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(elisp)Testing Accessibility

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Enter node , (file) or (file)node

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 and GNU/Linux,
     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.  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 third example below, `foo' is not writable because the
     parent directory does not exist, even though the user could create
     such a directory.

          (file-writable-p "~/foo")
               => t
          (file-writable-p "/foo")
               => nil
          (file-writable-p "~/no-such-dir/foo")
               => nil

 - 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 or the file name of a
     file which is a directory.

     Example: after the following,

          (file-accessible-directory-p "/foo")
               => nil

     we can deduce that any attempt to read a file in `/foo/' will give
     an error.

 - 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
     This function returns `t' if deleting the file FILENAME and then
     creating it anew would keep the file's owner unchanged.

 - Function: file-newer-than-file-p filename1 filename2
     This function 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, `aug-20' was written on the 20th, and the file
     `no-file' doesn't exist at all.

          (file-newer-than-file-p "aug-19" "aug-20")
               => nil
          (file-newer-than-file-p "aug-20" "aug-19")
               => t
          (file-newer-than-file-p "aug-19" "no-file")
               => t
          (file-newer-than-file-p "no-file" "aug-19")
               => nil

     You can use `file-attributes' to get a file's last modification
     time as a list of two numbers.  Note: File Attributes.