One of the various commands we mentioned was the who command which displays users who are currently logged on to a Linux system, including the terminals they are connecting from.
The basic syntax for using who command is as follows.
$ who who [OPTION]... [ FILE | ARG1 ARG2 ]
1. If you run who command without any arguments, it will display account information (user login name, user’s terminal, time of login as well as the host the user is logged in from) on your system similar to the one shown in the following output.
$ who vyga tty1 2017-08-24 20:08 rootadminz pts/0 2017-08-24 20:07 (78.15.145.82) root pts/1 2017-08-24 20:08 (78.15.145.82)
2. To print, the heading of the columns displayed, use the -H flag as shown.
$ who -H NAME LINE TIME COMMENT --------------------------------------------------------------------------------
vyga tty1 2017-08-24 20:08 rootadminz pts/0 2017-08-24 20:07 (78.15.145.82) root pts/1 2017-08-24 20:08 (78.15.145.82)
3. To print the login names and the total number of logged on users, use the -q flag.
$ who -q vyga rootadminz root # users=3
4. In case you want to show only hostname and user associated with stdin, use the -m switch.
$ who -m rootadminz pts/0 2017-08-24 20:07 (78.15.145.82)
5. Next, to add user’s message status as +, - or ?, use the -T option.
$ who -T vyga + tty1 2017-08-24 20:08 rootadminz + pts/0 2017-08-24 20:07 (78.15.145.82) root + pts/1 2017-08-24 20:08 (78.15.145.82)
The who command also helps you to view some useful system information such as last boot time, current runlevel (target under systemd), print dead processes as well as processes spawned by init.
6. To view the time of last system boot, use the -b flag and adding the -u option allows for listing of logged on users in the same output.
$ who -b system boot 2018-01-19 02:39
$ who -bu
system boot 2017-08-24 19:25
vyga tty1 2017-08-24 20:08 00:33 2366
rootadminz pts/0 2017-08-24 20:07 . 2332 (78.15.145.82)
root pts/1 2017-08-24 20:08 00:32 2423 (78.15.145.82)
7. You can check the current runlevel with the -r option.
$ who -r run-level 3 2017-08-24 02:39
8. The following command will print dead processes.
$ who -d pts/1 2017-08-24 11:10 9986 id=ts/1 term=0 exit=0
9. Furthermore, to see active processes spawned by init, use the -p option.
$ who -p
10. Last but not least, the -a flag allows for printing of default output combined with information from some of the options we have covered.
$ who -a
system boot 2018-06-16 02:39
run-level 3 2018-01-19 02:39
LOGIN tty1 2018-01-19 02:39 3258 id=1
LOGIN ttyS0 2018-01-19 02:39 3259 id=S0
tecmnt + pts/0 2018-03-16 05:33 . 20678 (208.snat-111-91-115.hns.net.in)
pts/1 2018-03-14 11:10 9986 id=ts/1 term=0 exit=0
You can find more options by consulting the who man page.
$ man who
