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This is Info file elisp, produced by Makeinfo-1.47 from the input file elisp.texi. This file documents GNU Emacs Lisp. This is edition 1.03 of the GNU Emacs Lisp Reference Manual, for Emacs Version 18. Published by the Free Software Foundation, 675 Massachusetts Avenue, Cambridge, MA 02139 USA Copyright (C) 1990 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. File: elisp, Node: Saving Buffers, Next: Reading from Files, Prev: Visiting Files, Up: Files Saving Buffers ============== When you edit a file in Emacs, you are actually working on a buffer that is visiting that file--that is, the contents of the file are copied into the buffer and the copy is what you edit. Changes to the buffer do not change the file until you "save" the buffer, which means copying the contents of the buffer into the file. -- Command: save-buffer &optional BACKUP-OPTION This function saves the contents of the current buffer in its visited file if the buffer has been modified since it was last visited or saved. Otherwise it does nothing. `save-buffer' is responsible for making backup files. Normally, BACKUP-OPTION is `nil', and `save-buffer' makes a backup file only if this is the first save or if the buffer was previously modified. Other values for BACKUP-OPTION request the making of backup files in other circumstances: * With an argument of 4 or 64, reflecting 1 or 3 `C-u''s, the `save-buffer' function marks this version of the file to be backed up when the buffer is next saved. * With an argument of 16 or 64, reflecting 2 or 3 `C-u''s, the `save-buffer' function unconditionally backs up the previous version of the file before saving it. -- Command: save-buffers-kill-emacs &optional KILL-SILENTLY-P This function offers to save each buffer that needs to be saved, and then kills the Emacs job. With a non-`nil' value of the optional KILL-SILENTLY-P argument, it unconditionally and silently saves all the file-visiting buffers, and then kills the job. The `save-buffers-kill-emacs' function is defined as follows: (defun save-buffers-kill-emacs (&optional arg) "Offer to save each buffer, then kill this Emacs fork..." (interactive "P") (save-some-buffers arg t) (kill-emacs)) -- Command: save-some-buffers &optional SAVE-SILENTLY-P EXITING This command saves some modified file-visiting buffers. Normally it asks the user about each buffer. If the function is called with a non-`nil' value of the optional SAVE-SILENTLY-P argument, this function saves all the file-visiting buffers without querying the user. The optional EXITING argument, if non-`nil', requests this function to offer also to save certain other buffers that are not visiting files. These are buffers that have a non-`nil' local value of `buffer-offer-save'. (A user who says yes to saving one of these will be asked to specify a file name to use.) The `save-buffers-kill-emacs' function passes a non-`nil' value for this argument. -- Variable: buffer-offer-save When this variable is non-`nil' in a buffer, Emacs offers to save the buffer on exit even if the buffer is not visiting a file. The variable is automatically local in all buffers. Normally, Mail mode (used for editing outgoing mail) sets this to `t'. -- Command: write-file FILENAME This function writes the current buffer into file FILENAME, makes the buffer visit that file, and marks it not modified. The buffer is renamed to correspond to FILENAME unless that name is already in use. -- Variable: write-file-hooks The value of this variable is a list of functions to be called before writing out a buffer to its visited file. If one of them returns non-`nil', the file is considered already written and the rest of the functions are not called, nor is the usual code for writing the file executed. If the `file-precious-flag' variable is `nil', the file is moved to the backup file before any of the hooks are called. If none of the hooks actually write the file, but one does return non-`nil', the file will not exist, although the backup will. Here is an example showing how to add an element to `write-file-hooks' but avoid adding it twice: (or (memq 'my-write-file-hook write-file-hooks) (setq write-file-hooks (cons 'my-write-file-hook write-file-hooks))) -- Variable: file-precious-flag If this variable is non-`nil', then `save-buffer' protects against I/O errors while saving by renaming the original file to a temporary name before writing the new contents of the file. If the new contents are successfully written, the renamed original file is deleted. Otherwise, it is renamed back to the original name. This procedure prevents problems such as a lack of disk space from resulting in an invalid file. Some modes set this non-`nil' locally in particular buffers. -- User Option: require-final-newline This variable determines whether files may be written out that do *not* end with a newline. If the value of the variable is `t', then Emacs silently puts a newline at the end of the file whenever the buffer being saved does not already end in one. If the value of the variable is non-`nil', but not `t', then Emacs asks the user whether to add a newline each time the case arises. If the value of the variable is `nil', then Emacs doesn't add newlines at all. `nil' is the default value, but a few major modes change it to `t'. File: elisp, Node: Reading from Files, Next: Writing to Files, Prev: Saving Buffers, Up: Files Reading from Files ================== You can copy a file directly from the disk and insert it into a buffer using the `insert-file-contents' function, or its interactive variant, `insert-file'. -- Command: insert-file FILENAME This function inserts the contents of file FILENAME into the current buffer after point, and sets the mark at the end of the inserted text. An error is signaled if FILENAME is not the name of a file that can be read. This function is for interactive use and does little more than call `insert-file-contents'. -- Function: insert-file-contents FILENAME &optional VISIT This function inserts the contents of file FILENAME into the current buffer after point. It returns a list of the absolute file name and the length of the data inserted. An error is signaled if FILENAME is not the name of a file that can be read. If VISIT is non-`nil', it also marks the buffer as unmodified and sets up various fields in the buffer so that it is visiting the file FILENAME: these include the buffer's visited file name and its last save file modtime. This feature is used by `find-file-noselect' and you should probably not use it yourself. File: elisp, Node: Writing to Files, Next: File Locks, Prev: Reading from Files, Up: Files Writing to Files ================ You can write the contents of a buffer, or part of a buffer, directly to a file on disk using the `append-to-file' and `write-region' functions. Don't use these functions to write to files that are being visited; that could cause confusion in the mechanisms for visiting. -- Command: append-to-file START END FILENAME This function appends the contents of the region delimited by START and END in the current buffer to the end of file FILENAME. If that file does not exist, it is created. This function returns `nil'. An error is signaled if FILENAME specifies a nonwritable file, or a nonexistent file in a directory where files cannot be created. -- Command: write-region START END FILENAME &optional APPEND VISIT This function writes the region (of the current buffer) delimited by START and END into the file specified by FILENAME. If APPEND is non-`nil', then the region is appended to the existing file contents (if any). If VISIT is `t', then Emacs establishes an association between the buffer and the file: the buffer is then visiting that file. It also sets the last file modification time for the current buffer to FILENAME's modtime, and marks the buffer as not modified. This feature is used by `write-file' and you should probably not use it yourself. Normally, this function displays a message `Wrote file FILENAME' in the echo area. If VISIT is neither `t' nor `nil', then this message is inhibited. This feature is useful for programs that use files for internal purposes, files which the user does not need to know about. File: elisp, Node: File Locks, Next: Information about Files, Prev: Writing to Files, Up: Files File Locks ========== When two users edit the same file at the same time, they are likely to interfere with each other. Emacs tries to prevent this situation from arising by recording a "file lock" when a file is being modified. Emacs can then detect the first attempt to modify a buffer visiting a file that is locked by another Emacs job, and ask the user what to do. File locks do not work properly when multiple machines can share filesystems, such as with NFS. Perhaps a better file locking system will be implemented in the future. When file locks do not work, it is possible for two users to make changes simultaneously, but Emacs will still be able to warn the user who saves second. Also, the detection of modification of a buffer visiting a file changed on disk catches some cases of simultaneous editing; see *Note Modification Time::. -- Function: file-locked-p FILENAME This function returns `nil' if the file FILENAME is not locked by this Emacs process. It returns `t' if it is locked by this Emacs, and it returns the name of the user who has locked it if it is locked by someone else. (file-locked-p "foo") => nil -- Function: lock-buffer &optional FILENAME This function locks the file FILENAME, if the current buffer is modified. The argument FILENAME defaults to the current buffer's visited file. Nothing is done if the current buffer is not visiting a file, or is not modified. -- Function: unlock-buffer This function unlocks the file being visited in the current buffer, if the buffer is modified. If the buffer is not modified, then the file should not be locked, so this function does nothing. It also does nothing if the current buffer is not visiting a file. -- Function: ask-user-about-lock FILE OTHER-USER This function is called when the user tries to modify FILE, but it is locked by another user name OTHER-USER. The value it returns controls what Emacs will do with the file: * A value of `t' tells Emacs to grab the lock on the file. Then this user may edit the file and OTHER-USER loses the lock. * A value of `nil' tells Emacs to ignore the lock and let this user edit the file anyway. * This function may instead signal a `file-locked' error, in which case the change to the buffer which the user was about to make does not take place. The error message for this error looks like this: error--> File is locked: FILE OTHER-USER where `file' is the name of the file and OTHER-USER is the name of the user who has locked the file. The default definition of this function asks the user to choose what to do. If you wish, you can replace the `ask-user-about-lock' function with your own version that decides in another way. The code for its usual definition is in `userlock.el'. File: elisp, Node: Information about Files, Next: Contents of Directories, Prev: File Locks, Up: Files Information about Files ======================= The functions described in this section are similar in as much as they all operate on strings which are interpreted as file names. All have names that begin with the word `file'. These functions all return information about actual files or directories, so their arguments must all exist as actual files or directories unless otherwise noted. Most of the file-oriented functions take a single argument, FILENAME, which must be a string. The file name is expanded using `expand-file-name', so `~' is handled correctly, as are relative file names (including `../'). Environment variable substitutions, such as `$HOME', are not recognized by these functions. * Menu: * Testing Accessibility:: Is a given file readable? Writable? * Kinds of Files:: Is it a directory? A link? * File Attributes:: How large is it? Any other names? Etc. File: elisp, Node: Testing Accessibility, Next: Kinds of Files, Prev: Information about Files, Up: Information about Files Testing Accessibility --------------------- These functions test for permission to access a file in specific ways. -- Function: file-exists-p FILENAME This function returns `t' if a file named FILENAME appears to exist. This does not mean you can necessarily read the file, only that you can find out its attributes. (On Unix, this is true if the file exists and you have execute permission on the containing directories, regardless of the protection of the file itself.) If the file does not exist, or if fascist access control policies prevent you from finding the attributes of the file, this function returns `nil'. -- Function: file-readable-p FILENAME This function returns `t' if a file named FILENAME exists and you can read it. It returns `nil' otherwise. (file-readable-p "files.texi") => t (file-exists-p "/usr/spool/mqueue") => t (file-readable-p "/usr/spool/mqueue") => nil -- Function: file-writable-p FILENAME This function returns `t' if FILENAME can be written or created by you. It is writable if the file exists and you can write it. It is creatable if the file does not exist, but the specified directory does exist and you can write in that directory. `file-writable-p' returns `nil' otherwise. In the third example below, `foo' is not writable because the parent directory does not exist, even though the user could create it. (file-writable-p "~rms/foo") => t (file-writable-p "/foo") => nil (file-writable-p "~rms/no-such-dir/foo") => nil -- Function: file-newer-than-file-p FILENAME1 FILENAME2 This functions returns `t' if the file FILENAME1 is newer than file FILENAME2. If FILENAME1 does not exist, it returns `nil'. If FILENAME2 does not exist, it returns `t'. In the following example, assume that the file `aug-19' was written on the 19th, and `aug-20' was written on the 20th. The file `no-file' doesn't exist at all. (file-newer-than-file-p "aug-19" "aug-20") => nil (file-newer-than-file-p "aug-20" "aug-19") => t (file-newer-than-file-p "aug-19" "no-file") => t (file-newer-than-file-p "no-file" "aug-19") => nil File: elisp, Node: Kinds of Files, Next: File Attributes, Prev: Testing Accessibility, Up: Information about Files Distinguishing Kinds of Files ----------------------------- This section describes how to distinguish directories and symbolic links from ordinary files. -- Function: file-symlink-p FILENAME If FILENAME is a symbolic link, the `file-symlink-p' function returns the file name to which it is linked. This may be the name of a text file, a directory, or even another symbolic link, or of no file at all. If FILENAME is not a symbolic link (or there is no such file), `file-symlink-p' returns `nil'. (file-symlink-p "foo") => nil (file-symlink-p "sym-link") => "foo" (file-symlink-p "sym-link2") => "sym-link" (file-symlink-p "/bin") => "/pub/bin" -- Function: file-directory-p FILENAME This function returns `t' if FILENAME is the name of an existing directory, `nil' otherwise. (file-directory-p "~rms") => t (file-directory-p "~rms/lewis/files.texi") => nil (file-directory-p "~rms/lewis/no-such-file") => nil (file-directory-p "$HOME") => nil (file-directory-p (substitute-in-file-name "$HOME")) => t File: elisp, Node: File Attributes, Prev: Kinds of Files, Up: Information about Files Other Information about Files ----------------------------- This section describes the functions for getting detailed information about a file, other than its contents. This information includes the mode bits that control access permission, the owner and group numbers, the number of names, the inode number, the size, and the times of access and modification. -- Function: file-modes FILENAME This function returns the mode bits of FILENAME, as an integer. The mode bits are also called the file permissions, and they specify access control in the usual Unix fashion. If the low-order bit is 1, then the file is executable by all users, if the second lowest-order bit is 1, then the file is writable by all users, etc. The highest value returnable is 4095 (7777 octal), meaning that everyone has read, write, and execute permission, that the SUID bit is set for both others and group, and that the sticky bit is set. (file-modes "~/junk/diffs") => 492 ; decimal integer (format "%o" 492) => 754 ; convert to octal (set-file-modes "~/junk/diffs" 438) => nil (format "%o" 438) => 666 ; convert to octal % ls -l diffs -rw-rw-rw- 1 lewis 0 3063 Oct 30 16:00 diffs -- Function: file-nlinks FILENAME This functions returns the number of names (i.e., hard links) that file FILENAME has. If the file does not exist, then this function returns `nil'. Note that symbolic links have no effect on this function, because they are not considered to be names of the files they link to. % ls -l foo* -rw-rw-rw- 2 rms 4 Aug 19 01:27 foo -rw-rw-rw- 2 rms 4 Aug 19 01:27 foo1 (file-nlinks "foo") => 2 (file-nlinks "doesnt-exist") => nil -- Function: file-attributes FILENAME This function returns a list of attributes of file FILENAME. If the specified file cannot be opened, it returns `nil'. The elements of the list, in order, are: 1. `t' for a directory, a string for a symbolic link (the name linked to), or `nil' for a text file. 2. The number of names the file has. Alternate names, also known as hard links, can be created with `add-name-to-file' (*note Changing File Attributes::.). 3. The file's UID. 4. The file's GID. 5. The time of last access, as a list of two integers. The first integer has the high-order 16 bits of time, the second has the low 16 bits. 6. The time of last modification as a list of two integers (as above). 7. The time of last status change as a list of two integers (as above). 8. The size of the file in bytes. 9. The file's modes, as a string of ten letters or dashes as in `ls -l'. 10. `t' if the file's GID would change if file were deleted and recreated; `nil' otherwise. 11. The file's inode number. For example, here are the file attributes for `files.texi': (file-attributes "files.texi") => (nil 1 2235 75 (8489 20284) (8489 20284) (8489 20285) 14906 "-rw-rw-rw-" nil 20920) and here is how the result is interpreted: `nil' is neither a directory nor a symbolic link. `1' has only one name (the name `files.texi' in the current default directory). `2235' is owned by the user with UID 2235. `75' is in the group with GID 75. `(8489 20284)' was last accessed on Aug 19 00:09. Unfortunately, you cannot convert this number into a time string in Emacs. `(8489 20284)' was last accessed on Aug 19 00:09. `(8489 20285)' last had its inode changed on Aug 19 00:09. `14906' is 14906 characters long. `"-rw-rw-rw-"' has a mode of read and write access for the owner, group, and world. `nil' would retain the same GID if it were recreated. `20920' has an inode number of 20920. File: elisp, Node: Contents of Directories, Next: Changing File Attributes, Prev: Information about Files, Up: Files Contents of Directories ======================= A directory is a kind of file that contains other files entered under various names. Directories are a feature of the file system. Emacs can list the names of the files in a directory as a Lisp list, or display the names in a buffer using the `ls' shell command. In the latter case, it can optionally display information about each file, depending on the value of switches passed to the `ls' command. -- Function: directory-files DIRECTORY &optional FULL-NAME MATCH-REGEXP This function returns a list of the names of the files in the directory DIRECTORY. If FULL-NAME is non-`nil', the function returns the files' absolute file names. Otherwise, it returns just the names relative to the specified directory. If MATCH-REGEXP is non-`nil', function returns only those file names that contain that regular expression--the other file names are discarded from the list. (directory-files "~lewis") => ("#foo#" "#foo.el#" "." ".." "dired-mods.el" "files.texi" "files.texi.~1~") An error is signaled if DIRECTORY is not the name of a directory that can be read. -- Function: file-name-all-versions FILE DIRNAME This function returns a list of all versions of the file named FILE in directory DIRNAME. -- Command: list-directory FILESPEC &optional VERBOSE This command displays a list of files in or matching FILESPEC. It calls the shell command `ls', passing it options under the control of VERBOSE. The argument FILESPEC may be either a directory name or a file name containing optional wildcards. Wildcards are processed by the shell. The options passed to `ls' are the value of `list-directory-verbose-switches' if VERBOSE is non-`nil'; `list-directory-brief-switches' otherwise. Interactively, the raw prefix argument is used for VERBOSE. -- Variable: list-directory-brief-switches This variable contains switches for `list-directory' to pass to `ls' for a short or "brief" listing. The default value is `"-CF"'. -- Variable: list-directory-verbose-switches This variable contains switches for `list-directory' to pass to `ls' for a verbose or "long" listing. The default value is `"-l"'. File: elisp, Node: Changing File Attributes, Next: File Names, Prev: Contents of Directories, Up: Files Changing File Names and Attributes ================================== The functions in this section rename, copy, delete, link, and set the modes of files. In the functions that have an argument NEWNAME, if a file by the name of NEWNAME already exists, the actions taken depend on the value of the argument OK-IF-ALREADY-EXISTS: * A `file-already-exists' error is signaled if OK-IF-ALREADY-EXISTS is `nil'. * Confirmation is requested if OK-IF-ALREADY-EXISTS is a number. * No confirmation is requested if OK-IF-ALREADY-EXISTS is any other value, in which case the old file is removed. -- Function: add-name-to-file OLDNAME NEWNAME &optional OK-IF-ALREADY-EXISTS This function gives the file named OLDNAME the additional name NEWNAME. This means that NEWNAME will be a new "hard link" to OLDNAME. In the first part of the following example, we list two files, `foo' and `foo3'. % ls -l fo* -rw-rw-rw- 1 rms 29 Aug 18 20:32 foo -rw-rw-rw- 1 rms 24 Aug 18 20:31 foo3 Then we evaluate the form `(add-name-to-file "~/lewis/foo" "~/lewis/foo2")'. Again we list the files. This shows two names, `foo' and `foo2'. (add-name-to-file "~/lewis/foo1" "~/lewis/foo2") => nil % ls -l fo* -rw-rw-rw- 2 rms 29 Aug 18 20:32 foo -rw-rw-rw- 2 rms 29 Aug 18 20:32 foo2 -rw-rw-rw- 1 rms 24 Aug 18 20:31 foo3 Finally, we evaluate `(add-name-to-file "~/lewis/foo" "~/lewis/foo3" t)', and list the files again. Now there are three names for one file: `foo', `foo2', and `foo3'. The old contents of `foo3' are lost. (add-name-to-file "~/lewis/foo1" "~/lewis/foo3") => nil % ls -l fo* -rw-rw-rw- 3 rms 29 Aug 18 20:32 foo -rw-rw-rw- 3 rms 29 Aug 18 20:32 foo2 -rw-rw-rw- 3 rms 29 Aug 18 20:32 foo3 This function is meaningless on VMS, where multiple names for one file are not allowed. See also `file-nlinks' in *Note Information about Files::. -- Command: rename-file FILENAME NEWNAME &optional OK-IF-ALREADY-EXISTS This command renames the file FILENAME as NEWNAME. If FILENAME has additional names aside from FILENAME, it continues to have those names. In fact, adding the name NEWNAME with `add-name-to-file' and then deleting FILENAME has the same effect as renaming, aside from momentary intermediate states. In an interactive call, this function prompts for FILENAME and NEWNAME in the minibuffer; also, it requests confirmation if NEWNAME already exists. -- Command: copy-file OLDNAME NEWNAME &optional OK-IF-EXISTS TIME This command copies the file OLDNAME to NEWNAME. An error is signaled if OLDNAME does not exist. If TIME is non-`nil', then this functions gives the new file the same last-modified time that the old one has. (This works on only some operating systems.) In an interactive call, this function prompts for FILENAME and NEWNAME in the minibuffer; also, it requests confirmation if NEWNAME already exists. -- Command: delete-file FILENAME This command deletes the file FILENAME, like the shell command `rm FILENAME'. If the file has multiple names, it continues to exist under the other names. A suitable kind of `file-error' error is signaled if the file does not exist, or is not deletable. (In Unix, a file is deletable if its directory is writable.) -- Command: make-symbolic-link FILENAME NEWNAME &optional OK-IF-EXISTS This command makes a symbolic link to FILENAME, named NEWNAME. This is like the shell command `ln -s FILENAME NEWNAME'. In an interactive call, FILENAME and NEWNAME are read in the minibuffer, and OK-IF-EXISTS is set to the numeric prefix argument. -- Function: set-file-modes FILENAME MODE This function sets mode bits of FILENAME to MODE (which must be a integer). Only the 12 low bits of MODE are used. File: elisp, Node: File Names, Prev: Changing File Attributes, Up: Files File Names ========== Files are generally referred to by their names, in Emacs as elsewhere. File names in Emacs are represented as strings. The functions that operate on a file all expect a file name argument. In addition to operating on files themselves, Emacs Lisp programs often need to operate on the names; i.e., to take them apart and to use part of a name to construct related file names. This section describes how to manipulate file names. The functions in this section do not actually access files, so they can operate on file names that do not refer to an existing file or directory. On VMS, all these functions understand both VMS file name syntax and Unix syntax. This is so that all the standard Lisp libraries can specify file names in Unix syntax and work properly on VMS without change. * Menu: * File Name Components:: The directory part of a file name, and the rest. * Directory Names:: A directory's name as a directory is different from its name as a file. * Relative File Names:: Some file names are relative to a current directory. * File Name Expansion:: Converting relative file names to absolute ones. * Unique File Names:: Generating names for temporary files. * File Name Completion:: Finding the completions for a given file name. File: elisp, Node: File Name Components, Next: Directory Names, Up: File Names File Name Components -------------------- The operating system groups files into directories. To specify a file, you must specify the directory, and the file's name in that directory. Therefore, a file name in Emacs is considered to have two main parts: the "directory name" part, and the "nondirectory" part (or "file name within the directory"). Either part may be empty. Concatenating these two parts reproduces the original file name. On Unix, the directory part is everything up to and including the last slash; the nondirectory part is the rest. The rules in VMS syntax are complicated. For some purposes, the nondirectory part is further subdivided into the name and the version number. On Unix, only backup files have version numbers in their names; on VMS, every file has a version number, but most of the time the file name actually used in Emacs omits the version number. Version numbers are found mostly in directory lists. -- Function: file-name-directory FILENAME This function returns the directory part of FILENAME (or `nil' if FILENAME does not include a directory part). On Unix, the function returns a string ending in a slash. On VMS, it returns a string ending in one of the three characters `:', `]', or `>'. (file-name-directory "lewis/foo") ; Unix example => "lewis/" (file-name-directory "foo") ; Unix example => nil (file-name-directory "[X]FOO.TMP") ; VMS example => "[X]" -- Function: file-name-nondirectory FILENAME This function returns the nondirectory part of FILENAME. (file-name-nondirectory "lewis/foo") => "foo" (file-name-nondirectory "foo") => "foo" ;; The following example is accurate only on VMS. (file-name-nondirectory "[X]FOO.TMP") => "FOO.TMP" -- Function: file-name-sans-versions FILENAME This function returns FILENAME without any file version numbers, backup version numbers, or trailing tildes. (file-name-sans-versions "~rms/foo.~1~") => "~rms/foo" (file-name-sans-versions "~rms/foo~") => "~rms/foo" (file-name-sans-versions "~rms/foo") => "~rms/foo" ;; The following example applies to VMS only. (file-name-sans-versions "foo;23") => "foo" File: elisp, Node: Directory Names, Next: Relative File Names, Prev: File Name Components, Up: File Names Directory Names --------------- A "directory name" is the name of a directory. A directory is a kind of file, and it has a file name, which is related to the directory name but not identical to it. (This is not quite the same as the usual Unix terminology.) These two different names for the same entity are related by a syntactic transformation. On Unix, this is simple: a directory name ends in a slash, whereas the directory's name as a file lacks that slash. On VMS, the relationship is more complicated. The difference between a directory name and its name as a file is subtle but crucial. When an Emacs variable or function argument is described as being a directory name, a file name of a directory is not acceptable. All of these functions take a single argument, FILENAME, which must be a string. Environment variable substitutions such as `$HOME', and the symbols `~', and `..', are *not* expanded. Use `expand-file-name' or `substitute-in-file-name' for that (*note File Name Expansion::.). -- Function: file-name-as-directory FILENAME This function returns a string representing FILENAME in a form that the operating system will interpret as the name of a directory. In Unix, this means that a slash is appended to the string. On VMS, the function converts a string of the form `[X]Y.DIR.1' to the form `[X.Y]'. (file-name-as-directory "~rms/lewis") => "~rms/lewis/" -- Function: directory-file-name DIRNAME This function returns a string representing DIRNAME in a form that the operating system will interpret as the name of a file. On Unix, this means that a final slash is removed from the string. On VMS, the function converts a string of the form `[X.Y]' to `[X]Y.DIR.1'. (directory-file-name "~lewis/") => "~lewis" File: elisp, Node: Relative File Names, Next: File Name Expansion, Prev: Directory Names, Up: File Names Absolute and Relative File Names -------------------------------- All the directories in the file system form a tree starting at the root directory. A file name can specify all the directory names starting from the root of the tree; then it is called an "absolute" file name. Or it can specify the position of the file in the tree relative to a default directory; then it is called an "relative" file name. On Unix, an absolute file name starts with a slash or a tilde (`~'), and a relative one does not. The rules on VMS are complicated. -- Function: file-name-absolute-p FILENAME This function returns `t' if file FILENAME is an absolute file name, `nil' otherwise. On VMS, this function understands both Unix syntax and VMS syntax. (file-name-absolute-p "~rms/foo") => t (file-name-absolute-p "rms/foo") => nil (file-name-absolute-p "$HOME") => nil (file-name-absolute-p "/user/rms/foo") => t File: elisp, Node: File Name Expansion, Next: Unique File Names, Prev: Relative File Names, Up: File Names Functions that Expand Filenames ------------------------------- "Expansion" of a file name means converting a relative file name to an absolute one. Since this is done relative to a default directory, you must specify the default directory name as well as the file name to be expanded. Expansion also canonicalizes file names by eliminating redundancies such as `./' and `NAME/../'. -- Function: expand-file-name FILENAME &optional DIRECTORY This function converts FILENAME to an absolute file name. If DIRECTORY is supplied, it is the directory to start with if FILENAME is relative. Otherwise, the current buffer's value of `default-directory' is used. For example: (expand-file-name "foo") => "/xcssun/users/rms/lewis/foo" (expand-file-name "../foo") => "/xcssun/users/rms/foo" (expand-file-name "foo" "/usr/spool") => "/usr/spool/foo" (expand-file-name "$HOME/foo") => "/xcssun/users/rms/lewis/$HOME/foo" Filenames containing `.' or `..' are simplified to their canonical form: (expand-file-name "bar/../foo") => "/xcssun/users/rms/lewis/foo" `~/' is expanded into the user's home directory. A `/' or `~' following a `/' is taken to be the start of an absolute file name that overrides what precedes it, so everything before that `/' or `~' is deleted. For example: (expand-file-name "/a1/gnu//usr/local/lib/emacs/etc/MACHINES") => "/usr/local/lib/emacs/etc/MACHINES" (expand-file-name "/a1/gnu/~/foo") => "/xcssun/users/rms/foo" In both cases, `/a1/gnu/' has been discarded because an absolute file name follows it. Note that `expand-file-name' does *not* expand environment variables; that is done only by `substitute-in-file-name'. -- Variable: default-directory The value of this buffer-local variable is the default directory for the current buffer. It is local in every buffer. `expand-file-name' uses the default directory when its second argument is `nil'. On Unix systems, the value is always a string ending with a slash. default-directory => "/user/lewis/manual/" -- Function: substitute-in-file-name FILENAME This function replaces environment variables names in FILENAME with the values to which they are set by the operating system. Following standard Unix shell syntax, `$' is the prefix to substitute an environment variable value. The environment variable name is the series of alphanumeric characters (including underscores) that follow the `$'. If the character following the `$' is a `{', then the variable name is everything up to the matching `}'. Here we assume that the environment variable `HOME', which holds the user's home directory name, has the value `/xcssun/users/rms'. (substitute-in-file-name "$HOME/foo") => "/xcssun/users/rms/foo" If a `~' or a `/' appears following a `/', after substitution, everything before the following `/' is discarded: (substitute-in-file-name "bar/~/foo") => "~/foo" (substitute-in-file-name "/usr/local/$HOME/foo") => "/xcssun/users/rms/foo" On VMS, `$' substitution is not done, so this function does nothing on VMS except discard superfluous initial components as shown above. File: elisp, Node: Unique File Names, Next: File Name Completion, Prev: File Name Expansion, Up: File Names Generating Unique File Names ---------------------------- Some programs need to write temporary files. Here is the usual way to construct a name for such a file: (concat "/tmp/" (make-temp-name NAME-OF-APPLICATION)) The directory `/tmp/' is chosen because that is the standard place on Unix for temporary files. The task of `make-temp-name' is to prevent two different users or two different jobs from trying to use the same name. -- Function: make-temp-name STRING This function generates string that can be used as a unique name. The name will start with the prefix STRING, and finish with a number that is different in each Emacs job. (make-temp-name "foo") => "foo021304" (make-temp-name "foo") => "foo021304" To prevent conflicts among different application libraries run in the same Emacs, each application should have its own STRING. The number added to the end of the name distinguishes between the same application running in different Emacses. File: elisp, Node: File Name Completion, Prev: Unique File Names, Up: File Names File Name Completion -------------------- This section describes low-level subroutines for completing a file name. For other completion functions, see *Note Completion::. -- Function: file-name-all-completions PARTIAL-FILENAME DIRECTORY This function returns a list of all possible completions for a file whose name starts with PARTIAL-FILENAME in directory DIRECTORY. The order of the completions is the order of the files in the directory, which is unpredictable and conveys no useful information. The argument PARTIAL-FILENAME must be a file name containing no directory part and no slash. The current buffer's default directory is prepended to DIRECTORY, if DIRECTORY is not an absolute file name. In the following example, suppose that the current default directory, `~rms/lewis', has five files whose names begin with `f': `foo', `file~', `file.c', `file.c.~1~', and `file.c.~2~'. (file-name-all-completions "f" "") => ("foo" "file~" "file.c.~2~" "file.c.~1~" "file.c") (file-name-all-completions "fo" "") => ("foo") -- Function: file-name-completion FILENAME DIRECTORY This function completes the file name FILENAME in directory DIRECTORY. It returns the longest prefix common to all file names in directory DIRECTORY that start with FILENAME. If only one match exists and FILENAME matches it exactly, the function returns `t'. The function returns `nil' if directory DIRECTORY contains no name starting with FILENAME. In the following example, suppose that the current default directory has five files whose names begin with `f': `foo', `file~', `file.c', `file.c.~1~', and `file.c.~2~'. (file-name-completion "fi" "") => "file" (file-name-completion "file.c.~1" "") => "file.c.~1~" (file-name-completion "file.c.~1~" "") => t (file-name-completion "file.c.~3" "") => nil -- User Option: completion-ignored-extensions `file-name-completion' usually ignores file names that end in any string in this list. It does not ignore them when all the possible completions end in one of these suffixes or when a buffer showing all possible completions is displayed. A typical value might look like this: completion-ignored-extensions => (".o" ".elc" "~" ".dvi") File: elisp, Node: Backups and Auto-Saving, Next: Buffers, Prev: Files, Up: Top Backups and Auto-Saving *********************** Backup files and auto-save files are two methods by which Emacs tries to protect the user from the consequences of crashes or of the user's own errors. Auto-saving preserves the text from earlier in the current editing session; backup files preserve file contents prior to the current session. * Menu: * Backup Files:: How backup files are made; how their names are chosen. * Auto-Saving:: How auto-save files are made; how their names are chosen. * Reverting:: `revert-buffer', and how to customize what it does. File: elisp, Node: Backup Files, Next: Auto-Saving, Prev: Backups and Auto-Saving, Up: Backups and Auto-Saving Backup Files ============ A "backup file" is a copy of the old contents of a file you are editing. Emacs makes a backup file the first time you save a buffer into its visited file. Normally, this means that the backup file contains the contents of the file as it was before the current editing session. The contents of the backup file normally remain unchanged once it exists. Backups are usually made by renaming the visited file to a new name. Optionally, you can specify that backup files should be made by copying the visited file. This choice makes a difference for files with multiple names; it also can affect whether the edited file remains owned by the original owner or becomes owned by the user editing it. By default, Emacs makes a single backup file for each file edited. You can alternatively request numbered backups; then each new backup file gets a new name. You can delete old numbered backups when you don't want them any more, or Emacs can delete them automatically. * Menu: * Making Backups:: How Emacs makes backup files, and when. * Rename or Copy:: Two alternatives: renaming the old file or copying it. * Numbered Backups:: Keeping multiple backups for each source file. * Backup Names:: How backup file names are computed; customization. File: elisp, Node: Making Backups, Next: Rename or Copy, Prev: Backup Files, Up: Backup Files Making Backup Files ------------------- -- Function: backup-buffer This function makes a backup of the file visited by the current buffer, if appropriate. It is called by `save-buffer' before saving the buffer the first time. -- Variable: buffer-backed-up This buffer-local variable indicates whether this buffer's file has been backed up on account of this buffer. If it is non-`nil', then the backup file has been written. Otherwise, the file should be backed up when it is next saved (if backup files are enabled). -- Variable: make-backup-files This variable determines whether or not backup files will be created. If it is non-`nil', then Emacs will create a backup of each file when it is saved for the first time. The following example shows how to change the `make-backup-files' variable only in the `RMAIL' buffer and not elsewhere. Setting it `nil' stops Emacs from making backups of the `RMAIL' file, which may save disk space. (You would put this code in your `.emacs' file.) (setq rmail-mode-hook (function (lambda () (make-local-variable 'make-backup-files) (setq make-backup-files nil))) File: elisp, Node: Rename or Copy, Next: Numbered Backups, Prev: Making Backups, Up: Backup Files Backup by Renaming or by Copying? --------------------------------- There are two ways that Emacs can make a backup file: * Emacs can rename the original file so that it becomes a backup file, and then write the buffer being saved into a new file. In this case, any other names (i.e., hard links) of the original file will now refer to the backup file. The new file will be owned by the user doing the editing, and its group will be the default for the user or the directory. * Emacs can copy the original file into a backup file, and then overwrite the original file with new contents. In this case, any other names (i.e., hard links) of the original file will still refer to the current version of the file. The file's owner and group will be unchanged. The first method, renaming, is the default. The variable `backup-by-copying', if non-`nil', says to use the second method, which is to copy the original file and overwrite it with the new buffer contents. The variable `file-precious-flag', if non-`nil', also has this effect (as a sideline of its main significance). *Note Saving Buffers::. The variables `backup-by-copying-when-linked' and `backup-by-copying-when-mismatch', if non-`nil', cause the second method to be used in certain special cases. They have no effect on the treatment of files that don't fall into the special cases. -- Variable: backup-by-copying This variable controls whether to make backup files by copying. If it is non-`nil', then Emacs always copies the current contents of the file into the backup file before writing the buffer to be saved to the file. (In many circumstances, this has the same effect as `file-precious-flag'.) -- Variable: backup-by-copying-when-linked This variable controls whether to make backups by copying for files with multiple names (hard links). If it is non-`nil', then Emacs will use copying to create backups for those files. This variable is significant only if `backup-by-copying' is `nil', since copying is always used when that variable is non-`nil'. -- Variable: backup-by-copying-when-mismatch This variable controls whether to make backups by copying when renaming would cause either the owner or the group of the file to change. If it is non-`nil' then Emacs will create backups by copying in such cases. The value has no effect when renaming would not result in changing the owner or group of the file; that is, for files which are owned by the user and whose group matches the default for a new file created there by the user. This variable is significant only if `backup-by-copying' is `nil', since copying is always used when that variable is non-`nil'.