3 basic Linux user management commands every sysadmin should know

I like logical commands; commands that are simple, straightforward, and just make sense. When I delivered Linux sysadmin training, I found Linux user management commands to be easy to explain.

I structured my explanation of account administration like this:

What three things must you do to manage user accounts?

• Create accounts
• Modify accounts
• Delete accounts

So, what three commands accomplish these tasks? (As in my overview on account administration, these commands are for Red Hat Enterprise Linux and RHEL-like distributions, but the concepts apply to any distribution of Linux.) 

• useradd
• usermod
• userdel

Pretty simple, right?

Here is a breakdown of how to use these three commands with some of their related options in RHEL.

useradd

The most basic task is to create an account to represent the user who will be working on the system. Each user must authenticate to Linux with an identity that can be used to control their resource access and consumption. User accounts are stored in the /etc/passwd file. That file should not be edited directly by tools such as Vim. Instead, there is useradd, a user-creation utility that adds an account but also accomplishes additional tasks.

Use the useradd command to create accounts:

$ sudo useradd dgarn

That's enough to create the account. However, there are some options you can add. As always, review the associated man page for details. Here are a few common options:

• --create-home (-m): Adds a home directory (this is a default on some distributions)
• --shell (-s): Sets the user's preferred shell if it's different from /bin/bash
• --uid (-u): Specifies a particular user ID (UID)
• --comment (-c): Populates the comment field (usually with the user's full name enclosed in quotes)

Settings for the useradd command are stored in the /etc/defaults/useradd file.

Also, don't forget to set a password for the account by using the passwd command.

Try a few exercises to test these commands:

1. Create a user named test1 with a home directory named /home/salesuser.
2. Create a user named test2 with zsh as the default shell.
3. Create a user named test3 with "Temp User" in the comment field.

Note: These commands require root or administrative privileges, so use the sudo before each command.

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usermod

Now that some user accounts exist on the system, you can modify their settings. You accomplish this with the usermod command and its related options. Modifications may be necessary when users change names, request different shells, or need updated password information.

Standard options for usermod include:

• --comment (-c): Modifies the comment field
• --home (-d): Modifies home directory information
• --expiredate (-d): Changes account-expiration settings
• --login (-l): Modifies the username
• --lock (-L): Locks a user account
• --unlock (-U): Unlocks a user account

Perhaps a user requests an account name change from test2 to testtwo. The command looks like this:

$ sudo usermod --login testtwo \
--comment "Test Two" test2

The test2 string is the argument in this command. The --login and --comment options act on that argument to modify the account.

Maybe a user is taking a leave of absence. The user will return, but the account should be inaccessible in the meantime. If an administrator deletes the account, the user's data, group memberships, and other unique information may be lost or more difficult to access. It's better to lock the account until their return.

Lock a user account by using the usermod command:

$ sudo usermod --lock test1

Upon the user's return, unlock the account:

$ sudo usermod --unlock test1

Interestingly, adding a user to a group modifies the user, not the group. Therefore, you manage group membership with the usermod command.

The two primary group membership scenarios are:

• Add a user to a group and remove the user from all other groups
• Add a user to a group and retain the user's membership in all other groups

Use the --groups (-G for short) option with usermod to accomplish the first scenario (add a user to a group and remove them from other groups). The --append (-a for short) option appends a group to the user, and when combined with -G, it retains its membership in other groups.

So, to add the test1 user account to the demo group and retain test1's membership in other groups, type:

$ sudo usermod --append --groups demo test1

Managing group membership is probably the most challenging use of the usermod command, but Tyler Carrigan's article Managing local group accounts in Linux covers this topic well.

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userdel

Finally, you might want to remove an account representing a user whose role has changed or is no longer with the organization.

To delete the account, type:

$ sudo userdel test3

However, before deleting the account, don't forget about resources such as the user's home directory or system mail. You will want to ensure those resources get handled according to the organization's written security policy.

Here are some common options for userdel that address these resources:

• --force (-f): Deletes the account (including mail and home directory), even if the user is still logged in
• --remove (-r): Deletes the account (including mail and home directory), but the user must be logged out

The userdel command is pretty simple. There aren't many options, but they can be displayed by typing userdel --help.

Wrap up

New Linux users sometimes have difficulty wrapping their heads around how many commands exist and how many of them sysadmins memorize. That memorization comes from years of use and experience. It's handy, however, when some commands have logical names and simple options.

In RHEL, it doesn't get much easier than:

• Add a user: useradd
• Modify a user: usermod
• Delete a user: userdel

It's worth noting that some Linux distributions provide front end commands to perform the same tasks. For example, the adduser command steps the sysadmin through a series of interactive prompts to create a new user. On a Linux system running a graphical desktop, there are also GUI applications to help manage user accounts.

The three commands to manage groups are similar, and I cover those in another article. Sysadmins really only need to remember these basic commands to manage users and groups.