In Emacs, you can find, create, view, save, and otherwise work with files and file directories. This chapter describes most of the file-related functions of Emacs Lisp, but a few others are described in section Buffers, and those related to backups and auto-saving are described in section Backups and Auto-Saving.
Many of the file functions take one or more arguments that are
file names. A file name is actually a string. Most of these
functions expand file name arguments by calling
expand-file-name, so that `~' is handled
correctly, as are relative file names (including
`../'). These functions don't recognize environment
variable substitutions such as `$HOME'. See section Functions that Expand Filenames.
Visiting a file means reading a file into a buffer. Once this is done, we say that the buffer is visiting that file, and call the file "the visited file" of the buffer.
A file and a buffer are two different things. A file is information recorded permanently in the computer (unless you delete it). A buffer, on the other hand, is information inside of Emacs that will vanish at the end of the editing session (or when you kill the buffer). Usually, a buffer contains information that you have copied from a file; then we say the buffer is visiting that file. The copy in the buffer is what you modify with editing commands. Such changes to the buffer do not change the file; therefore, to make the changes permanent, you must save the buffer, which means copying the altered buffer contents back into the file.
In spite of the distinction between files and buffers, people often refer to a file when they mean a buffer and vice-versa. Indeed, we say, "I am editing a file," rather than, "I am editing a buffer that I will soon save as a file of the same name." Humans do not usually need to make the distinction explicit. When dealing with a computer program, however, it is good to keep the distinction in mind.
This section describes the functions normally used to visit files. For historical reasons, these functions have names starting with `find-' rather than `visit-'. See section Buffer File Name, for functions and variables that access the visited file name of a buffer or that find an existing buffer by its visited file name.
In a Lisp program, if you want to look at the contents of a file
but not alter it, the fastest way is to use
insert-file-contents in a temporary buffer. Visiting
the file is not necessary and takes longer. See section Reading from Files.
The body of the find-file function is very simple
and looks like this:
(switch-to-buffer (find-file-noselect filename))
(See switch-to-buffer in section Displaying Buffers in Windows.)
When find-file is called interactively, it prompts
for filename in the minibuffer.
When find-file-noselect uses an existing buffer, it
first verifies that the file has not changed since it was last
visited or saved in that buffer. If the file has changed, then this
function asks the user whether to reread the changed file. If the
user says `yes', any changes previously made in the
buffer are lost.
This function displays warning or advisory messages in various
peculiar cases, unless the optional argument nowarn is
non-nil. For example, if it needs to create a buffer,
and there is no file named filename, it displays the
message `New file' in the echo area, and leaves the
buffer empty.
The find-file-noselect function normally calls
after-find-file after reading the file (see section Subroutines of Visiting). That
function sets the buffer major mode, parses local variables, warns
the user if there exists an auto-save file more recent than the
file just visited, and finishes by running the functions in
find-file-hooks.
If the optional argument rawfile is
non-nil, then after-find-file is not
called, and the find-file-not-found-hooks are not run
in case of failure. What's more, a non-nil
rawfile value suppresses coding system conversion (see
section Coding Systems) and format
conversion (see section File Format
Conversion).
The find-file-noselect function returns the buffer
that is visiting the file filename.
(find-file-noselect "/etc/fstab")
=> #<buffer fstab>
When this command is called interactively, it prompts for filename.
find-file, but it marks the buffer as read-only. See
section Read-Only Buffers, for
related functions and variables. When this command is called interactively, it prompts for filename.
view-mode-hook. See section Hooks. When view-file is called interactively, it prompts
for filename.
This variable works just like a normal hook, but we think that renaming it would not be advisable. See section Hooks.
find-file or
find-file-noselect is passed a nonexistent file name.
find-file-noselect calls these functions as soon as it
detects a nonexistent file. It calls them in the order of the list,
until one of them returns non-nil.
buffer-file-name is already set up. This is not a normal hook because the values of the functions are used, and in many cases only some of the functions are called.
The find-file-noselect function uses two important
subroutines which are sometimes useful in user Lisp code:
create-file-buffer and after-find-file.
This section explains how to use them.
Please note: create-file-buffer
does not associate the new buffer with a file and does not
select the buffer. It also does not use the default major mode.
(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. It
uses generate-new-buffer (see section Creating Buffers).
find-file-noselect and by the default revert
function (see section Reverting).
If reading the file got an error because the
file does not exist, but its directory does exist, the caller
should pass a non-nil value for error. In
that case, after-find-file issues a warning:
`(New File)'. For more serious errors, the caller
should usually not call after-find-file.
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 the list find-file-hooks.
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.
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 since visiting the file. Other values for
backup-option request the making of backup files in
other circumstances:
save-buffer function marks this
version of the file to be backed up when the buffer is next
saved.
save-buffer function
unconditionally backs up the previous version of the file before
saving it.
nil, it 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 buffer-local value of
buffer-offer-save. (A user who says yes to saving one
of these is asked to specify a file name to use.) The
save-buffers-kill-emacs function passes a
non-nil value for this argument.
set-visited-file-name (see section Buffer File Name) and
save-buffer.
Saving a buffer runs several hooks. It also performs format conversion (see section File Format Conversion), and may save text properties in "annotations" (see section Saving Text Properties in Files).
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 a function in write-file-hooks returns
non-nil, it is responsible for making a backup file
(if that is appropriate). To do so, execute the following code:
(or buffer-backed-up (backup-buffer))
You might wish to save the file modes value returned by
backup-buffer and use that to set the mode bits of the
file that you write. This is what save-buffer normally
does.
The hook functions in write-file-hooks are also
responsible for encoding the data (if desired): they must choose a
suitable coding system (see section Coding Systems in Lisp), perform the
encoding (see section Explicit Encoding
and Decoding), and set last-coding-system-used to
the coding system that was used (see section Encoding and I/O).
Do not make this variable buffer-local. To set up
buffer-specific hook functions, use
write-contents-hooks instead.
Even though this is not a normal hook, you can use
add-hook and remove-hook to manipulate
the list. See section Hooks.
write-file-hooks, but it is intended to be made
buffer-local in particular buffers, and used for hooks that pertain
to the file name or the way the buffer contents were obtained.
The variable is marked as a permanent local, so that changing the major mode does not alter a buffer-local value. This is convenient for packages that read "file" contents in special ways, and set up hooks to save the data in a corresponding way.
write-file-hooks, but it is intended for hooks that
pertain to the contents of the file, as opposed to hooks that
pertain to where the file came from. Such hooks are usually set up
by major modes, as buffer-local bindings for this variable. This variable automatically becomes buffer-local whenever it is
set; switching to a new major mode always resets this variable.
When you use add-hooks to add an element to this hook,
you should not specify a non-nil
local argument, since this variable is used
only buffer-locally.
nil, then save-buffer protects
against I/O errors while saving by writing the new file to a
temporary name instead of the name it is supposed to have, and then
renaming it to the intended name after it is clear there are no
errors. This procedure prevents problems such as a lack of disk
space from resulting in an invalid file. As a side effect, backups are necessarily made by copying. See section Backup by Renaming or by Copying?. Yet, at the same time, saving a precious file always breaks all hard links between the file you save and other file names.
Some modes give this variable a non-nil
buffer-local value in particular buffers.
t, then
save-buffer silently adds 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 save-buffer asks the user whether
to add a newline each time the case arises. If the value of the variable is nil, then
save-buffer doesn't add newlines at all.
nil is the default value, but a few major modes set it
to t in particular buffers.
See also the function set-visited-file-name (see
section Buffer File Name).
You can copy a file from the disk and insert it into a buffer
using the insert-file-contents function. Don't use the
user-level command insert-file in a Lisp program, as
that sets the mark.
The function insert-file-contents checks the file
contents against the defined file formats, and converts the file
contents if appropriate. See section File Format Conversion. It also calls
the functions in the list after-insert-file-functions;
see section Saving Text Properties in
Files.
If visit is non-nil, this function
additionally 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 probably should not use it
yourself.
If beg and end are non-nil,
they should be integers specifying the portion of the file to
insert. In this case, visit must be nil.
For example,
(insert-file-contents filename nil 0 500)
inserts the first 500 characters of a file.
If the argument replace is non-nil, it
means to replace the contents of the buffer (actually, just the
accessible portion) with the contents of the file. This is better
than simply deleting the buffer contents and inserting the whole
file, because (1) it preserves some marker positions and (2) it
puts less data in the undo list.
It is possible to read a special file (such as a FIFO or an I/O
device) with insert-file-contents, as long as
replace and visit are nil.
insert-file-contents except that it does not do format
decoding (see section File Format
Conversion), does not do character code conversion (see section
Coding Systems), does not run
find-file-hooks, does not perform automatic
uncompression, and so on.
If you want to pass a file name to another process so that
another program can read the file, use the function
file-local-copy; see section Making Certain File Names "Magic".
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.
nil. An error is signaled if filename specifies a nonwritable file, or a nonexistent file in a directory where files cannot be created.
If start is a string, then write-region
writes or appends that string, rather than text from the
buffer.
If append is non-nil, then the specified
text is appended to the existing file contents (if any).
If confirm is non-nil, then
write-region asks for confirmation if
filename names an existing file.
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
save-buffer, but you probably should not use it
yourself.
If visit is a string, it specifies the file name to
visit. This way, you can write the data to one file
(filename) while recording the buffer as visiting
another file (visit). The argument visit is
used in the echo area message and also for file locking;
visit is stored in buffer-file-name. This
feature is used to implement file-precious-flag; don't
use it yourself unless you really know what you're doing.
The function write-region converts the data which
it writes to the appropriate file formats specified by
buffer-file-format. See section File Format Conversion. It also calls
the functions in the list
write-region-annotate-functions; see section Saving Text Properties in Files.
Normally, write-region displays the message
`Wrote filename' in the echo area. If
visit is neither t nor nil nor
a string, then this message is inhibited. This feature is useful
for programs that use files for internal purposes, files that the
user does not need to know about.
with-temp-file macro evaluates
the body forms with a temporary buffer as the current
buffer; then, at the end, it writes the buffer contents into file
file. It kills the temporary buffer when finished,
restoring the buffer that was current before the
with-temp-file form. Then it returns the value of the
last form in body. The current buffer is restored even in case of an abnormal exit
via throw or error (see section Nonlocal Exits).
See also with-temp-buffer in section The Current Buffer.
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 are not completely reliable when multiple machines can share file systems. When file locks do not work, it is possible for two users to make changes simultaneously, but Emacs can still 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 section Comparison of Modification Time.
nil if the file filename is not locked. It
returns t if it is locked by this Emacs process, and
it returns the name of the user who has locked it if it is locked
by some other job.
(file-locked-p "foo")
=> nil
t says to grab the lock on the file.
Then this user may edit the file and other-user loses
the lock.
nil says to ignore the lock and let
this user edit the file anyway.
file-locked error, in which case the change
that 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-userwhere
file is the name of the file and
other-user is the name of the user who has locked the
file.
If you wish, you can replace the
ask-user-about-lock function with your own version
that makes the decision in another way. The code for its usual
definition is in `userlock.el'.
The functions described in this section all operate on strings that designate file names. All the functions 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.
These functions test for permission to access a file in specific ways.
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.
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
t if a file named filename exists and you
can execute it. It returns nil otherwise. If the file
is a directory, execute permission means you can check the
existence and attributes of files inside the directory, and open
those files if their modes permit.
t if the file filename can be written or
created by you, and nil otherwise. A file is writable
if the file exists and you can write it. It is creatable if it does
not exist, but the specified directory does exist and you can write
in that directory. In the third example below, `foo' is not writable because the parent directory does not exist, even though the user could create such a directory.
(file-writable-p "~/foo")
=> t
(file-writable-p "/foo")
=> nil
(file-writable-p "~/no-such-dir/foo")
=> nil
t if you have permission to open existing files in the
directory whose name as a file is dirname; otherwise (or
if there is no such directory), it returns nil. The
value of dirname may be either a directory name or the
file name of a file which is a directory. Example: after the following,
(file-accessible-directory-p "/foo")
=> nil
we can deduce that any attempt to read a file in `/foo/' will give an error.
nil. However,
if the open fails, it signals an error using string as
the error message text.
t if deleting the file filename and then
creating it anew would keep the file's owner unchanged.
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, `aug-20' was written on the 20th, and 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
You can use file-attributes to get a file's last
modification time as a list of two numbers. See section Other Information about Files.
This section describes how to distinguish various kinds of files, such as directories, symbolic links, and ordinary files.
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 it may be a
nonexistent file name. If the file 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"
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
t if the file filename exists and is a
regular file (not a directory, symbolic link, named pipe, terminal,
or other I/O device).
The truename of a file is the name that you get by following symbolic links until none remain, then simplifying away `.' and `..' appearing as components. Strictly speaking, a file need not have a unique truename; the number of distinct truenames a file has is equal to the number of hard links to the file. However, truenames are useful because they eliminate symbolic links as a cause of name variation.
file-truename returns the true name of the file
filename. This is the name that you get by following
symbolic links until none remain. The argument must be an absolute
file name.
See section Buffer File Name, for related information.
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.
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
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
nil. The elements of the list, in order, are:
t for a directory, a string for a symbolic link
(the name linked to), or nil for a text file.
add-name-to-file function (see section Changing File Names and
Attributes).
current-time; see section Time of Day.)
t if the file's GID would change if file were
deleted and recreated; nil otherwise.
(high .
low), where low holds the low 16
bits.
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 129500 -32252)
and here is how the result is interpreted:
nil1223575(8489 20284)(8489 20284)(8489 20285)14906"-rw-rw-rw-"nil129500-32252The 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:
file-already-exists error if
ok-if-already-exists is nil.
In the first part of the following example, we list two files, `foo' and `foo3'.
% ls -li fo* 81908 -rw-rw-rw- 1 rms 29 Aug 18 20:32 foo 84302 -rw-rw-rw- 1 rms 24 Aug 18 20:31 foo3
Now we create a hard link, by calling
add-name-to-file, then list the files again. This
shows two names for one file, `foo' and
`foo2'.
(add-name-to-file "foo" "foo2")
=> nil
% ls -li fo*
81908 -rw-rw-rw- 2 rms 29 Aug 18 20:32 foo
81908 -rw-rw-rw- 2 rms 29 Aug 18 20:32 foo2
84302 -rw-rw-rw- 1 rms 24 Aug 18 20:31 foo3
Finally, we evaluate the following:
(add-name-to-file "foo" "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 "foo1" "foo3")
=> nil
% ls -li fo*
81908 -rw-rw-rw- 3 rms 29 Aug 18 20:32 foo
81908 -rw-rw-rw- 3 rms 29 Aug 18 20:32 foo2
81908 -rw-rw-rw- 3 rms 29 Aug 18 20:32 foo3
This function is meaningless on operating systems where multiple names for one file are not allowed.
See also file-nlinks in section Other Information about Files.
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.
If time is non-nil, then this function
gives the new file the same last-modified time that the old one
has. (This works on only some operating systems.) If setting the
time gets an error, copy-file signals a
file-date-error error.
In an interactive call, this function prompts for filename and newname in the minibuffer; also, it requests confirmation if newname already exists.
A suitable kind of file-error error is signaled if
the file does not exist, or is not deletable. (On Unix, a file is
deletable if its directory is writable.)
See also delete-directory in section Creating and Deleting Directories.
In an interactive call, this function prompts for filename and newname in the minibuffer; also, it requests confirmation if newname already exists.
The argument mode must be an integer. On most systems, only the low 9 bits of mode are meaningful.
Saving a modified version of an existing file does not count as creating the file; it does not change the file's mode, and does not use the default file protection.
On MS-DOS, there is no such thing as an
"executable" file mode bit. So Emacs considers a file executable if
its name ends in `.com', `.bat' or
`.exe'. This is reflected in the values returned by
file-modes and file-attributes.
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 file 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. On MS-DOS and MS-Windows, these functions understand MS-DOS or MS-Windows file-name syntax as well as Unix syntax.
The operating system groups files into directories. To specify a file, you must specify the directory and the file's name within that directory. Therefore, Emacs considers a file name as having 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 proper 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, so that version numbers in Emacs are found mostly in directory lists.
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]"
(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"
(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-name-sans-extension "foo.lose.c")
=> "foo.lose"
(file-name-sans-extension "big.hack/foo")
=> "big.hack/foo"
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.
The following two functions convert between directory names and file names. They do nothing special with environment variable substitutions such as `$HOME', and the constructs `~', and `..'.
(file-name-as-directory "~rms/lewis")
=> "~rms/lewis/"
(directory-file-name "~lewis/")
=> "~lewis"
Directory name abbreviations are useful for directories that are normally accessed through symbolic links. Sometimes the users recognize primarily the link's name as "the name" of the directory, and find it annoying to see the directory's "real" name. If you define the link name as an abbreviation for the "real" name, Emacs shows users the abbreviation instead.
directory-abbrev-alist contains an alist of
abbreviations to use for file directories. Each element has the
form (from . to), and says to
replace from with to when it appears in a
directory name. The from string is actually a regular
expression; it should always start with `^'. The
function abbreviate-file-name performs these
substitutions. You can set this variable in `site-init.el' to describe the abbreviations appropriate for your site.
Here's an example, from a system on which file system `/home/fsf' and so on are normally accessed through symbolic links named `/fsf' and so on.
(("^/home/fsf" . "/fsf")
("^/home/gp" . "/gp")
("^/home/gd" . "/gd"))
To convert a directory name to its abbreviation, use this function:
directory-abbrev-alist to its
argument, and substitutes `~' for the user's home
directory.
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 a 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.
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 "/user/rms/foo")
=> t
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 simplifies file names by eliminating redundancies such as `./' and `name/../'.
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"
Note that expand-file-name does not expand
environment variables; only substitute-in-file-name
does that.
On some operating systems, an absolute file name begins with a
device name. On such systems, filename has no relative
equivalent based on directory if they start with two
different device names. In this case,
file-relative-name returns filename in
absolute form.
(file-relative-name "/foo/bar" "/foo/")
=> "bar"
(file-relative-name "/foo/bar" "/hack/")
=> "/foo/bar"
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/"
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 value
`/xcssun/users/rms'.
(substitute-in-file-name "$HOME/foo")
=> "/xcssun/users/rms/foo"
After substitution, if a `~' or a `/' appears following a `/', everything before the following `/' is discarded:
(substitute-in-file-name "bar/~/foo")
=> "~/foo"
(substitute-in-file-name "/usr/local/$HOME/foo")
=> "/xcssun/users/rms/foo"
;; `/usr/local/' has been discarded.
On VMS, `$' substitution is not done, so this function does nothing on VMS except discard superfluous initial components as shown above.
Some programs need to write temporary files. Here is the usual way to construct a name for such a file:
(make-temp-name
(expand-file-name name-of-application
temporary-file-directory))
The job of make-temp-name is to prevent two
different users or two different jobs from trying to use the exact
same file name. This example uses the variable
temporary-file-directory to decide where to put the
temporary file. All Emacs Lisp programs should use
temporary-file-directory for this purpose, to give the
user a uniform way to specify the directory for all temporary
files.
(make-temp-name "/tmp/foo")
=> "/tmp/foo232J6v"
To prevent conflicts among different libraries running in the
same Emacs, each Lisp program that uses make-temp-name
should have its own string. The number added to the end
of string distinguishes between the same application
running in different Emacs jobs. Additional added characters permit
a large number of distinct names even in one Emacs job.
expand-file-name is a good way to achieve that. The default value is determined in a reasonable way for your
operating system; on GNU and Unix systems it is based on the
TMP and TMPDIR environment variables.
Even if you do not use make-temp-name to choose the
temporary file's name, you should still use this variable to decide
which directory to put the file in.
This section describes low-level subroutines for completing a file name. For other completion functions, see section Completion.
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 absolute.
In the following example, suppose that `~rms/lewis' is the current default directory, and 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")
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
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")
Most of the file names used in Lisp programs are entered by the
user. But occasionally a Lisp program needs to specify a standard
file name for a particular use--typically, to hold customization
information about each user. For example, abbrev definitions are
stored (by default) in the file `~/.abbrev_defs'; the
completion package stores completions in the file
`~/.completions'. These are two of the many standard file
names used by parts of Emacs for certain purposes.
Various operating systems have their own conventions for valid
file names and for which file names to use for user profile data. A
Lisp program which reads a file using a standard file name ought to
use, on each type of system, a file name suitable for that system.
The function convert-standard-filename makes this easy
to do.
The recommended way to specify a standard file name in a Lisp
program is to choose a name which fits the conventions of GNU and
Unix systems, usually with a nondirectory part that starts with a
period, and pass it to convert-standard-filename
instead of using it directly. Here is an example from the
completion package:
(defvar save-completions-file-name
(convert-standard-filename "~/.completions")
"*The file name to save completions to.")
On GNU and Unix systems, and on some other systems as well,
convert-standard-filename returns its argument
unchanged. On some other systems, it alters the name to fit the
system's conventions.
For example, on MS-DOS the alterations made by this function include converting a leading `.' to `_', converting a `_' in the middle of the name to `.' if there is no other `.', inserting a `.' after eight characters if there is none, and truncating to three characters after the `.'. (It makes other changes as well.) Thus, `.abbrev_defs' becomes `_abbrev.def', and `.completions' becomes `_complet.ion'.
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 options passed to the
ls command.
If full-name is non-nil, the function
returns the files' absolute file names. Otherwise, it returns the
names relative to the specified directory.
If match-regexp is non-nil, this
function returns only those file names that contain a match for
that regular expression--the other file names are excluded from the
list.
If nosort is non-nil,
directory-files does not sort the list, so you get the
file names in no particular order. Use this if you want the utmost
possible speed and don't care what order the files are processed
in. If the order of processing is visible to the user, then the
user will probably be happier if you do sort the names.
(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.
ls according to
switches. It leaves point after the inserted text.
The argument file may be either a directory name or a
file specification including wildcard characters. If
wildcard is non-nil, that means treat
file as a file specification with wildcards.
If full-directory-p is non-nil, that
means the directory listing is expected to show the full contents
of a directory. You should specify t when
file is a directory and switches do not contain
`-d'. (The `-d' option to ls
says to describe a directory itself as a file, rather than showing
its contents.)
This function works by running a directory listing program whose
name is in the variable insert-directory-program. If
wildcard is non-nil, it also runs the shell
specified by shell-file-name, to expand the
wildcards.
insert-directory.
Most Emacs Lisp file-manipulation functions get errors when used
on files that are directories. For example, you cannot delete a
directory with delete-file. These special functions
exist to create and delete directories.
delete-file does not work for files that are
directories; you must use delete-directory for them.
If the directory contains any files, delete-directory
signals an error.
You can implement special handling for certain file names. This is called making those names magic. The principal use for this feature is in implementing remote file names (see section `Remote Files' in The GNU Emacs Manual).
To define a kind of magic file name, you must supply a regular expression to define the class of names (all those that match the regular expression), plus a handler that implements all the primitive Emacs file operations for file names that do match.
The variable file-name-handler-alist holds a list
of handlers, together with regular expressions that determine when
to apply each handler. Each element has this form:
(regexp . handler)
All the Emacs primitives for file access and file name
transformation check the given file name against
file-name-handler-alist. If the file name matches
regexp, the primitives handle that file by calling
handler.
The first argument given to handler is the name of the primitive; the remaining arguments are the arguments that were passed to that operation. (The first of these arguments is typically the file name itself.) For example, if you do this:
(file-exists-p filename)
and filename has handler handler, then handler is called like this:
(funcall handler 'file-exists-p filename)
Here are the operations that a magic file name handler gets to handle:
add-name-to-file, copy-file,
delete-directory, delete-file,
diff-latest-backup-file,
directory-file-name, directory-files,
dired-call-process, dired-compress-file,
dired-uncache, expand-file-name,
file-accessible-direc@discretionary{{}{}tory-p},
file-attributes,
file-direct@discretionary{{}{}ory-p},
file-executable-p, file-exists-p,
file-local-copy, file-modes,
file-name-all-completions,
file-name-as-directory,
file-name-completion,
file-name-directory,
file-name-nondirec@discretionary{{}{}tory},
file-name-sans-versions,
file-newer-than-file-p,
file-ownership-pre@discretionary{{}{}served-p},
file-readable-p, file-regular-p,
file-symlink-p, file-truename,
file-writable-p, find-backup-file-name,
get-file-buffer, insert-directory,
insert-file-contents, load,
make-direc@discretionary{{}{}tory},
make-symbolic-link, rename-file,
set-file-modes, set-visited-file-modtime,
shell-command,
unhandled-file-name-directory,
vc-regis@discretionary{{}{}tered},
verify-visited-file-modtime,
write-region.
Handlers for insert-file-contents typically need to
clear the buffer's modified flag, with (set-buffer-modified-p
nil), if the visit argument is
non-nil. This also has the effect of unlocking the
buffer if it is locked.
The handler function must handle all of the above operations, and possibly others to be added in the future. It need not implement all these operations itself--when it has nothing special to do for a certain operation, it can reinvoke the primitive, to handle the operation "in the usual way". It should always reinvoke the primitive for an operation it does not recognize. Here's one way to do this:
(defun my-file-handler (operation &rest args)
;; First check for the specific operations
;; that we have special handling for.
(cond ((eq operation 'insert-file-contents) ...)
((eq operation 'write-region) ...)
...
;; Handle any operation we don't know about.
(t (let ((inhibit-file-name-handlers
(cons 'my-file-handler
(and (eq inhibit-file-name-operation operation)
inhibit-file-name-handlers)))
(inhibit-file-name-operation operation))
(apply operation args)))))
When a handler function decides to call the ordinary Emacs
primitive for the operation at hand, it needs to prevent the
primitive from calling the same handler once again, thus leading to
an infinite recursion. The example above shows how to do this, with
the variables inhibit-file-name-handlers and
inhibit-file-name-operation. Be careful to use them
exactly as shown above; the details are crucial for proper behavior
in the case of multiple handlers, and for operations that have two
file names that may each have handlers.
nil
if there is none. The argument operation should be the
operation to be performed on the file--the value you will pass to
the handler as its first argument when you call it. The operation
is needed for comparison with
inhibit-file-name-operation.
If filename specifies a magic file name, which programs outside Emacs cannot directly read or write, this copies the contents to an ordinary file and returns that file's name.
If filename is an ordinary file name, not magic, then
this function does nothing and returns nil.
This is useful for running a subprocess; every subprocess must have a non-magic directory to serve as its current directory, and this function is a good way to come up with one.
The
variable format-alist defines a list of file
formats, which describe textual representations used in files
for the data (text, text-properties, and possibly other
information) in an Emacs buffer. Emacs performs format conversion
if appropriate when reading and writing files.
Each format definition is a list of this form:
(name doc-string regexp from-fn to-fn modify mode-fn)
Here is what the elements in a format definition mean:
(position . string),
where position is an integer specifying the relative
position in the text to be written, and string is the
annotation to add there. The list must be sorted in order of
position when to-fn returns it. When
write-region actually writes the text from the buffer
to the file, it intermixes the specified annotations at the
corresponding positions. All this takes place without modifying the
buffer.
t if the encoding function modifies the
buffer, and nil if it works by returning a list of
annotations.
The function insert-file-contents automatically
recognizes file formats when it reads the specified file. It checks
the text of the beginning of the file against the regular
expressions of the format definitions, and if it finds a match, it
calls the decoding function for that format. Then it checks all the
known formats over again. It keeps checking them until none of them
is applicable.
Visiting a file, with find-file-noselect or the
commands that use it, performs conversion likewise (because it
calls insert-file-contents); it also calls the mode
function for each format that it decodes. It stores a list of the
format names in the buffer-local variable
buffer-file-format.
When write-region writes data into a file, it first
calls the encoding functions for the formats listed in
buffer-file-format, in the order of appearance in the
list.
The argument format is a list of format names. If
format is nil, no conversion takes place.
Interactively, typing just RET for format
specifies nil.
nil, they specify which part of the file to read,
as in insert-file-contents (see section Reading from Files). The return value is like what insert-file-contents
returns: a list of the absolute file name and the length of the
data inserted (after conversion).
The argument format is a list of format names. If
format is nil, no conversion takes place.
Interactively, typing just RET for format
specifies nil.
buffer-file-format; however, it
is used instead of buffer-file-format for writing
auto-save files. This variable is always buffer-local in all
buffers.