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I'm trying to figure out a quick & easy way to see a list of everyone's effective user id... I would have thought 'w' or 'who' would be able to display if someone had switched user accounts... but it's only showing the real user ID they logged in with.

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    Doing an su generates logs, it is easier to look at that. You can and should disable su and force everybody to use sudo. If you have "rogue" root operators, then usually it is complicated trying to get technical solutions for political problems. – Rui F Ribeiro Mar 9 at 18:01
  • sudo creates logs, but ensure that the logs are stored off device, and are only appendable, and readable (not truncatable, deletable), from this device. So a compromised root can to change the logs. – ctrl-alt-delor Mar 9 at 18:50
  • A related question is unix.stackexchange.com/questions/441717 . – JdeBP Mar 10 at 10:20
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One approach could be to apply the fuser command to the pseudo-terminal devices in /dev/pts, e.g. sudo fuser -v /dev/pts/*.

As an example, I logged into a Linux system as user1 and ran sudo su user2, then logged in again (as user1) via another terminal. After running sudo fuser -v /dev/pts/*, I got the following output:

             USER        PID  ACCESS COMMAND
/dev/pts/0:  user1       5533 F....  bash
             root        6291 F....  sudo
/dev/pts/1:  user1       5655 F....  bash
             root        5748 F....  sudo
             root        5752 F....  su
             user2       5753 F....  bash

Looking at the second block of output here (corresponding to /dev/pts/1), you can see that user1 has switched to user2. For more information on this approach, you may want to consult the following post: How can we know who's at the other end of a pseudo-terminal device?

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The logins will show up in the /var/log/secure file (red hat\centos) or /var/log/auth.log (debian\ubuntu), and the format for a login contains the line text "session opened for", so cat /var/log/(secure OR auth.log) | grep "session opened for" should provide a list of logins like this:

Jan 9 07:07:07 hostname su:pam_unix(su:session): session opened for user user1 by user2(uid=2000)

ps aux | grep username should list shells running under "username," which is a quick way to check for "username" activity, and very eye-catching if you don't expect to find any "username" activity. This won't tell you WHO logged in as "username" though, so the log files would still need to be consulted for that.

For a large number of users these checks could get cumbersome, I hoped there was something like ps -eo ruid,euid for just users instead of all processes, but I found nothing that straightforward.

  • procstat -a -s -h | awk '{ if (!x[$3]++) print $3; if (!x[$4]++) print $4; }' – JdeBP Mar 10 at 10:19

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