GNU/Linux basic file permission overview

After creating a basic text file within a the GNU/Linux operating system:

jemurray@shell:~$ touch test

You may see a set of permission that look like this:

jemurray@shell:~$ ls -al test
-rw-r--r-- 1 jemurray jemurray 40 Jan 14 21:32 test

This section of the output above describes the type of file and the permissions associated with it:

-rw-r--r--

Broken out into individual components, each section has a specific meaning:

Type    User       Group       Others
-       rw-        r--         r--

• Type:
  • There are many different file types (complete list is here info ls -n "What information is listed"). The most common are:
  • - - Regular file
  • d - Directory
  • l - Link
• User:
  • Permissions for the owner of the file
• Group:
  • Permissions for users within the same /etc/group
• Others:
  • Permissions for everyone else

Each one of the - in the User, Group, or Others permission set specifies the none, read, write, or execute permission set:

Read      Write       Execute
-         -           -
r         w           x

If the file type is a regular file indicated by a - in the first field:

• r = Read the file
• w = Write to the file
• x = Execute the file if it is a program or script

If the file type is a directory indicated by a d in the first field:

• r = Files can be read within the directory
• w = Files can be written or modified in the directory
• x = Files can be accessed if the name is know, but not viewed

Changing permissions

The following command will change the User, Group, and Other permissions. But what is the black magic behind the numbers?

chmod 664 test

Each number above adjusts the User, Group, and Other section. The first 6 is for the User, the second 6 is for the Group, and the final 4 is for Other. This is known as the octal notation permission format.

Each - from left to right is assigned a number:

User          Group           Other
4   2   1     4   2   1       4    2    1
-   -   -     -   -   -       -    -    -
r   w   x     r   w   x       r    w    x

By adding the number of each section together, individual permissions are set. For example, to enable read and write for user and remove all other permissions add the numbers in the user column 4 + 2 = 6 and execute:

chmod 600 test

To set read and write for user, group, and other add each columns numbers 4 + 2 = 6 4 + 2 = 6 4 + 2 = 6:

chmod 666 test

To set permission that look like this rw-rw-r-- add the numbers 4 + 2 = 6 4 + 2 = 6 4 + 0 = 4:

chmod 664 test