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

 

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