Intro To 'who' Command In Linux

2023-06-01 - By Robert Elder

I use the who command to see who's logged in:

who

robert   :0           2023-05-29 09:28 (:0)

Process, User And Display Information

I can use the '-a' flag with the 'who' command to see other useful information as well, including the system boot time and the current run level:

who -a

           system boot  2023-05-29 09:28
           run-level 5  2023-05-29 09:28
robert   ? :0           2023-05-29 09:28   ?          2529 (:0)

If I open another terminal and start tmux as the postgres user, I'll see an additional entry for this user:

# In another terminal
sudo -u postgres -i
tmux

who -a

           system boot  2023-05-29 09:28
           run-level 5  2023-05-29 09:28
robert   ? :0           2023-05-29 09:28   ?          2562 (:0)
           pts/2        2023-05-29 09:28   ?         60014 id=ts/2  term=0 exit=0
postgres + pts/3        2023-05-29 11:51   .         63417 (tmux(63417).%0)

Here, I can see that the 'robert' user is currently using the graphical display session at ':0' with process ID 2562, and the postgres user is using pseudo terminal 3 and has process ID 63417.

Historical Login Information

By default, the 'who' command sources information from this file:

who -a /var/run/utmp

           system boot  2023-05-29 09:28
           run-level 5  2023-05-29 09:28
robert   ? :0           2023-05-29 09:28   ?          2529 (:0)
postgres + pts/4        2023-05-29 11:51 00:04       21567 (tmux(21567).%0)

but you can also specify this file:

who -a /var/log/wtmp

           run-level 5  2022-11-25 09:00
robert   ? :0           2022-11-25 09:00   ?          2585 (:0)
           run-level    2022-11-26 00:23
           system boot  2022-11-26 08:46
           run-level 5  2022-11-26 08:47
robert   ? :0           2022-11-26 08:47   ?          2613 (:0)
           run-level    2022-11-27 00:13
           system boot  2022-11-27 08:26
           run-level 5  2022-11-27 08:26
robert   ? :0           2022-11-27 08:27   ?          2625 (:0)
...Many hundreds of lines...

which includes more historical information.

Granular Information

The 'who' command supports many other flags to selectively display and format its output:

who -b
who -d
who --login
who -p
who -r
who -t
who -T
who -u

         system boot  2023-05-29 09:28
         run-level 5  2023-05-29 09:28
robert   ? :0           2023-05-29 09:28 (:0)
postgres + pts/4        2023-05-29 11:51 (tmux(21567).%0)
robert   :0           2023-05-29 09:28   ?          2529 (:0)
postgres pts/4        2023-05-29 11:51 00:05       21567 (tmux(21567).%0)

And that's why the 'who' command is my favourite Linux command.

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