cheat sheets.

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 \{\} \;