Guide to Unix/Commands/Multiuser Commands

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who[edit]

who gives information about the users logged into the machine. The information includes the user's terminal, login date ,login time and the location they are connecting from.

Examples:

$ who
alice    pts/0        Mar 23 08:05 (213.23.423.24)
bob      pts/2        Apr 10 22:06 (domain.aol.com)
carol    pts/3        Apr 10 18:34 (space.com)

The option "-w" shows wheter or not a user's tty is accessible with commands like write or talk. + indicates that the tty is accessible, and - that it's not:

$ who -w
root     - tty3         Jan 19 02:26
koppe    - tty4         Jan 19 17:10
bok      + pts/1        Jan 19 23:03 

Using "who" with two non-option words gives your username. On some systems, this gives your actual username, and using "su" or "sudo" to switch user does not change this name.

$ who am i
puffy

On other systems, this gives more information:

$ who am i
puffy    ttyp2    Oct 27 10:08

finger[edit]

finger finds out information about a user. If the user has created a .plan (several lines) and/or a .project (one line) file in their home directory this will also be displayed.

Examples:

$ finger alice
Login: alice                    Name: Alice Makemerry
Directory: /home/alice          Shell: /bin/bash
On since Sat Apr 10 18:34 (BST) on pts/3 from ip.fakedomain.com              
  1 hour 25 minutes idle
Mail last read Sat Apr 10 23:57 2004 (BST)
No Plan.

su[edit]

su switch user

Examples:

Become another user:

user> su bob
Password: 
bob>

Become root... then become another user:

user> su
Password:
root#
root# su bob
bob>
(Note: root is not asked for password to become bob!) 

Switching user and using the new user's environment (shell, shell-variables, home-directory) as if after a normal log-in:

user> su - bob
Password:
bob$

Run a program as another user (as root unless otherwise specified):

user> su -c 'apt-get update' 
Password:

Note: The permission and owner/group of su - as well as other config-files (typically /etc/su and /etc/login.defs) - may prevent users not belonging to certain groups from switching user even with the correct password, or even being able to execute su at all (E.g. in BSD it's traditionally been restricted to members of the wheel-group only).