cheat sheets.

Kernel.test is a handy ruby method when working with the filesystem:

Usage:
  Kernel.test(int_cmd, file1 [, file2] )

Here are tests for a single file:
  Test   Returns   Meaning
   ?A  | Time    | Last access time for file1
   ?b  | boolean | True if file1 is a block device
   ?c  | boolean | True if file1 is a character device
   ?C  | Time    | Last change time for file1
   ?d  | boolean | True if file1 exists and is a directory
   ?e  | boolean | True if file1 exists
   ?f  | boolean | True if file1 exists and is a regular file
   ?g  | boolean | True if file1 has the \CF{setgid} bit
       |         | set (false under NT)
   ?G  | boolean | True if file1 exists and has a group
       |         | ownership equal to the caller's group
   ?k  | boolean | True if file1 exists and has the sticky bit set
   ?l  | boolean | True if file1 exists and is a symbolic link
   ?M  | Time    | Last modification time for file1
   ?o  | boolean | True if file1 exists and is owned by
       |         | the caller's effective uid
   ?O  | boolean | True if file1 exists and is owned by
       |         | the caller's real uid
   ?p  | boolean | True if file1 exists and is a fifo
   ?r  | boolean | True if file1 is readable by the effective
       |         | uid/gid of the caller
   ?R  | boolean | True if file is readable by the real
       |         | uid/gid of the caller
   ?s  | int/nil | If file1 has nonzero size, return the size,
       |         | otherwise return nil
   ?S  | boolean | True if file1 exists and is a socket
   ?u  | boolean | True if file1 has the setuid bit set
   ?w  | boolean | True if file1 exists and is writable by
       |         | the effective uid/gid
   ?W  | boolean | True if file1 exists and is writable by
       |         | the real uid/gid
   ?x  | boolean | True if file1 exists and is executable by
       |         | the effective uid/gid
   ?X  | boolean | True if file1 exists and is executable by
       |         | the real uid/gid
   ?z  | boolean | True if file1 exists and has a zero length

Here are tests for multiple files:
?-  | boolean | True if file1 and file2 are identical
?=  | boolean | True if the modification times of file1
    |         | and file2 are equal
?<  | boolean | True if the modification time of file1
    |         | is prior to that of file2
?>  | boolean | True if the modification time of file1
    |         | is after that of file2