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How to Review sudo Command Usage on Linux

Linux laptop showing a bash prompt
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The sudo command gives a user superuser or root powers. No doubt you gave them the “with great power comes great responsibility” speech. Here’s how to check if they listened or not.

The sudo Command

The sudo command stands for “substitute user do.” It lets an authorized person execute a command as though they were another user. It can take command line parameters, one of which is the name of the user you wish to have the command executed as. The most common way sudo is used is to omit the command line options and use the default action. This effectively executes the command as the root user.

To use sudo in this way requires special permission. Only the privileged can use sudo. When you install a modern Linux distribution, you’re prompted to set up a root password that you can use with sudo.  Permission to do so is granted to the regular user that you create during installation. This is the preferred way to handle access to the capabilities of the root user. The old way was to create a root user and log in as them in order to administer your system.

This was a dangerous scenario. It was easy to forget to—or be too lazy to—log out and back in as your regular user when you no longer required root privileges. Any mistakes you made in the terminal window as root would be executed, no matter how drastic. Stuff that would be blocked by the shell if a regular user tried to do them would run without question when root requested them. Using the root account instead of a regular account is also a security risk.

How to Control sudo Access on Linux

RELATEDHow to Control sudo Access on Linux

Using sudo focuses the mind. You’re entering the same dangerous waters, but you’re consciously choosing to do so, and hopefully taking great care. You only invoke your superuser status when you need to do something that needs them.

If you open root access up to other users you want to know that they’re taking just as much care with them as you do. You don’t want them running commands recklessly or speculatively. The health and well-being of your Linux installation depend on privileged users behaving respectfully and responsibly.

Here are several ways to monitor their root usage.

The auth.log File

Some distributions maintain an authentication log, in a file called “auth.log.” With the advent and rapid uptake of systemd, the need for the “auth.log” file was removed. The systemd-journal daemon consolidates the system logs into a then-new binary format and journalctl provides a way for you to examine or interrogate the logs.

If you do have an “auth.log” file on your Linux computer, it’ll probably be in the “/var/log/” directory, although on some distributions the filename and path are “/var/log/audit/audit.log.”

You can open the file in less like this. Remember to adjust the path and filename to suit your distribution, and be prepared in case your Linux doesn’t even create an authentication file.

This command worked on Ubuntu 22.04.

less /var/log/auth.log

Looking at the /var/log/auth.log file with less

The logfile is opened, and you can scroll through the file or use the search facilities built into less to search for “sudo.”

The contents of the /var/log/auth.log file displayed in less

Even using the search facilities of less, it can take some time to locate the sudo entries you’re interested in.

Let’s say we want to see what a user called mary has used sudo for. We can search the logfile with grep for lines with “sudo” in them, and then pipe the output through grep again and look for lines with “mary” in them.

Note the sudo before grep and before the logfile name.

sudo grep sudo /var/log/auth.log | grep "mary"

Using grep to filter out entries that mention mary and sudo

This gives us lines that have “sudo” and “mary” in them.

We can see that the user mary was given sudo privileges at 15:25, and at 15:27 she’s opening the fstab file in an editor. That’s the type of activity that definitely warrants a deeper dive, starting with a chat with the user.

Using journalctl

The preferred method on systmd-based Linux distributions is to use the journalctl command to review system logs.

If we pass the name of a program to journalctl it will search the log files for entries that contain references to that program. Because sudo is a binary located at “/usr/bin/sudo” we can pass that to journactl. The -e (pager end) option tells journalctl to open the default file pager. Usually this will be less. The display is automatically scrolled to the bottom to show the most recent entries.

sudo journalctl -e /usr/bin/sudo

Using journalctl to search for entries that mention sudo

The log entries that feature sudo are listed in less.

journalctl displaying entries that contain sudo in the less file viewer

Use the “RightArrow” key to scroll to the right to see the command that was used with each of the invocations of sudo. (Or stretch your terminal window so that it is wider.)

Scrolling sideways to see the commands that were used with sudo

And because the output is displayed in less, you can search for text such as command names, user names, and time stamps.

RELATED: How to Use journalctl to Read Linux System Logs

Using the GNOME Logs Utility

Graphical desktop environments usually include a means of reviewing logs. We’ll look at the GNOME logs utility. To access the logs utility, press the “Super” key to the left of the “Spacebar.”

Type “logs” in the search field. The “Logs” icon appears.

Click the icon to launch the “Logs” application.

The GNOME Logs application

Clicking on the categories in the sidebar will filter the log messages by message type. To make more granular selections, click the “All” category in the sidebar, then click the magnifying glass icon on the toolbar. Enter some search text. We’re going to search for “sudo.”

Searching for entries that contain sudo in the GNOME Logs application

The list of events is filtered to display only those events that relate to the sudo command. A small gray block at the end of each line contains the number of entries in that event session. Click a line to expand it.

The gray block containing the number of entries in a sudo session

We clicked the top line to see the details of the 24 entries in that session.

The details of the events shown in an expanded view

With a little scrolling, we can see the same events that we saw when we used the journalctl command. User mary‘s unexplained editing session on the fstab file is quickly found. We could have searched for “mary”, but that would include entries other than her use of sudo.

Not Everyone Needs root Access

Where there’s a genuine, sensible requirement, giving sudo privileges to other users can make sense. Likewise, it only makes sense to check on their use—or abuse—of these powers, especially just after they’ve been given them.

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