Can you completely explain the nine permissions bits on UNIX or Linux files?

Each file in UNIX or Linux has the nine permission bits as follows:
ls -l /etc/hosts
Sample outputs:

-rw-r--r-- 1 root root 401 2009-12-20 12:08 /etc/hosts

The Entry Type

There are total 10 bits -rw-r–r–:
The file mode printed under the -l option consists of the entry type (1st bit) and the permissions (9 bits). The entry type character describes the type of file, as follows:

1. – Regular file.
2. b Block special file (stored in /dev).
3. c Character special file (stored in /dev).
4. d Directory.
5. l Symbolic link.
6. p FIFO.
7. s Socket.
8. w Whiteout.

Examples

Show directory type with the ls command:
ls -ld /etc
Sample outputs:

drwxr-xr-x 170 root root 12288 2010-02-24 11:39 /etc

Show block device type (hard disk) with the ls command:
ls -l /dev/sda
Sample outputs:

brw-rw---- 1 root disk 8, 0 2010-02-24 11:32 /dev/sda

The Permissions Bits

The next three fields are three characters each for

• File owner permissions
• File group permissions
• Other permissions (not part of both owner or group)

Each field has three character positions as follows:

1. r : The file is readable; if – it is not readable.
2. w : the file is writable; if -, it is not writable.
3. S : If in the owner permissions, the file is not executable and set-user-ID mode is set. If in the group permissions, the file is not executable and set-group-ID mode is set.
4. s : If in the owner permissions, the file is executable and set-user-ID mode is set. If in the group permissions, the file is executable and set group-ID mode is set.
5. x : The file is executable or the directory is searchable.
6. – : The file is neither readable, writable, executable, nor set-user-ID nor set-group-ID mode, nor sticky.

These next two apply only to the third character in the other permissions.

1. T The sticky bit is set (mode 1000), but not execute or search permission.
2. t The sticky bit is set (mode 1000), and is searchable or executable.

References:

• ls command man page

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