Linux Users – List Users Created by Current User

Parsing the output of w is probably a better approach than who. Here are some representative data, which shows the login time:

$ who
tom      pts/1        2015-11-15 06:39 (michener:S.0)
$ w
 06:40:10 up  1:04,  1 user,  load average: 0.00, 0.01, 0.05
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
tom      pts/1    michener:S.0     06:39    2.00s  0.03s  0.00s w

Those are more widely available than finger. Since this is a classroom exercise, parsing the data is left to OP. As a hint, awk can do more than print its fields in a one-liner:

• Typically, one would handle the output of w by having in the awk script a BEGIN section (to set a line-number or state).
• Then, a default action for each line (just curly braces with no pattern) would increment the line number.
• Using the line number, handle the first line specially (skip it in this case: OP may need the number of users for a report header, but that is not used in OP's example), and skip the line with USER.
• After that, each line can be printed as OP needs. awk will quit when there is no more data; it is not necessary to know the number of users to do this.

If OP is told to use who, that has options to list more information, e.g.,

$ who -l -u
LOGIN    tty5         2015-11-15 05:36              3670 id=5
LOGIN    tty6         2015-11-15 05:36              3671 id=6
LOGIN    tty4         2015-11-15 05:36              3669 id=4
LOGIN    tty3         2015-11-15 05:36              3668 id=3
LOGIN    tty2         2015-11-15 05:36              3667 id=2
LOGIN    tty1         2015-11-15 05:36              3666 id=1
tom      pts/1        2015-11-15 06:39 00:06        5780 (michener:S.0)
tom      pts/2        2015-11-15 06:52   .          6078 (michener:S.1)

again, showing the terminal name and the login times.

Given the context of wanting to reboot a Linux system, I would take a multi-pronged approach.

First, disable future logins by creating an /etc/nologin file. You could leave it blank or enter informative text in there, such as:

"Logins to this system have been temporarily disabled in preparation for a server reboot, scheduled for (time and date). Please try again after (expected end time)."

Don't forget to remove /etc/nologin when you're done!

Additionally, since a reboot will clear all processes, whether they're interactive or not, I would use ps to look for processes owned by users. This will take some manual investigation to determine whether the processes are worth keeping or not, but should narrow the field some. I've hard-coded 1000 here as the value of UID_MIN from /etc/login.defs as the cutoff for "system" vs "user" UIDs. If any of your users have UIDs below 1000, you'll need to adjust that number.

ps -eo pid,uid,args | awk '$2 >= 1000'

Of course, you could adjust the ps columns to taste, perhaps to add the translated username and process start time (ps -eo pid,uid,user,start,args) or others -- just be careful to keep the ps UID and awk field in sync with each other.

To get the list of unique user names, use:

ps -eo user,uid | awk 'NR>1 && $2 >= 1000 && ++seen[$2]==1{print $1}'