
----------------------------
   When you visit a file that is maintained using version control, the
mode line displays `RCS' or `SCCS' to inform you that version control
is in use, and also (in case you care) which low-level system the file
is actually stored in.  Normally, such a source file is read-only, and
the mode line indicates this with `%%'.
   These are the commands for editing a file maintained with version
control:
`C-x C-q'
     Check the visited file in or out.
`C-x v u'
     Revert the buffer and the file to the last checked in version.
`C-x v c'
     Remove the last-entered change from the master for the visited
     file.  This undoes your last check-in.
`C-x v i'
     Register the visited file in version control.
(`C-x v' is the prefix key for version control commands; all of these
commands except for `C-x C-q' start with `C-x v'.)
   When you want to modify a file maintained with version control, type
`C-x C-q' (`vc-toggle-read-only').  This "checks out" the file, and
tells RCS or SCCS to lock the file.  This means making the file
writable for you (but not for anyone else).
   When you are finished editing the file, type `C-x C-q' again.  When
used on a file that is checked out, this command checks the file in.
But check-in does not start immediately; first, you must enter the "log
entry"--a description of the changes in the new version.  `C-x C-q'
pops up a buffer for you to enter this in.  When you are finished
typing in the log entry, type `C-c C-c' to terminate it; this is when
actual check-in takes place.
   Once you have checked in your changes, the file is unlocked, so that
other users can lock it and modify it.
   Emacs does not save backup files for source files that are maintained
with version control.  If you want to make backup files despite version
control, set the variable `vc-make-backup-files' to a non-`nil' value.
   Normally the work file exists all the time, whether it is locked or
not.  If you set `vc-keep-workfiles' to `nil', then checking in a new
version with `C-x C-q' deletes the work file; but any attempt to visit
the file with Emacs creates it again.
   It is not impossible to lock a file that someone else has locked.  If
you try to check out a file that is locked, `C-x C-q' asks you whether
you want to "steal the lock."  If you say yes, the file becomes locked
by you, but a message is sent to the person who had formerly locked the
file, to inform him of what has happened.
   If you want to discard your current set of changes and revert to the
last version checked in, use `C-x v u' (`vc-revert-buffer').  This
cancels your last check-out, leaving the file unlocked.  If you want to
make a different set of changes, you must first check the file out
again.  `C-x v u' requires confirmation, unless it sees that you
haven't made any changes since the last checked-in version.
   `C-x v u' is also the command to use if you lock a file and then
don't actually change it.
   You can cancel a change after checking it in, with `C-x v c'
(`vc-cancel-version').  This command discards all record of the most
recent checked in version, so be careful about using it.  It requires
confirmation with `yes'.  By default, `C-x v c' reverts your workfile
and buffer to the previous version (the one that precedes the version
that is deleted), but you can prevent the reversion by giving the
command a prefix argument.  Then the buffer does not change.
   This command with a prefix argument is useful when you have checked
in a change and then discover a trivial error in it; you can cancel the
erroneous check-in, fix the error, and repeat the check-in.
   Be careful when invoking `C-x v c', as it is easy to throw away a
lot of work with it.  To help you be careful, this command always asks
for confirmation with `yes'.
   You can register the visited file for version control using
`C-x v i' (`vc-register').  This uses RCS if RCS is installed on your
system; otherwise, it uses SCCS.  After `C-x v i', the file is unlocked
and read-only.  Type `C-x C-q' if you wish to edit it.
   By default, the initial version number is 1.1.  If you want to use a
different number, give `C-x v i' a prefix argument; then it reads the
initial version number using the minibuffer.
   If `vc-initial-comment' is non-`nil', `C-x v i' reads an initial
comment (much like a log entry) to describe the purpose of this source
file.
File: emacs,  Node: Variables for Check-in/out,  Next: Log Entries,  Prev: Editing with VC,  Up: Version Control
Variables Affecting Check-in and Check-out
------------------------------------------
   If `vc-suppress-confirm' is non-`nil', then `C-x C-q' and `C-x v i'
can save the current buffer without asking, and `C-x v u' also operates
without asking for confirmation.  (This variable does not affect `C-x v
c'; that is so drastic that it should always ask for confirmation.)
   VC mode does much of its work by running the shell commands for RCS
and SCCS.  If `vc-command-messages' is non-`nil', VC displays messages
to indicate which shell commands it runs, and additional messages when
the commands finish.
   Normally, VC assumes that it can deduce the locked/unlocked state of
files by looking at the file permissions of the work file; this is
fast.  However, if the `RCS' or `SCCS' subdirectory is actually a
symbolic link, then VC does not trust the file permissions to reflect
this status.
   You can specify the criterion for whether to trust the file
permissions by setting the variable `vc-mistrust-permissions'.  Its
value may be `t' (always mistrust the file permissions and check the
master file), `nil' (always trust the file permissions), or a function
of one argument which makes the decision.  The argument is the directory
name of the `RCS' or `SCCS' subdirectory.  A non-`nil' value from the
function says to mistrust the file permissions.
   If you find that the file permissions of work files are changed
erroneously, set `vc-mistrust-permissions' to `t'.  Then VC always
checks the master file to determine the file's status.
File: emacs,  Node: Log Entries,  Next: Change Logs and VC,  Prev: Variables for Check-in/out,  Up: Version Control
Log Entries
-----------
   When you're editing an initial comment or log entry for inclusion in
a master file, finish your entry by typing `C-c C-c'.
`C-c C-c'
     Finish the comment edit normally (`vc-finish-logentry').  This
     finishes check-in.
   To abort check-in, just don't type `C-c C-c' in that buffer.  You
can switch buffers and do other editing.  As long as you don't try to
check in another file, the entry you were editing remains in its
buffer, and you can go back to that buffer at any time to complete the
check-in.
   If you change several source files for the same reason, it is often
convenient to specify the same log entry for many of the files.  To do
this, use the history of previous log entries.  The commands `M-n',
`M-p', `M-s' and `M-r' for doing this work just like the minibuffer
history commands (except that these versions are used outside the
minibuffer).
   Each time you check in a file, the log entry buffer is put into VC
Log mode, which involves running two hooks: `text-mode-hook' and
`vc-log-mode-hook'.  *Note Hooks::.