Linux Command Line Tutorial #7 - the sudo command

What Will I Learn?

You will learn how to use and configure the sudo command.

Requirements

• Basic linux knowledge
• Knowledge of using a text editor to open and edit files

Difficulty

• Intermediate

Tutorial Contents

Why do we need the sudo command?

When you use a linux system, you will most likely need to run important commands that are not available for the new user. Now, we could give everyone the root password, but that increases the chances of the root password leaking.

Instead, we could allow certain users, or certain groups to run a few(or all) commands as root, without them knowing the root password. This is exactly what the sudo tool lets us achieve. Basically, the sudo command lets the root account delegate the power to run a few(or all) commands with administrator(root) privileges to other users.

The sudo command is obviously the subject of a lot of memes, so I think most people are aware of how its actually used. This is the syntax:

sudo cmd

So if i wanted to see the status of NetworkManager using the systemctl status command, this is what i'd use:

But there are a few other command line arguments that are useful.

For example, if you have commands that depend on your environment, you can use the -E flag to preserve them when running the command, like so:

sudo -E cmd

If you actually want to check what commands you're allowed to run with the sudo command, you can use the -l argument. If i run it, this is what i get:

[h@laptop ~]$ sudo -l
User h may run the following commands on laptop:
    (ALL) ALL
[h@laptop ~]$ 

The (ALL)ALL means that can run any command with sudo.

Another really interesting feature of sudo is that you can run a command as any user, not just root. You can do this using the -u flag. Like so:

So in the first line, the prompt says that the file /etc/fstab is write-protected , ie the user myuser doesn't have the permission to write to the file. But if there was no -u flag set, it would have deleted the file right away.

In the following lines, running the whoami command gives the value myuser when the -u flag is set.

Configuring the sudo command by editing the /etc/sudoers file

One of the biggest Linux memes out there is the one where you run a command, but your user isn't in the sudoers file, and thus the "incident will be reported":

So where ARE the incidents ACTUALLY logged?

On ubuntu/debian systems, it's stored at:

/var/log/auth.log

On redhat systems, it's stored at:

/var/log/secure

On other systems it's simply stored in the system logs(sysconf or journald).

Editing the sudoers file using visudo

The file /etc/sudoers is used for configuring the sudo command. But this is a very important file, and if you get the syntax wrong, you could end up with a broken system. So we have to use the visudo command to edit this file. The visudo command makes a copy of the file, and checks the syntax before saving it.

So if you want to use the nano editor, you can use this command:

EDITOR=nano visudo -f /etc/sudoers

You'll see this:

As you can see, we're using the sudoers.tmp file, not the actual sudoers file.

Allowing a user to use sudo

You can let a user use the sudo command, by adding the following line to the /etc/sudoers command:

USER ALL=(ALL) ALL

So in the case of myuser, this will be:

myuser ALL=(ALL) ALL

So after adding this, myuser can use the sudo command:

To allow all the users in a group to use the sudo command, use this command:

%GROUP ALL=(ALL) ALL

Allowing a user to only use certain commands with sudo

Let's say that you want myuser to be able to use the sudo command to only use the systemctl command, but not any other command. You can do that using this:

myuser ALL=/usr/bin/systemctl

With that set, myuser can use the systemctl command, but when you do anything else that requires root, we get an error message:

Disable password prompt for a user

Let's say that we totally trust myuser to be completely secure, and hence, there is no need to ask myuser for a password when it uses the sudo command. You can do so like this:

myuser ALL= NOPASSWD: /usr/bin/systemctl

After setting this, if we use the sudo command:

So as you can see, the systemctl command doesn't need a password, but other commands do(and if the command isn't specified in the sudoers file, it won't be allowed to be run).

Curriculum

• Part 6.2
• Part 5
• Part 4

Sources

1. Header image from xkcd(https://xkcd.com/149/)
2. xkcd - incident image - https://xkcd.com/838/
3. All screenshots are of my own system.

harshallele

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Posted on Utopian.io - Rewarding Open Source Contributors

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