New Features for Files

• The new function file-accessible-directory-p tells you whether you can open files in a particular directory. Specify as an argument either a directory name or a file name which names a directory file. The function returns t if you can open existing files in that directory.
• The new function file-executable-p returns t if its argument is the name of a file you have permission to execute.
• The function file-truename returns the "true name" of a specified file. This is the name that you get by following symbolic links until none remain. The argument must be an absolute file name.
• New functions make-directory and delete-directory create and delete directories. They both take one argument, which is the name of the directory as a file.
• The function read-file-name now takes an additional argument which specifies an initial file name. If you specify this argument, read-file-name inserts it along with the directory name. It puts the cursor between the directory and the initial file name. The user can then use the initial file name unchanged, modify it, or simply kill it with C-k. If the variable insert-default-directory is nil, then the default directory is not inserted, and the new argument is ignored.
• The function file-relative-name 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.)
• The function file-newest-backup returns the name of the most recent backup file for filename, or nil that file has no backup files.
• The list returned by file-attributes now has 12 elements. The 12th element is the file system number of the file system that the file is in. This element together with the file's inode number, which is the 11th element, give enough information to distinguish any two files on the system--no two files can have the same values for both of these numbers.
• The new function set-visited-file-modtime updates the current buffer's recorded modification time from the visited file's time. This is useful if the buffer was not read from the file normally, or if the file itself has been changed for some known benign reason. If you give the function an argument, that argument specifies the new value for the recorded modification time. The argument should be a list of the form (high . low) or (high low) containing two integers, each of which holds 16 bits of the time. (This is the same format that file-attributes uses to return time values.) The new function visited-file-modtime returns the recorded last modification time, in that same format.
• The function directory-files now takes an optional fourth argument which, if non-nil, inhibits sorting the file names. 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 at all visible to the user, then the user will probably be happier if you do sort the names.
• 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. This replacement is done when setting up the default directory of a newly visited file. The from string is actually a regular expression; it should always start with `^'. You can set this variable in `site-init.el' to describe the abbreviations appropriate for your site.
• The function abbreviate-file-name applies abbreviations from directory-abbrev-alist to its argument, and substitutes `~' for the user's home directory. Abbreviated directory names are useful for directories that are normally accessed through symbolic links. If you think of the link's name as "the name" of the directory, you can define it as an abbreviation for the directory's official name; then ordinarily Emacs will call that directory by the link name you normally use.
• write-region can write a given string instead of text from the buffer. Use the string as the first argument (in place of the starting character position). You can supply a second file name as the fifth argument (visit). Use this to write the data to one file (the first argument, filename) while nominally visiting a different file (the fifth argument, visit). The argument visit is used in the echo area message and also for file locking; visit is stored in buffer-file-name.
• The value of write-file-hooks does not change when you switch to a new major mode. The intention is that these hooks have to do with where the file came from, and not with what it contains.
• There is a new hook variable for saving files: write-contents-hooks. It works just like write-file-hooks except that switching to a new major mode clears it back to nil. Major modes should use this hook variable rather than write-file-hooks.
• The hook after-save-buffer-hook runs just after a buffer has been saved in its visited file.
• The new function set-default-file-modes sets the file protection for new files created with Emacs. The argument must be an integer. (It would be better to permit symbolic arguments like the chmod program, but that would take more work than this function merits.) Use the new function default-file-modes to read the current default file mode.
• Call the new function unix-sync to force all pending disk output to happen as soon as possible.