Go to the previous, next section.

Files

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.

Visiting Files

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 which 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.

Functions for Visiting Files

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.

Command: find-file filename

This function reads the file filename into a buffer and displays that buffer in the selected window so that the user can edit it.

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.

Function: find-file-noselect filename

This function is the guts of all the file-visiting functions. It reads a file into a buffer and returns the buffer. You may then make the buffer current or display it in a window if you wish, but this function does not do so.

If no buffer is currently visiting filename, then one is created and the file is visited. If filename does not exist, the buffer is left empty, and find-file-noselect displays the message `New file' in the echo area.

If a buffer is already visiting filename, then the find-file-noselect function uses that buffer rather than creating a new one. However, it does verify 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.

The find-file-noselect function calls after-find-file after the file is read in (see section Subroutines of Visiting). The after-find-file 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.

The find-file-noselect function returns the buffer that is visiting the file filename.

(find-file-noselect "/etc/fstab")
     => #<buffer fstab>

Command: find-alternate-file filename

This function reads the file filename into a buffer and selects it, killing the buffer current at the time the command is run. It is useful if you have visited the wrong file by mistake, so that you can get rid of the buffer that you did not want to create, at the same time as you visit the file you intended.

When this function is called interactively, it prompts for filename.

Command: find-file-other-window filename

This function visits the file filename and displays its buffer in a window other than the selected window. It may use another existing window or split a window; see section Displaying Buffers in Windows.

When this function is called interactively, it prompts for filename.

Command: find-file-read-only filename

This function visits the file named filename and selects its buffer, just like find-file, but it marks the buffer as read-only. See section Read-Only Buffers, for related functions and variables.

When this function is called interactively, it prompts for filename.

Command: view-file filename

This function views filename in View mode, returning to the previous buffer when done. View mode is a mode that allows you to skim rapidly through the file but does not let you modify it.

After loading the file, view-file runs the normal hook view-hook using run-hooks. See section Hooks.

When this function is called interactively, it prompts for filename.

Variable: find-file-hooks

The value of this variable is a list of functions to be called after a file is visited. The file's local-variables specification (if any) will have been processed before the hooks are run. The buffer visiting the file is current when the hook functions are run.

This variable could be a normal hook, but we think that renaming it would not be advisable.

Variable: find-file-not-found-hooks

The value of this variable is a list of functions to be called when find-file or find-file-noselect is passed a nonexistent filename. These functions are called as soon as the error is detected. buffer-file-name is already set up. The functions are called in the order given, until one of them returns non-nil.

This is not a normal hook because the values of the functions are used and they may not all be run.

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 section Creating Buffers.

Please 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. It uses generate-new-buffer (see section Creating Buffers).

Function: after-find-file &optional error warn

This function is called by find-file-noselect and by the default revert function (see section Reverting). It sets the buffer major mode, and parses local variables (see section How Emacs Chooses a 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.

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:

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. But if save-silently-p is non-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 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.

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 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 basic-save-buffer does when it writes a file in the usual way.

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: local-write-file-hooks

This works just like write-file-hooks, but it is intended to be made local to particular buffers. It's not a good idea to make write-file-hooks local to a buffer--use this variable instead.

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.

Variable: write-contents-hooks

This works just like write-file-hooks, but it is intended to be used for hooks that pertain to the contents of the file, as opposed to hooks that pertain to where the file came from.

Variable: after-save-hook

This normal hook runs after a buffer has been saved in its visited file.

Variable: file-precious-flag

If this variable is non-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.

(This feature worked differently in older Emacs versions.)

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 set it to t in particular buffers.

Reading from Files

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.

Function: insert-file-contents filename &optional visit beg end

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.

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 you want to pass a file name to another process so that another program can read the file, see the function file-local-copy in section Making Certain File Names "Magic".

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 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 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.

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.

Normally, write-region displays a message `Wrote file 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 which the user does not need to know about.

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 file systems, 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 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.

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 tells Emacs what to do next:

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'.

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. See section Functions that Expand Filenames.

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-executable-p filename

This function returns 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 access files inside the directory.

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-accessible-directory-p dirname

This function returns t if you have permission to open existing files in directory dirname; otherwise (and 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 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.

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.

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.

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

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

Truenames

The truename of a file is the name that you get by following symbolic links until none remain, then expanding to get rid of `.' and `..' 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.

Function: file-truename filename

The function 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.

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 by using the add-name-to-file function (see section Changing File Names and 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. (This is similar to the value of current-time; see section Time of Day.)

  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.

  12. The file system number of the file system that the file is in. This element together with the file's inode number, give enough information to distinguish any two files on the system--no two files can have the same values for both of these numbers.

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:

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 modified 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.

129500
has an inode number of 129500.
-32252
is on file system number -32252.

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 nosort

This function returns a list of the names of the files in the directory directory. By default, the list is in alphabetical order.

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, this function returns only those file names that contain that regular expression--the other file names are discarded from the list.

If nosort is non-nil, that inhibits sorting 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.

Function: file-name-all-versions file dirname

This function returns a list of all versions of the file named file in directory dirname.

Function: insert-directory file switches &optional wildcard full-directory-p

This function inserts a directory listing for directory dir, formatted according to switches. It leaves point after the inserted text.

The argument dir 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 file is a directory and switches do not contain `d', so that a full listing is expected.

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.

Variable: insert-directory-program

This variable's value is the program to run to generate a directory listing for the function insert-directory.

Creating and Deleting Directories

Function: make-directory dirname

This function creates a directory named dirname.

Function: delete-directory dirname

This function deletes the directory named dirname. The function delete-file does not work for files that are directories; you must use delete-directory in that case.

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:

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 becomes 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 the following:

(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 section Other 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.)

See also delete-directory in section Creating and Deleting Directories.

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: define-logical-name varname string

This function defines the logical name name to have the value string. It is available only on VMS.

Function: set-file-modes filename mode

This function sets mode bits of filename to mode (which must be an integer). Only the 12 low bits of mode are used.

Function: set-default-file-modes mode

This function sets the default file protection for new files created by Emacs and its subprocesses. Every file created with Emacs initially has this protection. On Unix, the default protection is the bitwise complement of the "umask" value.

The argument mode must be an integer. Only the 9 low bits of mode are used.

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.

Function: default-file-modes

This function returns the current default protection value.

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.

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 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. 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"

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.

These two 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 (see section Functions that Expand Filenames).

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 removing a final slash 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"

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.

If you wish to convert a directory name to its abbreviation, use this function:

Function: abbreviate-file-name dirname

This function applies abbreviations from directory-abbrev-alist to its argument, and substitutes `~' for the user's home directory.

Variable: directory-abbrev-alist

The variable 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"))

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 "/user/rms/foo")
     => t

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 simplifies 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. (The value of directory should itself be an absolute, expanded file name; it should not start with `~'.) 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/' is 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.

Function: file-relative-name filename directory

This function does the inverse of expansion--it tries to return a relative name which is equivalent to filename when interpreted relative to directory. (If such a relative name would be longer than the absolute name, it returns the absolute name instead.)

(file-relative-name "/foo/bar" "/foo/")
     => "bar")
(file-relative-name "/foo/bar" "/hack/")
     => "/foo/bar")

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 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.

Generating Unique File Names

Some programs need to write temporary files. Here is the usual way to construct a name for such a file:

(make-temp-name (concat "/tmp/" name-of-application))

Here we use the directory `/tmp/' because that is the standard place on Unix for temporary files. The job 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 starts with the prefix string, and ends with a number that is different in each Emacs job.

(make-temp-name "/tmp/foo")
     => "/tmp/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 Emacs jobs.

File Name Completion

This section describes low-level subroutines for completing a file name. For other completion functions, see section 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")

Making Certain File Names "Magic"

You can implement special handling for certain file names. This is called making those names magic. You must supply a regular expression to define the class of names (all those which match the regular expression), plus a handler that implements all the primitive Emacs file operations for file names that do match.

The value of file-name-handler-alist is a list of handlers, together with regular expressions that decide 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 you can handle for a magic file name:

add-name-to-file, copy-file, delete-directory,
delete-file, directory-file-name, directory-files,
dired-compress-file, dired-uncache,
expand-file-name, file-accessible-directory-p,
file-attributes, file-directory-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-nondirectory,
file-name-sans-versions, file-newer-than-file-p,
file-readable-p, file-symlink-p, file-writable-p,
insert-directory, insert-file-contents, load,
make-directory, make-symbolic-link, rename-file,
set-file-modes, set-visited-file-modtime,
unhandled-file-name-directory,
verify-visited-file-modtime, write-region.

The handler function must handle all of the above operations, and possibly others to be added in the future. Therefore, it should always reinvoke the ordinary Lisp primitive when it receives 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 (file-name-handler-alist)
             (apply operation args)))))

Function: find-file-name-handler file

This function returns the handler function for file name file, or nil if there is none.

Function: file-local-copy filename

This function copies file filename to the local site, if it isn't there already. 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.

Function: unhandled-file-name-directory filename

This function returns the name of a directory that is not magic. It uses the directory part of filename if that is not magic. Otherwise, it asks the handler what to do.

This is used for running a subprocess; any subprocess must have a non-magic directory to serve as its current directory.

Go to the previous, next section.