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24.6.1 Testing Accessibility

These functions test for permission to access a file for reading, writing, or execution. Unless explicitly stated otherwise, they recursively follow symbolic links for their file name arguments, at all levels (at the level of the file itself and at all levels of parent directories).

On some operating systems, more complex sets of access permissions can be specified, via mechanisms such as Access Control Lists (ACLs). See Extended Attributes, for how to query and set those permissions.

— 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 and GNU/Linux, this is true if the file exists and you have execute permission on the containing directories, regardless of the permissions of the file itself.)

If the file does not exist, or if access control policies prevent you from finding its attributes, this function returns nil.

Directories are files, so file-exists-p returns t when given a directory name. However, symbolic links are treated specially; file-exists-p returns t for a symbolic link name only if the target file exists.

— Function: file-readable-p filename

This function returns t if a file named filename exists and you can read it. It returns nil otherwise.

— Function: file-executable-p filename

This function returns t if a file named filename exists and you can execute it. It returns nil otherwise. On Unix and GNU/Linux, if the file is a directory, execute permission means you can check the existence and attributes of files inside the directory, and open those files if their modes permit.

— Function: file-writable-p filename

This function returns t if the file filename can be written or created by you, and nil otherwise. A file is writable if the file exists and you can write it. It is creatable if it does not exist, but the specified directory does exist and you can write in that directory.

In the example below, foo is not writable because the parent directory does not exist, even though the user could create such a directory.

          (file-writable-p "~/no-such-dir/foo")
               ⇒ nil
— Function: file-accessible-directory-p dirname

This function returns t if you have permission to open existing files in the directory whose name as a file is dirname; otherwise (or if there is no such directory), it returns nil. The value of dirname may be either a directory name (such as /foo/) or the file name of a file which is a directory (such as /foo, without the final slash).

For example, from the following we deduce that any attempt to read a file in /foo/ will give an error:

          (file-accessible-directory-p "/foo")
               ⇒ nil
— Function: access-file filename string

This function opens file filename for reading, then closes it and returns nil. However, if the open fails, it signals an error using string as the error message text.

— Function: file-ownership-preserved-p filename &optional group

This function returns t if deleting the file filename and then creating it anew would keep the file's owner unchanged. It also returns t for nonexistent files.

If the optional argument group is non-nil, this function also checks that the file's group would be unchanged.

If filename is a symbolic link, then, unlike the other functions discussed here, file-ownership-preserved-p does not replace filename with its target. However, it does recursively follow symbolic links at all levels of parent directories.

— Function: file-modes filename

This function returns the mode bits of filename—an integer summarizing its read, write, and execution permissions. Symbolic links in filename are recursively followed at all levels. If the file does not exist, the return value is nil.

See File permissions, for a description of mode bits. For example, if the low-order bit is 1, the file is executable by all users; if the second-lowest-order bit is 1, the file is writable by all users; etc. The highest possible value is 4095 (7777 octal), meaning that everyone has read, write, and execute permission, the SUID bit is set for both others and group, and the sticky bit is set.

See Changing Files, for the set-file-modes function, which can be used to set these permissions.

          (file-modes "~/junk/diffs")
               ⇒ 492               ; Decimal integer.
          (format "%o" 492)
               ⇒ "754"             ; Convert to octal.
          
          (set-file-modes "~/junk/diffs" #o666)
               ⇒ nil
          
          $ ls -l diffs
          -rw-rw-rw- 1 lewis lewis 3063 Oct 30 16:00 diffs

MS-DOS note: On MS-DOS, there is no such thing as an executable file mode bit. So file-modes considers a file executable if its name ends in one of the standard executable extensions, such as .com, .bat, .exe, and some others. Files that begin with the Unix-standard ‘#!’ signature, such as shell and Perl scripts, are also considered executable. Directories are also reported as executable, for compatibility with Unix. These conventions are also followed by file-attributes (see File Attributes).