How to Add a New Sudo User in Ubuntu

The acronym sudo stands for "substitute user and do." The sudo program allows regular users in the system to perform tasks that would typically require the superuser (root), who has full privileges and access rights.

This approach enables system management under a user with limited privileges, reducing the risk of errors or unauthorized access to critical system functions.

Thus, you can create a separate user with access to the sudo utility but without access to many system functions, the misuse of which could harm the system.

The key difference between sudo and su (substitute user) is that sudo switches users temporarily, without asking for the user's password.

In this guide, we'll go over how to create a new user in Ubuntu 22.04 and add them to the sudo group, thus providing extended privileges for system management.

Creating a New User for Sudo

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Before creating a new user with special privileges, you need to log into the system as the superuser.

If you're using a server running Ubuntu, connect to it via SSH as the root user:

ssh root@IP_ADDRESS

For example, the connection command might look like this:

ssh root@166.1.227.189

After that, the terminal will prompt you to enter the root password.  For security reasons, the terminal won't display the password characters as you type them.

Next, create a new user by assigning them a chosen name:

adduser hostman

The terminal will show a few messages indicating the creation of the new user, a new group to which they are automatically added, and a directory associated with the user:

Adding user `hostman' ...
Adding new group `hostman' (1001) ...
Adding new user `hostman' (1000) with group `hostman' ...
Creating home directory `/home/hostman' ...
Copying files from `/etc/skel' ...

Next, the terminal will ask you to set a password for the new user and provide additional information about them:

Changing the user information for hostman
Enter the new value, or press ENTER for the default
    Full Name []:
    Room Number []:
    Work Phone []:
    Home Phone []:
    Other []:
Is the information correct? [Y/n]

After filling out the information, press y to confirm. You have created a new user.

Adding a User to the Sudo Group

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Now, you need to add the new user to the special sudo group, which will grant them extended privileges:

usermod -aG sudo hostman

The -a flag is necessary to ensure that the specified group does not replace other groups the user is already a part of. In this case, the user hostman is at least part of the previously created hostman group.

The -G flag is used to specify additional groups we want to add the user to. It is different from the -g flag, which sets the user's primary group. In this case, the primary group for the user hostman is the hostman group.

Now, you can switch to the new user:

su - hostman

Immediately after switching, the terminal will display a message stating that commands can now be executed as the administrator (root) using sudo:

To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

By the way, you can always check which user is currently executing commands:

whoami

The terminal will display the active user's name:

hostman

Running Commands with Sudo

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Sudo As root

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To test the new user's privileges, try listing the contents of the system directory /root:

sudo ls -la /root

The -la flag is a combination of two flags:

• -l specifies a detailed (long) format for listing the contents of the filesystem.

• -a includes directories whose names start with a dot.

Thus, the ls command will show all the contents of the /root directory in detail.

When running this command for the first time, the terminal will ask for the password set for the user hostman:

[sudo] password for hostman:

After entering the password, you will see the contents of the /root directory:

total 48
drwx------  7 root root 4096 Nov 20 05:30 .
drwxr-xr-x 20 root root 4096 Nov 20 12:09 ..
drwx------  3 root root 4096 Nov 11 12:17 .ansible
-rw-r--r--  1 root root 4078 Nov 20 10:12 .bash_history
-rw-r--r--  1 root root 3106 Oct 15  2021 .bashrc
drwx------  2 root root 4096 Nov 11 12:17 .cache
drwxr-xr-x  3 root root 4096 Nov 19 05:36 .local
-rw-------  1 root root  214 Nov 18 04:26 .mysql_history
-rw-r--r--  1 root root  161 Jul  9  2019 .profile
-rw-r--r--  1 root root 1372 Nov 18 04:16 resize.log
drwx------  3 root root 4096 Nov 11 12:17 snap
drwx------  2 root root 4096 Nov 18 04:16 .ssh

Note that using sudo does not require wrapping the command in quotes or anything else. The target command is written naturally right after sudo.

If you enter the above command without using sudo:

ls -la /root

You will see an access denied message:

ls: cannot open directory '/root': Permission denied

Another basic command that is run with sudo is updating the list of available repositories:

sudo apt update

Similarly, if you try to update repositories without sudo, you'll get an access restriction message:

Reading package lists... Done
E: Could not open lock file /var/lib/apt/lists/lock - open (13: Permission denied)
E: Unable to lock directory /var/lib/apt/lists/
W: Problem unlinking the file /var/cache/apt/pkgcache.bin - RemoveCaches (13: Permission denied)
W: Problem unlinking the file /var/cache/apt/srcpkgcache.bin - RemoveCaches (13: Permission denied)

Thus, any attempt to use a command requiring administrator privileges will result in an access denied message in the terminal.

Sudo As Another User

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With sudo, you can execute commands not only as root but also as any other user.

First, let's switch back to the root user:

su - root

The terminal will prompt for the root user's password.

Now, let's try executing a command that requires administrative privileges as the user hostman, using the -u flag:

sudo -u hostman ls -la /root

The terminal will display the familiar access denied message:

ls: cannot open directory '/root': Permission denied

Next, let's switch back to the hostman user:

su - hostman

For clarity, we can perform the same action under the hostman user:

sudo -u root ls -la /root

First, the terminal asks for the hostman user's password and then displays the contents of the specified directory:

total 52
drwx------  7 root root 4096 Nov 20 15:39 .
drwxr-xr-x 20 root root 4096 Nov 20 12:09 ..
drwx------  3 root root 4096 Nov 11 12:17 .ansible
-rw-r--r--  1 root root 4171 Nov 20 15:21 .bash_history
-rw-r--r--  1 root root 3106 Oct 15  2021 .bashrc
drwx------  2 root root 4096 Nov 11 12:17 .cache
drwxr-xr-x  3 root root 4096 Nov 19 05:36 .local
-rw-------  1 root root  214 Nov 18 04:26 .mysql_history
-rw-r--r--  1 root root  161 Jul  9  2019 .profile
-rw-r--r--  1 root root 1372 Nov 18 04:16 resize.log
drwx------  3 root root 4096 Nov 11 12:17 snap
drwx------  2 root root 4096 Nov 18 04:16 .ssh
-rw-r--r--  1 root root    0 Nov 20 15:39 .sudo_as_admin_successful

Configuring Sudo Access Permissions

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You can restrict the permissions of a particular user in the sudo group to only executing specific allowed commands.

To check this, let's first switch back to the root user:

su - root

Setting Access Permissions

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To configure unique access permissions for each sudo user, we need to open the /etc/sudoers file:

sudo nano /etc/sudoers

Then, we can add the description of allowed commands using the following format:

USER HOST=(AVATAR:GROUP) COMMANDS

Where:

• USER: The user that will initiate the sudo command.
• HOST: The hostname where the sudo command will be executed. This is relevant when using multiple machines.
• AVATAR: The user under whose name the allowed commands will be executed via sudo.
• GROUP: The group the user belongs to.
• COMMANDS: The set of commands (which may consist of just one command) that the user can execute via sudo.

In the simplest case, you can allow to execute all commands under any user:

hostman ALL=(ALL:ALL) ALL

In a more complex case, only specific commands can be allowed from a limited set of users:

hostman ALL=(root:ALL) /usr/bin/apt,/usr/bin/rm,/bin/nano

Note that command sets are listed comma-separated without spaces.

To find the full paths to the binaries of necessary commands, you can use the whereis utility:

whereis apt rm nano

The terminal will display information about the specified commands:

apt: /usr/bin/apt /usr/lib/apt /etc/apt /usr/share/man/man8/apt.8.gz
rm: /usr/bin/rm /usr/share/man/man1/rm.1.gz
nano: /usr/bin/nano /usr/share/nano /usr/share/man/man1/nano.1.gz /usr/share/info/nano.info.gz

The first path listed after the command name is the actual address of the binary file.

To activate the specified restrictions, you need to replace the line that allows sudo group users to execute any root commands:

%sudo ALL=(ALL:ALL) ALL

With a similar one but with a comment symbol at the beginning, to disable the setting:

#%sudo ALL=(ALL:ALL) ALL

Now you can switch back to the hostman user:

su - hostman

And let's try running the familiar command to list the contents of the /root directory:

sudo ls -la /root

The terminal will display a message indicating that the specified command is prohibited on this host:

Sorry, user hostman is not allowed to execute '/usr/bin/ls /root' as root on <hostname>.

However, the command to update repositories will still work:

sudo apt update

Checking Access Rights

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Of course, you can find out the details of a user's privileges by simply viewing the contents of the /etc/sudoers file. However, there's an easier way, by using the sudo command itself:

sudo -l -U hostman

• The -l flag lists all commands the user is allowed to use.

• The -U flag specifies the target username. If omitted, the terminal will display access rights for the root user.

In the terminal, you will see a message detailing the access rights for the specified user:

Matching Defaults entries for hostman on <hostname>:
    env_reset, mail_badpass, secure_path=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin,
    use_pty

User hostman may run the following commands on <hostname>:
    (root : ALL) /usr/bin/apt, /usr/bin/rm, /bin/nano

The key part of the output is:

User hostman may run the following commands on <hostname>:
   (root : ALL) /usr/bin/apt, /usr/bin/rm, /bin/nano

This indicates that the user hostman can run the following commands as root:

• /usr/bin/apt
• /usr/bin/rm
• /bin/nano

These are exactly the commands specified in the /etc/sudoers configuration file. This way, you can quickly review the privileges of a specific user without the need to search through the /etc/sudoers file manually.

Additionally, you can simplify the process of obtaining user privilege information by checking the permission for executing a specific command:

sudo -l -U hostman ls

If the command is not allowed, there will be no output in the terminal. However, if it is allowed:

sudo -l -U hostman apt

The terminal will display the full path to the command's binary:

/usr/bin/apt

This way, you can check whether the current user can execute a specific command when unsure about their access rights.

Disabling Password Prompt

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The sudo utility allows running commands without explicitly entering a password. However, disabling the password prompt is not considered secure, so perform this configuration at your own risk.

To disable the password prompt, you need to open the /etc/sudoers file:

sudo nano /etc/sudoers

Then, add a new line containing the NOPASSWD keyword and a list of commands for which the password is not required:

hostman ALL=(root:ALL) NOPASSWD: /usr/bin/apt

You should also separate commands that require a password from those that don't. For example, the allowed commands with a password prompt should be listed separately from the ones without:

hostman ALL=(root:ALL) /usr/bin/rm,/bin/nano
hostman ALL=(root:ALL) NOPASSWD: /usr/bin/apt

This way, you'll have two sections for allowed commands: one requiring a password and one that doesn't.

Conclusion

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Although the sudo command resembles the su command, there is a key difference between them:

• su stands for "substitute user".
• sudo stands for "substitute user and do".

Thus, su performs a full user switch, requiring an explicit password input, while sudo only simulates executing a command as another user, without switching the user entirely.

For this reason, sudo is much safer when granting extended privileges to another user. The user won't need the root password, as they can execute administrator commands under their own user account.

Additionally, unique permissions (access rights) for each individual user in the sudo group can be configured in a special configuration file. In this file, you can also specify whether a password is necessary to run certain commands.

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