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14.3 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, and record that file
     as the one visited in the buffer (`write-file').

`M-x set-visited-file-name'
     Change file the name under which the current buffer will be saved.

   To save a 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), Emacs does not save it
because it would have no effect.  Instead, `C-x C-s' prints a message
in the echo area saying:

     (No changes need to be saved)

   The command `C-x s' (`save-some-buffers') can save any or all
modified buffers.  First it asks, for each modified buffer, whether to
save it.  The questions should be answered with `y' or `n'.  `C-x C-c',
the key that kills 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, you are liable to
save it by mistake each time you use `save-some-buffers' or a related
command.  One thing you can do is type `M-~' (`not-modified'), which
removes 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
`Meta-~' is `not', metafied.)  You could also use
`set-visited-file-name' (see below) to mark the buffer as visiting a
different file name, not in use for anything important.

   You can also undo 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.  Alternatively, you can undo all the
changes by repeating the undo command `C-x u'; but this only works if
you have not made more changes than the undo mechanism can remember.

   `M-x set-visited-file-name' alters the name of the file that the
current buffer is visiting.  It prompts you for the new file name in the
minibuffer.  You can also use `set-visited-file-name' on a buffer that
is not visiting a file.  The buffer's name is changed to correspond to
the file it is now visiting unless the new name is already used by a
different buffer; in that case, the buffer name is not changed.
`set-visited-file-name' does not save the buffer in the newly visited
file; it just alters the records inside Emacs so that it will save the
buffer in that file.  It also marks the buffer as "modified" so that
`C-x C-s' will save.

   If you wish to mark a 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  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.

   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.

   Use the hook variable `write-file-hooks' to implement other ways to
write files, and specify things to be done before files are written.
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. Use the hook variable `after-save-hook' to list all the
functions to be called after writing out a buffer to a file.

Backup
How Emacs saves the old version of your file.
Interlocking
How Emacs protects against simultaneous editing
of one file by two users.

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