Is there a command to list all users? Also to add, delete, modify users, in the terminal?

To list

To list all local users you can use:

cut -d: -f1 /etc/passwd

To list all users capable of authenticating (in some way), including non-local, see this reply.

Some more useful user-management commands (also limited to local users):

To add

To add a new user you can use:

sudo adduser new_username

or:

sudo useradd new_username

See also: What is the difference between adduser and useradd?

To remove/delete

To remove/delete a user, first you can use:

sudo userdel username

Then you may want to delete the home directory for the deleted user account :

sudo rm -r /home/username

^{Please use with caution the above command!}

To modify

To modify the username of a user:

usermod -l new_username old_username

To change the password for a user:

sudo passwd username

To change the shell for a user:

sudo chsh username

To change the details for a user (for example real name):

sudo chfn username

To add a user to the sudo group:

adduser username sudo

or

usermod -aG sudo username

And, of course, see also: man adduser, man useradd, man userdel... and so on.

Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:

cat /etc/passwd

OR

less /etc/passwd
more /etc/passwd

You can also use awk:^awk

awk -F':' '{ print $1}' /etc/passwd