A deeper dive into Linux permissions

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:


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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