Users and groups
Users and groups are used on GNU/Linux for access control—that is, to control access to the system’s files, directories, and peripherals. Linux offers relatively simple/coarse access control mechanisms by default. For more advanced options, see ACL, Capabilities and PAM#Configuration How-Tos.
Overview
A user is anyone who uses a computer. In this case, we are describing the names which represent those users. It may be Mary or Bill, and they may use the names Dragonlady or Pirate in place of their real name. All that matters is that the computer has a name for each account it creates, and it is this name by which a person gains access to use the computer. Some system services also run using restricted or privileged user accounts.
Managing users is done for the purpose of security by limiting access in certain specific ways. The superuser (root) has complete access to the operating system and its configuration; it is intended for administrative use only. Unprivileged users can use several programs for controlled privilege elevation.
Any individual may have more than one account as long as they use a different name for each account they create. Further, there are some reserved names which may not be used such as «root».
Users may be grouped together into a «group», and users may be added to an existing group to utilize the privileged access it grants.
Note: The beginner should use these tools carefully and stay away from having anything to do with any other existing user account, other than their own.
Permissions and ownership
The UNIX operating system crystallizes a couple of unifying ideas and concepts that shaped its design, user interface, culture and evolution. One of the most important of these is probably the mantra: «everything is a file,» widely regarded as one of the defining points of UNIX. This key design principle consists of providing a unified paradigm for accessing a wide range of input/output resources: documents, directories, hard-drives, CD-ROMs, modems, keyboards, printers, monitors, terminals and even some inter-process and network communications. The trick is to provide a common abstraction for all of these resources, each of which the UNIX fathers called a «file.» Since every «file» is exposed through the same API, you can use the same set of basic commands to read/write to a disk, keyboard, document or network device.
A fundamental and very powerful, consistent abstraction provided in UNIX and compatible operating systems is the file abstraction. Many OS services and device interfaces are implemented to provide a file or file system metaphor to applications. This enables new uses for, and greatly increases the power of, existing applications — simple tools designed with specific uses in mind can, with UNIX file abstractions, be used in novel ways. A simple tool, such as cat, designed to read one or more files and output the contents to standard output, can be used to read from I/O devices through special device files, typically found under the /dev directory. On many systems, audio recording and playback can be done simply with the commands, » cat /dev/audio > myfile » and » cat myfile > /dev/audio ,» respectively.
Every file on a GNU/Linux system is owned by a user and a group. In addition, there are three types of access permissions: read, write, and execute. Different access permissions can be applied to a file’s owning user, owning group, and others (those without ownership). One can determine a file’s owners and permissions by viewing the long listing format of the ls command:
total 13740 drwxr-xr-x 2 root root 4096 Jan 12 00:33 grub -rw-r--r-- 1 root root 8570335 Jan 12 00:33 initramfs-linux-fallback.img -rw-r--r-- 1 root root 1821573 Jan 12 00:31 initramfs-linux.img -rw-r--r-- 1 root root 1457315 Jan 8 08:19 System.map26 -rw-r--r-- 1 root root 2209920 Jan 8 08:19 vmlinuz-linux
The first column displays the file’s permissions (for example, the file initramfs-linux.img has permissions -rw-r—r— ). The third and fourth columns display the file’s owning user and group, respectively. In this example, all files are owned by the root user and the root group.
total 16 drwxrwx--- 1 root vboxsf 16384 Jan 29 11:02 sf_Shared
In this example, the sf_Shared directory is owned by the root user and the vboxsf group. It is also possible to determine a file’s owners and permissions using the stat command:
How to Add and Delete Users on Ubuntu 20.04
Adding and removing users on a Linux system is one of the most important system administration tasks to familiarize yourself with. When you create a new system, you are often only given access to the root account by default.
While running as the root user gives you complete control over a system and its users, it is also dangerous and possibly destructive. For common system administration tasks, it’s a better idea to add an unprivileged user and carry out those tasks without root privileges. You can also create additional unprivileged accounts for any other users you may have on your system. Each user on a system should have their own separate account.
For tasks that require administrator privileges, there is a tool installed on Ubuntu systems called sudo . Briefly, sudo allows you to run a command as another user, including users with administrative privileges. In this guide, you’ll learn how to create user accounts, assign sudo privileges, and delete users.
Prerequisites
To complete this tutorial, you will need access to a server running Ubuntu 20.04. Ensure that you have root access to the server and firewall enabled. To set this up, follow our Initial Server Setup Guide for Ubuntu 20.04.
Adding a User
If you are signed in as the root user, you can create a new user at any time by running the following:
If you are signed in as a non-root user who has been given sudo privileges, you can add a new user with the following command:
Either way, you will be required to respond to a series of questions:
- Assign and confirm a password for the new user.
- Enter any additional information about the new user. This is optional and can be skipped by pressing ENTER if you don’t wish to utilize these fields.
- Finally, you’ll be asked to confirm that the information you provided was correct. Press Y to continue.
Your new user is now ready for use and can be logged into with the password that you entered.
If you need your new user to have administrative privileges, continue on to the next section.
Granting a User Sudo Privileges
If your new user should have the ability to execute commands with root (administrative) privileges, you will need to give the new user access to sudo . Let’s examine two approaches to this task: first, adding the user to a pre-defined sudo user group, and second, specifying privileges on a per-user basis in sudo ’s configuration.
Adding the New User to the Sudo Group
By default, sudo on Ubuntu 20.04 systems is configured to extend full privileges to any user in the sudo group.
You can view what groups your new user is in with the groups command:
By default, a new user is only in their own group because adduser creates this in addition to the user profile. A user and its own group share the same name. In order to add the user to a new group, you can use the usermod command:
The -aG option tells usermod to add the user to the listed groups.
Please note that the usermod command itself requires sudo privileges. This means that you can only add users to the sudo group if you’re signed in as the root user or as another user that has already been added as a member of the sudo group. In the latter case, you will have to precede this command with sudo , as in this example:
Specifying Explicit User Privileges in /etc/sudoers
As an alternative to putting your user in the sudo group, you can use the visudo command, which opens a configuration file called /etc/sudoers in the system’s default editor, and explicitly specify privileges on a per-user basis.
Using visudo is the only recommended way to make changes to /etc/sudoers because it locks the file against multiple simultaneous edits and performs a validation check on its contents before overwriting the file. This helps to prevent a situation where you misconfigure sudo and cannot fix the problem because you have lost sudo privileges.
If you are currently signed in as root, run the following:
If you are signed in as a non-root user with sudo privileges, run the same command with the sudo prefix:
Traditionally, visudo opened /etc/sudoers in the vi editor, which can be confusing for inexperienced users. By default on new Ubuntu installations, visudo will use the nano text editor, which provides a more convenient and accessible text editing experience. Use the arrow keys to move the cursor, and search for the line that reads like the following:
Below this line, add the following highlighted line. Be sure to change newuser to the name of the user profile that you would like to grant sudo privileges:
root ALL=(ALL:ALL) ALL newuser ALL=(ALL:ALL) ALL Add a new line like this for each user that should be given full sudo privileges. When you’re finished, save and close the file by pressing CTRL + X , followed by Y , and then ENTER to confirm.
Testing Your User’s Sudo Privileges
Now your new user is able to execute commands with administrative privileges.
When signed in as the new user, you can execute commands as your regular user by typing commands as normal:
You can execute the same command with administrative privileges by typing sudo ahead of the command:
When doing this, you will be prompted to enter the password of the regular user account you are signed in as.
Deleting a User
In the event that you no longer need a user, it’s best to delete the old account.
You can delete the user itself, without deleting any of their files, by running the following command as root:
If you are signed in as another non-root user with sudo privileges, you would use the following:
If, instead, you want to delete the user’s home directory when the user is deleted, you can issue the following command as root:
If you’re running this as a non-root user with sudo privileges, you would run the same command with the sudo prefix:
If you previously configured sudo privileges for the user you deleted, you may want to remove the relevant line again:
Or use the following command if you are a non-root user with sudo privileges:
root ALL=(ALL:ALL) ALL newuser ALL=(ALL:ALL) ALL # DELETE THIS LINE This will prevent a new user created with the same name from being accidentally given sudo privileges.
Conclusion
You should now have a fairly good handle on how to add and remove users from your Ubuntu 20.04 system. Effective user management will allow you to separate users and give them only the access that they are required to do their job.
For more information about how to configure sudo , check out our guide on how to edit the sudoers file.
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