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Info file elisp, produced by Makeinfo, -*- Text -*- from input file
elisp.texi.
This file documents GNU Emacs Lisp.
This is edition 1.03 of the GNU Emacs Lisp Reference Manual, for
Emacs Version 18.
Published by the Free Software Foundation, 675 Massachusetts
Avenue, Cambridge, MA 02139 USA
Copyright (C) 1990 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.
File: elisp, Node: Subroutines of Visiting, Prev: Visiting Functions, Up: Visiting Files
Subroutines of Visiting
-----------------------
The `find-file-noselect' function uses the `create-file-buffer'
and `after-find-file' functions as subroutines. Sometimes it is
useful to call them directly.
* Function: create-file-buffer FILENAME
This function creates a suitably named buffer for visiting
FILENAME, and returns it. The string FILENAME (sans directory)
is used unchanged if that name is free; otherwise, a string such
as `<2>' is appended to get an unused name. See also *Note
Creating Buffers::.
*Note:* `create-file-buffer' does *not* associate the new buffer
with a file and does not make it the current buffer.
(create-file-buffer "foo")
=> #<buffer foo>
(create-file-buffer "foo")
=> #<buffer foo<2>>
(create-file-buffer "foo")
=> #<buffer foo<3>>
This function is used by `find-file-noselect'.
* Function: after-find-file &optional ERROR WARN
This function is called by `find-file-noselect' and by the
default revert function (*note Reverting::.). It sets the
buffer major mode, and parses local variables (*note Auto Major
Mode::.).
If there was an error in opening the file, the calling function
should pass ERROR a non-`nil' value. In that case,
`after-find-file' issues a warning: `(New File)'. Note that,
for serious errors, you would not even call `after-find-file'.
Only "file not found" errors get here with a non-`nil' ERROR.
If WARN is non-`nil', then this function issues a warning if an
auto-save file exists and is more recent than the visited file.
The last thing `after-find-file' does is call all the functions
in `find-file-hooks'.
File: elisp, Node: Saving Buffers, Next: Reading from Files, Prev: Visiting Files, Up: Files
Saving Buffers
==============
When you edit a file in Emacs, you are actually working on a
buffer that is visiting that file--that is, the contents of the file
are copied into the buffer and the copy is what you edit. Changes to
the buffer do not change the file until you "save" the buffer, which
means copying the contents of the buffer into the file.
* Command: save-buffer &optional BACKUP-OPTION
This function saves the contents of the current buffer in its
visited file if the buffer has been modified since it was last
visited or saved. Otherwise it does nothing.
`save-buffer' is responsible for making backup files. Normally,
BACKUP-OPTION is `nil', and `save-buffer' makes a backup file
only if this is the first save or if the buffer was previously
modified. Other values for BACKUP-OPTION request the making of
backup files in other circumstances:
* With an argument of 4 or 64, reflecting 1 or 3 `C-u''s, the
`save-buffer' function marks this version of the file to be
backed up when the buffer is next saved.
* With an argument of 16 or 64, reflecting 2 or 3 `C-u''s,
the `save-buffer' function unconditionally backs up the
previous version of the file before saving it.
* Command: save-buffers-kill-emacs &optional KILL-SILENTLY-P
This function offers to save each buffer that needs to be saved,
and then kills the Emacs job. With a non-`nil' value of the
optional KILL-SILENTLY-P argument, it unconditionally and
silently saves all the file-visiting buffers, and then kills the
job.
The `save-buffers-kill-emacs' function is defined as follows:
(defun save-buffers-kill-emacs (&optional arg)
"Offer to save each buffer, then kill this Emacs fork..."
(interactive "P")
(save-some-buffers arg t)
(kill-emacs))
* Command: save-some-buffers &optional SAVE-SILENTLY-P EXITING
This command saves some modified file-visiting buffers.
Normally it asks the user about each buffer. If the function is
called with a non-`nil' value of the optional SAVE-SILENTLY-P
argument, this function saves all the file-visiting buffers
without querying the user.
The optional EXITING argument, if non-`nil', requests this
function to offer also to save certain other buffers that are
not visiting files. These are buffers that have a non-`nil'
local value of `buffer-offer-save'. (A user who says yes to
saving one of these will be asked to specify a file name to
use.) The `save-buffers-kill-emacs' function passes a non-`nil'
value for this argument.
* Variable: buffer-offer-save
When this variable is non-`nil' in a buffer, Emacs offers to
save the buffer on exit even if the buffer is not visiting a
file. The variable is automatically local in all buffers.
Normally, Mail mode (used for editing outgoing mail) sets this
to `t'.
* Command: write-file FILENAME
This function writes the current buffer into file FILENAME,
makes the buffer visit that file, and marks it not modified.
The buffer is renamed to correspond to FILENAME unless that name
is already in use.
* Variable: write-file-hooks
The value of this variable is a list of functions to be called
before writing out a buffer to its visited file. If one of them
returns non-`nil', the file is considered already written and
the rest of the functions are not called, nor is the usual code
for writing the file executed.
If the `file-precious-flag' variable is `nil', the file is moved
to the backup file before any of the hooks are called. If none
of the hooks actually write the file, but one does return
non-`nil', the file will not exist, although the backup will.
Here is an example showing how to add an element to
`write-file-hooks' but avoid adding it twice:
(or (memq 'my-write-file-hook write-file-hooks)
(setq write-file-hooks
(cons 'my-write-file-hook write-file-hooks)))
* Variable: file-precious-flag
If this variable is non-`nil', then `save-buffer' protects
against I/O errors while saving by renaming the original file to
a temporary name before writing the new contents of the file.
If the new contents are successfully written, the renamed
original file is deleted. Otherwise, it is renamed back to the
original name. This procedure prevents problems such as a lack
of disk space from resulting in an invalid file.
Some modes set this non-`nil' locally in particular buffers.
* User Option: require-final-newline
This variable determines whether files may be written out that
do *not* end with a newline. If the value of the variable is
`t', then Emacs silently puts a newline at the end of the file
whenever the buffer being saved does not already end in one. If
the value of the variable is non-`nil', but not `t', then Emacs
asks the user whether to add a newline each time the case arises.
If the value of the variable is `nil', then Emacs doesn't add
newlines at all. `nil' is the default value, but a few major
modes change it to `t'.
File: elisp, Node: Reading from Files, Next: Writing to Files, Prev: Saving Buffers, Up: Files
Reading from Files
==================
You can copy a file directly from the disk and insert it into a
buffer using the `insert-file-contents' function, or its interactive
variant, `insert-file'.
* Command: insert-file FILENAME
This function inserts the contents of file FILENAME into the
current buffer after point, and sets the mark at the end of the
inserted text. An error is signaled if FILENAME is not the name
of a file that can be read. This function is for interactive
use and does little more than call `insert-file-contents'.
* Function: insert-file-contents FILENAME &optional VISIT
This function inserts the contents of file FILENAME into the
current buffer after point. It returns a list of the absolute
file name and the length of the data inserted. An error is
signaled if FILENAME is not the name of a file that can be read.
If VISIT is non-`nil', it also marks the buffer as unmodified
and sets up various fields in the buffer so that it is visiting
the file FILENAME: these include the buffer's visited file name
and its last save file modtime. This feature is used by
`find-file-noselect' and you should probably not use it yourself.
File: elisp, Node: Writing to Files, Next: File Locks, Prev: Reading from Files, Up: Files
Writing to Files
================
You can write the contents of a buffer, or part of a buffer,
directly to a file on disk using the `append-to-file' and
`write-region' functions. Don't use these functions to write to
files that are being visited; that could cause confusion in the
mechanisms for visiting.
* Command: append-to-file START END FILENAME
This function appends the contents of the region delimited by
START and END in the current buffer to the end of file FILENAME.
If that file does not exist, it is created. This function
returns `nil'.
An error is signaled if FILENAME specifies a nonwritable file,
or a nonexistent file in a directory where files cannot be
created.
* Command: write-region START END FILENAME &optional APPEND VISIT
This function writes the region (of the current buffer)
delimited by START and END into the file specified by FILENAME.
If APPEND is non-`nil', then the region is appended to the
existing file contents (if any).
If VISIT is `t', then Emacs establishes an association between
the buffer and the file: the buffer is then visiting that file.
It also sets the last file modification time for the current
buffer to FILENAME's modtime, and marks the buffer as not
modified. This feature is used by `write-file' and you should
probably not use it yourself.
Normally, this function displays a message `Wrote file FILENAME'
in the echo area. If VISIT is neither `t' nor `nil', then this
message is inhibited. This feature is useful for programs that
use files for internal purposes, files which the user does not
need to know about.
File: elisp, Node: File Locks, Next: Information about Files, Prev: Writing to Files, Up: Files
File Locks
==========
When two users edit the same file at the same time, they are
likely to interfere with each other. Emacs tries to prevent this
situation from arising by recording a "file lock" when a file is
being modified. Emacs can then detect the first attempt to modify a
buffer visiting a file that is locked by another Emacs job, and ask
the user what to do.
File locks do not work properly when multiple machines can share
filesystems, such as with NFS. Perhaps a better file locking system
will be implemented in the future. When file locks do not work, it
is possible for two users to make changes simultaneously, but Emacs
will still be able to warn the user who saves second. Also, the
detection of modification of a buffer visiting a file changed on disk
catches some cases of simultaneous editing; see *Note Modification
Time::.
* Function: file-locked-p FILENAME
This function returns `nil' if the file FILENAME is not locked
by this Emacs process. It returns `t' if it is locked by this
Emacs, and it returns the name of the user who has locked it if
it is locked by someone else.
(file-locked-p "foo")
=> nil
* Function: lock-buffer &optional FILENAME
This function locks the file FILENAME, if the current buffer is
modified. The argument FILENAME defaults to the current
buffer's visited file. Nothing is done if the current buffer is
not visiting a file, or is not modified.
* Function: unlock-buffer
This function unlocks the file being visited in the current
buffer, if the buffer is modified. If the buffer is not
modified, then the file should not be locked, so this function
does nothing. It also does nothing if the current buffer is not
visiting a file.
* Function: ask-user-about-lock FILE OTHER-USER
This function is called when the user tries to modify FILE, but
it is locked by another user name OTHER-USER. The value it
returns controls what Emacs will do with the file:
* A value of `t' tells Emacs to grab the lock on the file.
Then this user may edit the file and OTHER-USER loses the
lock.
* A value of `nil' tells Emacs to ignore the lock and let
this user edit the file anyway.
* This function may instead signal a `file-locked' error, in
which case the change to the buffer which the user was
about to make does not take place.
The error message for this error looks like this:
error--> File is locked: FILE OTHER-USER
where `file' is the name of the file and OTHER-USER is the
name of the user who has locked the file.
The default definition of this function asks the user to choose
what to do. If you wish, you can replace the
`ask-user-about-lock' function with your own version that
decides in another way. The code for its usual definition is in
`userlock.el'.
File: elisp, Node: Information about Files, Next: Contents of Directories, Prev: File Locks, Up: Files
Information about Files
=======================
The functions described in this section are similar in as much as
they all operate on strings which are interpreted as file names. All
have names that begin with the word `file'. These functions all
return information about actual files or directories, so their
arguments must all exist as actual files or directories unless
otherwise noted.
Most of the file-oriented functions take a single argument,
FILENAME, which must be a string. The file name is expanded using
`expand-file-name', so `~' is handled correctly, as are relative file
names (including `../'). Environment variable substitutions, such as
`$HOME', are not recognized by these functions.
* Menu:
* Testing Accessibility:: Is a given file readable? Writable?
* Kinds of Files:: Is it a directory? A link?
* File Attributes:: How large is it? Any other names? Etc.
File: elisp, Node: Testing Accessibility, Next: Kinds of Files, Prev: Information about Files, Up: Information about Files
Testing Accessibility
---------------------
These functions test for permission to access a file in specific
ways.
* Function: file-exists-p FILENAME
This function returns `t' if a file named FILENAME appears to
exist. This does not mean you can necessarily read the file,
only that you can find out its attributes. (On Unix, this is
true if the file exists and you have execute permission on the
containing directories, regardless of the protection of the file
itself.)
If the file does not exist, or if fascist access control
policies prevent you from finding the attributes of the file,
this function returns `nil'.
* Function: file-readable-p FILENAME
This function returns `t' if a file named FILENAME exists and
you can read it. It returns `nil' otherwise.
(file-readable-p "files.texi")
=> t
(file-exists-p "/usr/spool/mqueue")
=> t
(file-readable-p "/usr/spool/mqueue")
=> nil
* Function: file-writable-p FILENAME
This function returns `t' if FILENAME can be written or created
by you. It is writable if the file exists and you can write it.
It is creatable if the file does not exist, but the specified
directory does exist and you can write in that directory.
`file-writable-p' returns `nil' otherwise.
In the third example below, `foo' is not writable because the
parent directory does not exist, even though the user could
create it.
(file-writable-p "~rms/foo")
=> t
(file-writable-p "/foo")
=> nil
(file-writable-p "~rms/no-such-dir/foo")
=> nil
* Function: file-newer-than-file-p FILENAME1 FILENAME2
This functions returns `t' if the file FILENAME1 is newer than
file FILENAME2. If FILENAME1 does not exist, it returns `nil'.
If FILENAME2 does not exist, it returns `t'.
In the following example, assume that the file `aug-19' was
written on the 19th, and `aug-20' was written on the 20th. The
file `no-file' doesn't exist at all.
(file-newer-than-file-p "aug-19" "aug-20")
=> nil
(file-newer-than-file-p "aug-20" "aug-19")
=> t
(file-newer-than-file-p "aug-19" "no-file")
=> t
(file-newer-than-file-p "no-file" "aug-19")
=> nil
File: elisp, Node: Kinds of Files, Next: File Attributes, Prev: Testing Accessibility, Up: Information about Files
Distinguishing Kinds of Files
-----------------------------
This section describes how to distinguish directories and symbolic
links from ordinary files.
* Function: file-symlink-p FILENAME
If FILENAME is a symbolic link, the `file-symlink-p' function
returns the file name to which it is linked. This may be the
name of a text file, a directory, or even another symbolic link,
or of no file at all.
If FILENAME is not a symbolic link (or there is no such file),
`file-symlink-p' returns `nil'.
(file-symlink-p "foo")
=> nil
(file-symlink-p "sym-link")
=> "foo"
(file-symlink-p "sym-link2")
=> "sym-link"
(file-symlink-p "/bin")
=> "/pub/bin"
* Function: file-directory-p FILENAME
This function returns `t' if FILENAME is the name of an existing
directory, `nil' otherwise.
(file-directory-p "~rms")
=> t
(file-directory-p "~rms/lewis/files.texi")
=> nil
(file-directory-p "~rms/lewis/no-such-file")
=> nil
(file-directory-p "$HOME")
=> nil
(file-directory-p (substitute-in-file-name "$HOME"))
=> t
File: elisp, Node: File Attributes, Prev: Kinds of Files, Up: Information about Files
Other Information about Files
-----------------------------
This section describes the functions for getting detailed
information about a file, other than its contents. This information
includes the mode bits that control access permission, the owner and
group numbers, the number of names, the inode number, the size, and
the times of access and modification.
* Function: file-modes FILENAME
This function returns the mode bits of FILENAME, as an integer.
The mode bits are also called the file permissions, and they
specify access control in the usual Unix fashion. If the
low-order bit is 1, then the file is executable by all users, if
the second lowest-order bit is 1, then the file is writable by
all users, etc.
The highest value returnable is 4095 (7777 octal), meaning that
everyone has read, write, and execute permission, that the SUID
bit is set for both others and group, and that the sticky bit is
set.
(file-modes "~/junk/diffs")
=> 492 ; decimal integer
(format "%o" 492)
=> 754 ; convert to octal
(set-file-modes "~/junk/diffs" 438)
=> nil
(format "%o" 438)
=> 666 ; convert to octal
% ls -l diffs
-rw-rw-rw- 1 lewis 0 3063 Oct 30 16:00 diffs
* Function: file-nlinks FILENAME
This functions returns the number of names (i.e., hard links)
that file FILENAME has. If the file does not exist, then this
function returns `nil'. Note that symbolic links have no effect
on this function, because they are not considered to be names of
the files they link to.
% ls -l foo*
-rw-rw-rw- 2 rms 4 Aug 19 01:27 foo
-rw-rw-rw- 2 rms 4 Aug 19 01:27 foo1
(file-nlinks "foo")
=> 2
(file-nlinks "doesnt-exist")
=> nil
* Function: file-attributes FILENAME
This function returns a list of attributes of file FILENAME. If
the specified file cannot be opened, it returns `nil'.
The elements of the list, in order, are:
1. `t' for a directory, a string for a symbolic link (the name
linked to), or `nil' for a text file.
2. The number of names the file has. Alternate names, also
known as hard links, can be created with `add-name-to-file'
(*note Changing File Attributes::.).
3. The file's UID.
4. The file's GID.
5. The time of last access, as a list of two integers. The
first integer has the high-order 16 bits of time, the
second has the low 16 bits.
6. The time of last modification as a list of two integers (as
above).
7. The time of last status change as a list of two integers
(as above).
8. The size of the file in bytes.
9. The file's modes, as a string of ten letters or dashes as
in `ls -l'.
10. `t' if the file's GID would change if file were deleted and
recreated; `nil' otherwise.
11. The file's inode number.
For example, here are the file attributes for `files.texi':
(file-attributes "files.texi")
=> (nil
1
2235
75
(8489 20284)
(8489 20284)
(8489 20285)
14906
"-rw-rw-rw-"
nil
20920)
and here is how the result is interpreted:
`nil'
is neither a directory nor a symbolic link.
`1'
has only one name (the name `files.texi' in the current
default directory).
`2235'
is owned by the user with UID 2235.
`75'
is in the group with GID 75.
`(8489 20284)'
was last accessed on Aug 19 00:09. Unfortunately, you
cannot convert this number into a time string in Emacs.
`(8489 20284)'
was last accessed on Aug 19 00:09.
`(8489 20285)'
last had its inode changed on Aug 19 00:09.
`14906'
is 14906 characters long.
`"-rw-rw-rw-"'
has a mode of read and write access for the owner, group,
and world.
`nil'
would retain the same GID if it were recreated.
`20920'
has an inode number of 20920.
File: elisp, Node: Contents of Directories, Next: Changing File Attributes, Prev: Information about Files, Up: Files
Contents of Directories
=======================
A directory is a kind of file that contains other files entered
under various names. Directories are a feature of the file system.
Emacs can list the names of the files in a directory as a Lisp
list, or display the names in a buffer using the `ls' shell command.
In the latter case, it can optionally display information about each
file, depending on the value of switches passed to the `ls' command.
* Function: directory-files DIRECTORY &optional FULL-NAME MATCH-REGEXP
This function returns a list of the names of the files in the
directory DIRECTORY.
If FULL-NAME is non-`nil', the function returns the files'
absolute file names. Otherwise, it returns just the names
relative to the specified directory.
If MATCH-REGEXP is non-`nil', function returns only those file
names that contain that regular expression--the other file names
are discarded from the list.
(directory-files "~lewis")
=> ("#foo#" "#foo.el#" "." ".."
"dired-mods.el" "files.texi" "files.texi.~1~")
An error is signaled if DIRECTORY is not the name of a directory
that can be read.
* Function: file-name-all-versions FILE DIRNAME
This function returns a list of all versions of the file named
FILE in directory DIRNAME.
* Command: list-directory FILESPEC &optional VERBOSE
This command displays a list of files in or matching FILESPEC.
It calls the shell command `ls', passing it options under the
control of VERBOSE.
The argument FILESPEC may be either a directory name or a file
name containing optional wildcards. Wildcards are processed by
the shell.
The options passed to `ls' are the value of
`list-directory-verbose-switches' if VERBOSE is non-`nil';
`list-directory-brief-switches' otherwise. Interactively, the
raw prefix argument is used for VERBOSE.
* Variable: list-directory-brief-switches
This variable contains switches for `list-directory' to pass to
`ls' for a short or "brief" listing. The default value is
`"-CF"'.
* Variable: list-directory-verbose-switches
This variable contains switches for `list-directory' to pass to
`ls' for a verbose or "long" listing. The default value is
`"-l"'.
File: elisp, Node: Changing File Attributes, Next: File Names, Prev: Contents of Directories, Up: Files
Changing File Names and Attributes
==================================
The functions in this section rename, copy, delete, link, and set
the modes of files.
In the functions that have an argument NEWNAME, if a file by the
name of NEWNAME already exists, the actions taken depend on the value
of the argument OK-IF-ALREADY-EXISTS:
* A `file-already-exists' error is signaled if
OK-IF-ALREADY-EXISTS is `nil'.
* Confirmation is requested if OK-IF-ALREADY-EXISTS is a number.
* No confirmation is requested if OK-IF-ALREADY-EXISTS is any
other value, in which case the old file is removed.
* Function: add-name-to-file OLDNAME NEWNAME &optional
OK-IF-ALREADY-EXISTS
This function gives the file named OLDNAME the additional name
NEWNAME. This means that NEWNAME will be a new "hard link" to
OLDNAME.
In the first part of the following example, we list two files,
`foo' and `foo3'.
% ls -l fo*
-rw-rw-rw- 1 rms 29 Aug 18 20:32 foo
-rw-rw-rw- 1 rms 24 Aug 18 20:31 foo3
Then we evaluate the form `(add-name-to-file "~/lewis/foo"
"~/lewis/foo2")'. Again we list the files. This shows two
names, `foo' and `foo2'.
(add-name-to-file "~/lewis/foo1" "~/lewis/foo2")
=> nil
% ls -l fo*
-rw-rw-rw- 2 rms 29 Aug 18 20:32 foo
-rw-rw-rw- 2 rms 29 Aug 18 20:32 foo2
-rw-rw-rw- 1 rms 24 Aug 18 20:31 foo3
Finally, we evaluate `(add-name-to-file "~/lewis/foo"
"~/lewis/foo3" t)', and list the files again. Now there are
three names for one file: `foo', `foo2', and `foo3'. The old
contents of `foo3' are lost.
(add-name-to-file "~/lewis/foo1" "~/lewis/foo3")
=> nil
% ls -l fo*
-rw-rw-rw- 3 rms 29 Aug 18 20:32 foo
-rw-rw-rw- 3 rms 29 Aug 18 20:32 foo2
-rw-rw-rw- 3 rms 29 Aug 18 20:32 foo3
This function is meaningless on VMS, where multiple names for
one file are not allowed.
See also `file-nlinks' in *Note Information about Files::.
* Command: rename-file FILENAME NEWNAME &optional OK-IF-ALREADY-EXISTS
This command renames the file FILENAME as NEWNAME.
If FILENAME has additional names aside from FILENAME, it
continues to have those names. In fact, adding the name NEWNAME
with `add-name-to-file' and then deleting FILENAME has the same
effect as renaming, aside from momentary intermediate states.
In an interactive call, this function prompts for FILENAME and
NEWNAME in the minibuffer; also, it requests confirmation if
NEWNAME already exists.
* Command: copy-file OLDNAME NEWNAME &optional OK-IF-EXISTS TIME
This command copies the file OLDNAME to NEWNAME. An error is
signaled if OLDNAME does not exist.
If TIME is non-`nil', then this functions gives the new file the
same last-modified time that the old one has. (This works on
only some operating systems.)
In an interactive call, this function prompts for FILENAME and
NEWNAME in the minibuffer; also, it requests confirmation if
NEWNAME already exists.
* Command: delete-file FILENAME
This command deletes the file FILENAME, like the shell command
`rm FILENAME'. If the file has multiple names, it continues to
exist under the other names.
A suitable kind of `file-error' error is signaled if the file
does not exist, or is not deletable. (In Unix, a file is
deletable if its directory is writable.)
* Command: make-symbolic-link FILENAME NEWNAME &optional OK-IF-EXISTS
This command makes a symbolic link to FILENAME, named NEWNAME.
This is like the shell command `ln -s FILENAME NEWNAME'.
In an interactive call, FILENAME and NEWNAME are read in the
minibuffer, and OK-IF-EXISTS is set to the numeric prefix
argument.
* Function: set-file-modes FILENAME MODE
This function sets mode bits of FILENAME to MODE (which must be
a integer). Only the 12 low bits of MODE are used.
File: elisp, Node: File Names, Prev: Changing File Attributes, Up: Files
File Names
==========
Files are generally referred to by their names, in Emacs as
elsewhere. File names in Emacs are represented as strings. The
functions that operate on a file all expect a file name argument.
In addition to operating on files themselves, Emacs Lisp programs
often need to operate on the names; i.e., to take them apart and to
use part of a name to construct related file names. This section
describes how to manipulate file names.
The functions in this section do not actually access files, so
they can operate on file names that do not refer to an existing file
or directory.
On VMS, all these functions understand both VMS file name syntax
and Unix syntax. This is so that all the standard Lisp libraries can
specify file names in Unix syntax and work properly on VMS without
change.
* Menu:
* File Name Components:: The directory part of a file name, and the rest.
* Directory Names:: A directory's name as a directory
is different from its name as a file.
* Relative File Names:: Some file names are relative to a current directory.
* File Name Expansion:: Converting relative file names to absolute ones.
* Unique File Names:: Generating names for temporary files.
* File Name Completion:: Finding the completions for a given file name.
File: elisp, Node: File Name Components, Next: Directory Names, Up: File Names
File Name Components
--------------------
The operating system groups files into directories. To specify a
file, you must specify the directory, and the file's name in that
directory. Therefore, a file name in Emacs is considered to have two
main parts: the "directory name" part, and the "nondirectory" part
(or "file name within the directory"). Either part may be empty.
Concatenating these two parts reproduces the original file name.
On Unix, the directory part is everything up to and including the
last slash; the nondirectory part is the rest. The rules in VMS
syntax are complicated.
For some purposes, the nondirectory part is further subdivided
into the name and the version number. On Unix, only backup files
have version numbers in their names; on VMS, every file has a version
number, but most of the time the file name actually used in Emacs
omits the version number. Version numbers are found mostly in
directory lists.
* Function: file-name-directory FILENAME
This function returns the directory part of FILENAME (or `nil'
if FILENAME does not include a directory part). On Unix, the
function returns a string ending in a slash. On VMS, it returns
a string ending in one of the three characters `:', `]', or `>'.
(file-name-directory "lewis/foo") ; Unix example
=> "lewis/"
(file-name-directory "foo") ; Unix example
=> nil
(file-name-directory "[X]FOO.TMP") ; VMS example
=> "[X]"
* Function: file-name-nondirectory FILENAME
This function returns the nondirectory part of FILENAME.
(file-name-nondirectory "lewis/foo")
=> "foo"
(file-name-nondirectory "foo")
=> "foo"
;; The following example is accurate only on VMS.
(file-name-nondirectory "[X]FOO.TMP")
=> "FOO.TMP"
* Function: file-name-sans-versions FILENAME
This function returns FILENAME without any file version numbers,
backup version numbers, or trailing tildes.
(file-name-sans-versions "~rms/foo.~1~")
=> "~rms/foo"
(file-name-sans-versions "~rms/foo~")
=> "~rms/foo"
(file-name-sans-versions "~rms/foo")
=> "~rms/foo"
;; The following example applies to VMS only.
(file-name-sans-versions "foo;23")
=> "foo"
File: elisp, Node: Directory Names, Next: Relative File Names, Prev: File Name Components, Up: File Names
Directory Names
---------------
A "directory name" is the name of a directory. A directory is a
kind of file, and it has a file name, which is related to the
directory name but not identical to it. (This is not quite the same
as the usual Unix terminology.) These two different names for the
same entity are related by a syntactic transformation. On Unix, this
is simple: a directory name ends in a slash, whereas the directory's
name as a file lacks that slash. On VMS, the relationship is more
complicated.
The difference between a directory name and its name as a file is
subtle but crucial. When an Emacs variable or function argument is
described as being a directory name, a file name of a directory is
not acceptable.
All of these functions take a single argument, FILENAME, which
must be a string. Environment variable substitutions such as
`$HOME', and the symbols `~', and `..', are *not* expanded. Use
`expand-file-name' or `substitute-in-file-name' for that (*note File
Name Expansion::.).
* Function: file-name-as-directory FILENAME
This function returns a string representing FILENAME in a form
that the operating system will interpret as the name of a
directory. In Unix, this means that a slash is appended to the
string. On VMS, the function converts a string of the form
`[X]Y.DIR.1' to the form `[X.Y]'.
(file-name-as-directory "~rms/lewis")
=> "~rms/lewis/"
* Function: directory-file-name DIRNAME
This function returns a string representing DIRNAME in a form
that the operating system will interpret as the name of a file.
On Unix, this means that a final slash is removed from the
string. On VMS, the function converts a string of the form
`[X.Y]' to `[X]Y.DIR.1'.
(directory-file-name "~lewis/")
=> "~lewis"
File: elisp, Node: Relative File Names, Next: File Name Expansion, Prev: Directory Names, Up: File Names
Absolute and Relative File Names
--------------------------------
All the directories in the file system form a tree starting at the
root directory. A file name can specify all the directory names
starting from the root of the tree; then it is called an "absolute"
file name. Or it can specify the position of the file in the tree
relative to a default directory; then it is called an "relative" file
name. On Unix, an absolute file name starts with a slash or a tilde
(`~'), and a relative one does not. The rules on VMS are complicated.
* Function: file-name-absolute-p FILENAME
This function returns `t' if file FILENAME is an absolute file
name, `nil' otherwise. On VMS, this function understands both
Unix syntax and VMS syntax.
(file-name-absolute-p "~rms/foo")
=> t
(file-name-absolute-p "rms/foo")
=> nil
(file-name-absolute-p "$HOME")
=> nil
(file-name-absolute-p "/user/rms/foo")
=> t
File: elisp, Node: File Name Expansion, Next: Unique File Names, Prev: Relative File Names, Up: File Names
Functions that Expand Filenames
-------------------------------
"Expansion" of a file name means converting a relative file name
to an absolute one. Since this is done relative to a default
directory, you must specify the default directory name as well as the
file name to be expanded. Expansion also canonicalizes file names by
eliminating redundancies such as `./' and `NAME/../'.
* Function: expand-file-name FILENAME &optional DIRECTORY
This function converts FILENAME to an absolute file name. If
DIRECTORY is supplied, it is the directory to start with if
FILENAME is relative. Otherwise, the current buffer's value of
`default-directory' is used. For example:
(expand-file-name "foo")
=> "/xcssun/users/rms/lewis/foo"
(expand-file-name "../foo")
=> "/xcssun/users/rms/foo"
(expand-file-name "foo" "/usr/spool")
=> "/usr/spool/foo"
(expand-file-name "$HOME/foo")
=> "/xcssun/users/rms/lewis/$HOME/foo"
Filenames containing `.' or `..' are simplified to their
canonical form:
(expand-file-name "bar/../foo")
=> "/xcssun/users/rms/lewis/foo"
`~/' is expanded into the user's home directory. A `/' or `~'
following a `/' is taken to be the start of an absolute file
name that overrides what precedes it, so everything before that
`/' or `~' is deleted. For example:
(expand-file-name "/a1/gnu//usr/local/lib/emacs/etc/MACHINES")
=> "/usr/local/lib/emacs/etc/MACHINES"
(expand-file-name "/a1/gnu/~/foo")
=> "/xcssun/users/rms/foo"
In both cases, `/a1/gnu/' has been discarded because an absolute
file name follows it.
Note that `expand-file-name' does *not* expand environment
variables; that is done only by `substitute-in-file-name'.
* Variable: default-directory
The value of this buffer-local variable is the default directory
for the current buffer. It is local in every buffer.
`expand-file-name' uses the default directory when its second
argument is `nil'.
On Unix systems, the value is always a string ending with a slash.
default-directory
=> "/user/lewis/manual/"
* Function: substitute-in-file-name FILENAME
This function replaces environment variables names in FILENAME
with the values to which they are set by the operating system.
Following standard Unix shell syntax, `$' is the prefix to
substitute an environment variable value.
The environment variable name is the series of alphanumeric
characters (including underscores) that follow the `$'. If the
character following the `$' is a `{', then the variable name is
everything up to the matching `}'.
Here we assume that the environment variable `HOME', which holds
the user's home directory name, has the value `/xcssun/users/rms'.
(substitute-in-file-name "$HOME/foo")
=> "/xcssun/users/rms/foo"
If a `~' or a `/' appears following a `/', after substitution,
everything before the following `/' is discarded:
(substitute-in-file-name "bar/~/foo")
=> "~/foo"
(substitute-in-file-name "/usr/local/$HOME/foo")
=> "/xcssun/users/rms/foo"
On VMS, `$' substitution is not done, so this function does
nothing on VMS except discard superfluous initial components as
shown above.
File: elisp, Node: Unique File Names, Next: File Name Completion, Prev: File Name Expansion, Up: File Names
Generating Unique File Names
----------------------------
Some programs need to write temporary files. Here is the usual
way to construct a name for such a file:
(concat "/tmp/" (make-temp-name NAME-OF-APPLICATION))
The directory `/tmp/' is chosen because that is the standard place on
Unix for temporary files. The task of `make-temp-name' is to prevent
two different users or two different jobs from trying to use the same
name.
* Function: make-temp-name STRING
This function generates string that can be used as a unique name.
The name will start with the prefix STRING, and finish with a
number that is different in each Emacs job.
(make-temp-name "foo")
=> "foo021304"
(make-temp-name "foo")
=> "foo021304"
To prevent conflicts among different application libraries run
in the same Emacs, each application should have its own STRING.
The number added to the end of the name distinguishes between
the same application running in different Emacses.
File: elisp, Node: File Name Completion, Prev: Unique File Names, Up: File Names
File Name Completion
--------------------
This section describes low-level subroutines for completing a file
name. For other completion functions, see *Note Completion::.
* Function: file-name-all-completions PARTIAL-FILENAME DIRECTORY
This function returns a list of all possible completions for a
file whose name starts with PARTIAL-FILENAME in directory
DIRECTORY. The order of the completions is the order of the
files in the directory, which is unpredictable and conveys no
useful information.
The argument PARTIAL-FILENAME must be a file name containing no
directory part and no slash. The current buffer's default
directory is prepended to DIRECTORY, if DIRECTORY is not an
absolute file name.
In the following example, suppose that the current default
directory, `~rms/lewis', has five files whose names begin with
`f': `foo', `file~', `file.c', `file.c.~1~', and `file.c.~2~'.
(file-name-all-completions "f" "")
=> ("foo" "file~" "file.c.~2~" "file.c.~1~" "file.c")
(file-name-all-completions "fo" "")
=> ("foo")
* Function: file-name-completion FILENAME DIRECTORY
This function completes the file name FILENAME in directory
DIRECTORY. It returns the longest prefix common to all file
names in directory DIRECTORY that start with FILENAME.
If only one match exists and FILENAME matches it exactly, the
function returns `t'. The function returns `nil' if directory
DIRECTORY contains no name starting with FILENAME.
In the following example, suppose that the current default
directory has five files whose names begin with `f': `foo',
`file~', `file.c', `file.c.~1~', and `file.c.~2~'.
(file-name-completion "fi" "")
=> "file"
(file-name-completion "file.c.~1" "")
=> "file.c.~1~"
(file-name-completion "file.c.~1~" "")
=> t
(file-name-completion "file.c.~3" "")
=> nil
* User Option: completion-ignored-extensions
`file-name-completion' usually ignores file names that end in
any string in this list. It does not ignore them when all the
possible completions end in one of these suffixes or when a
buffer showing all possible completions is displayed.
A typical value might look like this:
completion-ignored-extensions
=> (".o" ".elc" "~" ".dvi")
File: elisp, Node: Backups and Auto-Saving, Next: Buffers, Prev: Files, Up: Top
Backups and Auto-Saving
***********************
Backup files and auto-save files are two methods by which Emacs
tries to protect the user from the consequences of crashes or of the
user's own errors. Auto-saving preserves the text from earlier in
the current editing session; backup files preserve file contents
prior to the current session.
* Menu:
* Backup Files:: How backup files are made; how their names are chosen.
* Auto-Saving:: How auto-save files are made; how their names are chosen.
* Reverting:: `revert-buffer', and how to customize what it does.
File: elisp, Node: Backup Files, Next: Auto-Saving, Prev: Backups and Auto-Saving, Up: Backups and Auto-Saving
Backup Files
============
A "backup file" is a copy of the old contents of a file you are
editing. Emacs makes a backup file the first time you save a buffer
into its visited file. Normally, this means that the backup file
contains the contents of the file as it was before the current
editing session. The contents of the backup file normally remain
unchanged once it exists.
Backups are usually made by renaming the visited file to a new name.
Optionally, you can specify that backup files should be made by
copying the visited file. This choice makes a difference for files
with multiple names; it also can affect whether the edited file
remains owned by the original owner or becomes owned by the user
editing it.
By default, Emacs makes a single backup file for each file edited.
You can alternatively request numbered backups; then each new backup
file gets a new name. You can delete old numbered backups when you
don't want them any more, or Emacs can delete them automatically.
* Menu:
* Making Backups:: How Emacs makes backup files, and when.
* Rename or Copy:: Two alternatives: renaming the old file or copying it.
* Numbered Backups:: Keeping multiple backups for each source file.
* Backup Names:: How backup file names are computed; customization.
File: elisp, Node: Making Backups, Next: Rename or Copy, Prev: Backup Files, Up: Backup Files
Making Backup Files
-------------------
* Function: backup-buffer
This function makes a backup of the file visited by the current
buffer, if appropriate. It is called by `save-buffer' before
saving the buffer the first time.
* Variable: buffer-backed-up
This buffer-local variable indicates whether this buffer's file
has been backed up on account of this buffer. If it is
non-`nil', then the backup file has been written. Otherwise,
the file should be backed up when it is next saved (if backup
files are enabled).
* Variable: make-backup-files
This variable determines whether or not backup files will be
created. If it is non-`nil', then Emacs will create a backup of
each file when it is saved for the first time.
The following example shows how to change the
`make-backup-files' variable only in the `RMAIL' buffer and not
elsewhere. Setting it `nil' stops Emacs from making backups of
the `RMAIL' file, which may save disk space. (You would put
this code in your `.emacs' file.)
(setq rmail-mode-hook
(function (lambda ()
(make-local-variable 'make-backup-files)
(setq make-backup-files nil)))