A deeper dive into Linux permissions

Sometimes you see more than just the ordinary r, w, x and – designations when looking at file permissions on Linux. Instead of rwx for the owner, group and other fields in the permissions string, you might see an s or t, as in this example:

drwxrwsrwt

One way to get a little more clarity on this is to look at the permissions with the stat command. The fourth line of stat’s output displays the file permissions both in octal and string format:

$ stat /var/mail
  File: /var/mail
  Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: 801h/2049d      Inode: 1048833     Links: 2
Access: (3777/drwxrwsrwt)  Uid: (    0/    root)   Gid: (    8/    mail)
Access: 2019-05-21 19:23:15.769746004 -0400
Modify: 2019-05-21 19:03:48.226656344 -0400
Change: 2019-05-21 19:03:48.226656344 -0400
 Birth: -

This output reminds us that there are more than nine bits assigned to file permissions. In fact, there are 12. And those extra three bits provide a way to assign permissions beyond the usual read, write and execute — 3777 (binary 011111111111), for example, indicates that two extra settings are in use.

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