Linux command of the day, 10 of 31 - who

I’m looking briefly at a Linux command every day for a month

Nice simple one today. who tells me who is logged in to a system. It reads by default from a file (because Unix!) at /var/run/utmp

$ who
pi       pts/0        2020-08-12 06:35 (192.168.1.159)

Here I’m logged in to a raspberry pi as user ‘pi’. I get a lot more info with the -a flag and -H to add headers, plus I log in again so I’ve got two users in:

$ who -aH
NAME       LINE         TIME             IDLE          PID COMMENT  EXIT
           system boot  1970-01-01 01:00
           run-level 5  2020-08-09 18:38
LOGIN      tty1         2020-08-09 18:38               501 id=tty1
pi       + pts/0        2020-08-12 06:35   .         30980 (192.168.1.159)
pi       + pts/1        2020-08-12 06:37   .         31058 (192.168.1.159)

Notes:

• I see the system was apparently ‘booted’ on the Unix epoch (?!)
• The + is the user pi‘s mesg status, i.e. whether the current user can write to that user’s tty. Let’s dig into that tomorrow|!

I have commands mesg and tty to quickly look those up:

$ mesg
is y
$ tty
/dev/pts/1

I log in twice, and for the user in pts/1 I set mesg n then :

$ who -aH --lookup
NAME       LINE         TIME             IDLE          PID COMMENT  EXIT
           system boot  1970-01-01 01:00
           run-level 5  2020-08-09 18:38
LOGIN      tty1         2020-08-09 18:38               501 id=tty1
pi       + pts/0        2020-08-12 06:35   .         30980 (192.168.1.159)
pi       - pts/1        2020-08-12 06:37   .         31058 (192.168.1.159)

I tried to message the pi on pts/1 and get:

$ write  pi pts/1
write: pi has messages disabled on pts/1