Unix Numeric (Octal) Permissions
================================

    $ chmod 640 foo.txt

|         | User |  Group | Other
|---------|------|--------|-------
| Read    | +400 |  +40   | +4
| Write   | +200 |  +20   | +2
| Execute | +100 |  +10   | +1

Show permissions on linux:

    $ stat foo.txt
    $ stat -c '%a %n' foo.txt

Show permissions on BSD:

    $ stat -x foo.txt
    $ stat -f '%OLp' foo.txt


- Setuid `+4000` executes with the privileges of the file's owner.
- Setgid `+2000` executes with the privileges of the file's group.
- Sticky `+1000` for directories, allows the contents to only be moved or deleted by the directory or file owner (or root). Typically set on /tmp to keep users from messing with each other's files.

(The sticky bit was originally introduced to keep the text segment of an executable remain resident in memory, to make subsequent execution of that program start faster. This behavior is no longer widely supported.)