Using special permissions

As we’ve seen in the previous section, there are special permissions that could be applied to files and directories. Let’s start by reviewing Set-UID (or SUID) and Set-GUID (or SGID).

Understanding and applying SUID

Let’s review how SUID applies to files and directories:

• SUID permission applied to a file: When applied to an executable file, this file will run as if the owner of the file was running it, applying the permissions.
• SUID permission applied to a directory: No effect.

Let’s check a file with SUID:

[root@rhel-instance ~]# ls -l /usr/bin/passwd
-rwsr-xr-x. 1 root root 32648 Aug 10  2021 /usr/bin/passwd

Tip

In this example, s in the executable bit of the user block is lowercase because the executable bit is set – if the executable bit isn’t set, it will be in uppercase (S).

The passwd command requires root permissions to change hashes in the /etc/shadow file.

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