Linux permissions: SUID, SGID, and sticky bit

Linux permissions are a concept that every user becomes intimately familiar with early on in their development. We need to execute scripts, modify files, and run processes in order to administer systems effectively, but what happens when we see Permission denied? Do you know why we see this message? If you know the cause of the problem, do you know how to implement the solution?

I will give a quick explanation of the various ways to calculate permissions, and then we will focus on the special permissions within Linux. If you want an in-depth look at the chmod command, check out this article from Sudoer Shashank Hegde, Linux permissions: An introduction to chmod.

The TL;DR is that there are two main ways of assigning permissions.

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Symbolic method

The symbolic method uses the following syntax:

[tcarrigan@server ~]$ chmod WhoWhatWhich file | directory

Where:

• Who - represents identities: u,g,o,a (user, group, other, all)
• What - represents actions: +, -, = (add, remove, set exact)
• Which - represents access levels: r, w, x (read, write, execute)

An example of this is if I want to add the read and write permissions to a file named test.txt for user and group, I use the following command:

[tcarrigan@server ~]$ chmod ug+rw test.txt

Full disclosure, this is not my preferred method of assigning permissions, and if you would like more information around this method, I recommend your nearest search engine.

Numeric method

The numeric method is, in my experience, the best way to learn and practice permissions. It is based on the following syntax:

[tcarrigan@server ~]$ chmod ### file | directory

Here, from left to right, the character # represents an access level. There are three access levels—user, group, and others. To determine what each digit is, we use the following:

• Start at 0
• If the read permission should be set, add 4
• If the write permission should be set, add 2
• If the execute permission should be set, add 1

This is calculated on a per access level basis. Let's interpret this permissions example:

-rw-r-x---

The permissions are represented as 650. How did I arrive at those numbers?

• The user's permissions are: rw- or 4+2=6
• The group's permissions are: r-x or 4+1=5
• The others's permissions are: --- or 0

To put this into the command syntax, it looks like this:

[tcarrigan@server ~]$ chmod 650 test.txt

Now that you understand the basics of permission calculation in Linux, let's look at the special permissions included in the OS.

[ You might also like An introduction to Linux Access Control Lists (ACLs). ]

Special permission explained

Special permissions make up a fourth access level in addition to user, group, and other. Special permissions allow for additional privileges over the standard permission sets (as the name suggests). There is a special permission option for each access level discussed previously. Let's take a look at each one individually, beginning with Set UID:

user + s (pecial)

Commonly noted as SUID, the special permission for the user access level has a single function: A file with SUID always executes as the user who owns the file, regardless of the user passing the command. If the file owner doesn't have execute permissions, then use an uppercase S here.

Now, to see this in a practical light, let's look at the /usr/bin/passwd command. This command, by default, has the SUID permission set:

[tcarrigan@server ~]$ ls -l /usr/bin/passwd 
-rwsr-xr-x. 1 root root 33544 Dec 13  2019 /usr/bin/passwd

Note the s where x would usually indicate execute permissions for the user.

group + s (pecial)

Commonly noted as SGID, this special permission has a couple of functions:

• If set on a file, it allows the file to be executed as the group that owns the file (similar to SUID)
• If set on a directory, any files created in the directory will have their group ownership set to that of the directory owner

[tcarrigan@server article_submissions]$ ls -l 
total 0
drwxrws---. 2 tcarrigan tcarrigan  69 Apr  7 11:31 my_articles

This permission set is noted by a lowercase s where the x would normally indicate execute privileges for the group. It is also especially useful for directories that are often used in collaborative efforts between members of a group. Any member of the group can access any new file. This applies to the execution of files, as well. SGID is very powerful when utilized properly.

As noted previously for SUID, if the owning group does not have execute permissions, then an uppercase S is used.

other + t (sticky)

The last special permission has been dubbed the "sticky bit." This permission does not affect individual files. However, at the directory level, it restricts file deletion. Only the owner (and root) of a file can remove the file within that directory. A common example of this is the /tmp directory:

[tcarrigan@server article_submissions]$ ls -ld /tmp/
drwxrwxrwt. 15 root root 4096 Sep 22 15:28 /tmp/

The permission set is noted by the lowercase t, where the x would normally indicate the execute privilege.

Setting special permissions

To set special permissions on a file or directory, you can utilize either of the two methods outlined for standard permissions above: Symbolic or numerical.

Let's assume that we want to set SGID on the directory community_content.

To do this using the symbolic method, we do the following:

[tcarrigan@server article_submissions]$ chmod g+s community_content/

Using the numerical method, we need to pass a fourth, preceding digit in our chmod command. The digit used is calculated similarly to the standard permission digits:

• Start at 0
• SUID = 4
• SGID = 2
• Sticky = 1

The syntax is:

[tcarrigan@server ~]$ chmod X### file | directory

Where X is the special permissions digit.

Here is the command to set SGID on community_content using the numerical method:

[tcarrigan@server article_submissions]$ chmod 2770 community_content/
[tcarrigan@server article_submissions]$ ls -ld community_content/
drwxrws---. 2 tcarrigan tcarrigan 113 Apr  7 11:32 community_content/

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Summary

In closing, permissions are fundamentally important to being an effective Linux administrator. There are two defined ways to set permissions using the chmod command: Symbolic and numerical. We examined the syntax and calculations required for both methods. We also considered the special permissions and their role in the system. Now that you understand permissions and the underlying concepts, you can solve the ever-annoying Permission denied error when it tries to impede your work.