cheat sheets.

--- chmod version 2	Sat Oct 28 13:38:37 +0000 2006
+++ chmod version 3	Mon Feb 19 08:45:28 +0000 2007
@@ -1,24 +1,42 @@
 chmod: a Unix utility for managing file permissions
 
 Modes can be expressed in octal or human-readable:
 
                           Description | Octal  | Human-readable
     ----------------------------------+--------+---------------
                        Set user write | 200    | u=w
     Set user rwx, group and others rx | 755    | u=rwx,go=rx
                        Add group read | -      | g+w
                    Remove all execute | 111    | a-x
      Set user rws, everyone else none | 4700   | u=rwxs,go-rwx
                       Add user sticky | -      | u+s
                      Add group sticky | -      | g+s
+Octal
+-----
+With three-digit octal notation, each numeral represents a different component
+of the permission set: user class, group class, and "others" class respectively.
+
+Each of these digits is the sum of its component bits. As a result, specific
+bits add to the sum as it is represented by a numeral:
+
+ * The read bit adds 4 to its total,
+ * The write bit adds 2 to its total,
+ * The execute bit adds 1 to its total.
+
+Examples
+--------
+
+"-rwxr-xr-x" would be represented as 755 in three-digit octal.
+"-rw-rw-r--" would be represented as 664 in three-digit octal.
+"-r-x------" would be represented as 500 in three-digit octal.
 
 Example: Make foo u=rwx and go=rx:
   $ chmod u=rwx,go=rx foo
 
 Example: Make foo u=rwx and go=rx (in octal):
   $ chmod 755 foo
 
 Example: Find all subdirectories, put them in 'mygroup', make them group sticky:
   $ find . -type d -exec chgrp mygroup \{\} \;
   $ find . -type d -exec chmod g+s \{\} \;