introduction to users

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/; Bert Van Vreckem, https://github.com/bertvv/)

This little chapter will teach you how to identify your user account on a Linux computer using commands like who am i, id, and more.

In a second part you will learn how to become another user with the su command.

Finally, you will learn how to run a command as another user with sudo.

whoami

The whoami command tells you your username.

student@debian:~$ whoami
student

who

The who command will give you information about who is logged on the system.

student@debian:~$ who
yanina   tty1         2024-10-15 18:59
student  pts/0        2024-10-15 18:55 (192.168.56.1)
vagrant  pts/1        2024-10-15 18:56 (10.0.2.2)
serena   pts/2        2024-10-15 18:57 (192.168.56.11)
venus    pts/3        2024-10-15 18:57 (192.168.56.15)

In this example, the who command shows that the user yanina is logged in on the physical machine (no IP address is shown), the other students are logged in via SSH. The IP addresses are shown in the output.

In the second column, tty is short for teletype. This term refers to a teleprinter, a device that was in the past used to interact with a computer. The pts stands for pseudo terminal slave and refers to a virtual terminal that is used to interact with the system over a network connection.

who am i

With who am i the who command will display only the line pointing to your current session.

student@debian:~$ who am i
student  pts/0        2024-10-15 18:55 (192.168.56.1)

In fact, any two words would work here with the same result. The following is also common:

student@debian:~$ who mom loves
student  pts/0        2024-10-15 18:55 (192.168.56.1)

w

The w command shows you who is logged on and what they are doing.

student@debian:~$ w
 19:13:30 up 18 min,  5 users,  load average: 0.00, 0.00, 0.00
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
yanina   tty1     -                18:59   14:34   0.03s  0.01s -bash
student  pts/0    192.168.56.1     18:55    1.00s  0.06s  0.01s w
vagrant  pts/1    10.0.2.2         18:56    7.00s  0.02s   ?    pager
serena   pts/2    192.168.56.1     18:57   16:29   0.02s  0.02s -zsh
venus    pts/3    192.168.56.1     18:57   42.00s  0.02s   ?    nano README.md

id

The id command will give you your user id, primary group id, and a list of the groups that you belong to.

student@debian:~$ id
uid=1001(student) gid=1001(student) groups=1001(student),27(sudo),100(users)

On Enterprise Linux you will also get SELinux context information with this command.

[student@el ~]$ id
uid=1001(student) gid=1001(student) groups=1001(student),10(wheel) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

su to another user

The su command (substitute user) allows a user to run a shell as another user. You need to know the password of the user you want to become.

student@debian:~$ su venus
Password: 
venus@debian:/home/student$ pwd
/home/student

In this example, the user student becomes the user venus. The prompt changes to reflect the new user. The pwd command shows that the current directory is still the home directory of the original user. In fact, the only thing that changed is the current user id, the rest of the environment is still the same.

Used like this, the su command is actually not very useful. Use su - instead (see below).

su - $username

To become another user and also get the target user's environment, as you would when you log in as that user, issue the su - command followed by the target username.

venus@debian:/home/student$ su - venus
Password: 
venus@debian:~$ pwd
/home/venus

su -

When no username is provided to su or su -, the command will assume root is the target.

student@debian:~$ su -
Password: 
root@debian:~# pwd
/root

Remark that this assumes that the root user has a password set. On modern Linux distributions, more often than not, the root user does not have a password and you will not be able to use su to become root. Instead, use sudo (see below).

run a program as another user

The sudo program allows a user to start a program with the credentials of another user. Before this works, the system administrator has to set up the /etc/sudoers file. This can be useful to delegate administrative tasks to another user (without giving the root password). Nowadays, this is the preferred way to run commands with superuser privileges instead of logging in as the root user.

The screenshot below shows the usage of sudo. User student received the right to run useradd with the credentials of root. This allows student to create new users on the system without becoming root and without knowing the root password.

First the command fails:

student@debian:~$ /sbin/useradd -m -s /bin/bash valentina
useradd: Permission denied.
useradd: cannot lock /etc/passwd; try again later.

But with sudo it works. The first time a user executes sudo, they have to enter their own password to confirm the action. The sudo command will remember the password for a short time (usually 15 minutes) so that they don't have to enter the password for every command.

student@debian:~$ sudo useradd -m -s /bin/bash valentina
student@debian:~$ getent passwd valentina
valentina:x:1006:1006::/home/valentina:/bin/bash

For more information about the commands used in this example, see the chapter about user management.

visudo

The /etc/sudoers file can be edited with the visudo command. This command will check the syntax of the file before saving it.

Check the man page of visudo(8) before playing with the /etc/sudoers file. Editing the sudoers is out of scope for this fundamentals book.

The default configuration of the sudoers file on Enterprise Linux is to give all users in the wheel group the right to use sudo. On Debian-based systems, users in the group sudo get these rights. In both cases, users have to enter their own password on first use. VMs created with Vagrant are set up in such a way that the default user vagrant can use sudo without entering a password.

sudo su -

On most modern Linux systems, the root user does not have a password set. This means that it is not possible to login as root from the login screen, or via SSH. In order to get a root prompt, one of the users with sudo rights can type sudo su - and become root without having to enter the root password.

student@linux:~$ sudo su -
Password:
root@linux:~#

sudo logging

Using sudo without authorization will result in a severe warning:

paul@linux:~$ sudo su -

We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:

    #1) Respect the privacy of others.
    #2) Think before you type.
    #3) With great power comes great responsibility.

[sudo] password for paul:
paul is not in the sudoers file.  This incident will be reported.
paul@linux:~$

On systemd based distributions, use journalctl (with superuser privileges) to see the logs:

student@debian:~$ sudo journalctl -et sudo 
[... some output omitted ...]
Oct 15 19:21:30 debian sudo[1669]:    paul : user NOT in sudoers ; TTY=pts/0 ; PWD=/home/paul ; USER=root ; COMMAND=/usr/bin/su -

On Enterprise Linux, there is a separate log file for sudo, /var/log/secure:

[vagrant@el ~]$ sudo grep 'NOT in sudoers' /var/log/secure 
Oct 16 07:00:19 el sudo[7458]:  paul : user NOT in sudoers ; TTY=pts/1 ; PWD=/home/paul ; USER=root ; COMMAND=/bin/su -

On older Debian systems, the log file is /var/log/auth.log, with similar content. If this file does not exist, this is an indication that you're on a newer system and that journalct should be used.

practice: introduction to users

1. Run a command that displays only your currently logged on user name.

2. Display a list of all logged on users.

3. Display a list of all logged on users including the command they are running at this very moment.

4. Display your user name and your unique user identification (userid).

5. Use su to switch to another user account (unless you are root, you will need the password of the other account). And get back to the previous account.

6. Now use su - to switch to another user and notice the difference.

7. Try to create a new user account (when using your normal user account). This should fail. (Details on adding user accounts are explained in the chapter about user management.)

8. Now try the same, but with sudo before your command.

solution: introduction to users

1. Run a command that displays only your currently logged on user name.

   laura@linux:~$ whoami
   laura
   laura@linux:~$ echo $USER
   laura
   

2. Display a list of all logged on users.

   laura@linux:~$ who
   laura     pts/0        2014-10-13 07:22 (10.104.33.101)
   laura@linux:~$
   

3. Display a list of all logged on users including the command they are running at this very moment.

   laura@linux:~$ w
    07:47:02 up 16 min,  2 users,  load average: 0.00, 0.00, 0.00
   USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
   root     pts/0    10.104.33.101    07:30    6.00s  0.04s  0.00s w
   root     pts/1    10.104.33.101    07:46    6.00s  0.01s  0.00s sleep 42
   laura@linux:~$
   

4. Display your user name and your unique user identification (userid).

   laura@linux:~$ id
   uid=1005(laura) gid=1007(laura) groups=1007(laura)
   laura@linux:~$
   

5. Use su to switch to another user account (unless you are root, you will need the password of the other account). And get back to the previous account.

   laura@linux:~$ su tania
   Password:
   tania@linux:/home/laura$ id
   uid=1006(tania) gid=1008(tania) groups=1008(tania)
   tania@linux:/home/laura$ exit
   laura@linux:~$
   

6. Now use su - to switch to another user and notice the difference.

   laura@linux:~$ su - tania
   Password:
   tania@linux:~$ pwd
   /home/tania
   tania@linux:~$ logout
   laura@linux:~$
   

   Note that su - gets you into the home directory of tania.

7. ry to create a new user account (when using your normal user account). This should fail. (Details on adding user accounts are explained in the chapter about user management.)

   laura@linux:~$ useradd valentina
   -su: useradd: command not found
   laura@linux:~$ /usr/sbin/useradd valentina
   useradd: Permission denied.
   useradd: cannot lock /etc/passwd; try again later.
   laura@linux:~$
   

   It is possible that useradd is located in /sbin/useradd on your computer.

8. Now try the same, but with sudo before your command.

   laura@linux:~$ sudo /usr/sbin/useradd valentina
   [sudo] password for laura:
   laura is not in the sudoers file.  This incident will be reported.
   laura@linux:~$
   

   Notice that laura has no permission to use the sudo on this system.