Today's post is yet another in a somewhat disjointed series of posts on "stuff you might not know and you might find interesting" regarding very common commands. And they don't get much more common than the "who" command.
Generally, "who" is used like the last command that we looked at in our previous post. It's generally issued at the command line to determine who (yes, it's not just a clever name ;) is logged on "right now," if anyone is at all.
Unlike "last," however, the "who" command has quite a number of options that make it a great troubleshooting, and statistics gathering, command. And, as luck would have it, the four options that we're going to look at today are exactly the same on SUSE Linux 9.x, Solaris 9 Unix and even Solaris 10 :) We'll go through the options from most to least used (in my experience). Not that it matters. We're only looking at four options, so it's going to be hard to get lost ;) All example output will be from SUSE Linux 9.x
1. "who -r" - Prints the current runlevel. This is somewhat similar to the functionality of the last command that we posted about before, but it gives more limited information. This command is excellent for a quick overview of the system's current runlevel, previous state and last state-transition time. For instance, take the following example:
host # who -r
run-level 3 Feb 27 16:06 last=S
This shows us that our system is currently at "run level 3," was in "Single User" mode (S) previous to that, and that the transition from "Single User" to "run level 3" occurred approximately February 27th at 16:06. I say approximately, because (if we look at last's output, as we did in our previous post on using last to its full potential, we could see that this was actually a reboot).
The last state will usually appear as "S" on a reboot, since it's the last recorded state the system is at before it switches to "run level 3" (Of course run level 2 is executed on a normal boot to run level 3). All the information about switching from "run level 3" to "run level 6," and from "run level 6" to "run level S", and all the reboot and shutdown commands are not reported. Again, we don't know the year, but, since this command reads from wtmpx, you can check out a few older posts on user deletion with Perl and the relevant mods for Linux if you want to use Perl to grab that information, as well.
2. "who -b" - Prints the system boot time. Didn't I just get through a really long-winded explanation of all the information missing from "who -r"? ;) Well, here's some of that. This invocation of "who" prints out the last time the system was booted. Note that this doesn't differentiate between a reboot and a power-cycle:
host # who -b
system boot Feb 27 16:06
3. "who -d" - Prints out a list of all the dead processes on your system. This invocation of the who command is really only useful if you're looking for a problem process and can't seem to find it. Generally, you'd use either lsof or ptree/pfiles to find the rogue process, but, if you don't have those (or find them too messy), this command can sometimes help. Mostly though, it's just a listing of processes which are no longer running and still in memory. Note that, for our example below, all of these processes aren't even in the process table anymore!
host # who -d
Feb 27 16:06 2134 id=si term=0 exit=0
Feb 27 16:07 4410 id=l3 term=0 exit=0
pts/2 Apr 14 10:40 24532 id=ts/2 term=0 exit=0
pts/1 May 2 20:29 20407 id=ts/1 term=0 exit=0
4. "who -t" - Prints out the last time the System Clock was changed. Like I mentioned, I saved the least used, and/or obvious, invocation of who for last. You may never have to run the who command with this argument. Still, it's nice to know it's there. As far as I can tell, this setting is not affected by the NTP protocol or any similar software you might have running on your machine (xnptd, etc) to keep the OS clock set correctly. If someone with root (or equivalent) privilege decides to run the "date" command on the server to set an incorrect (or correct) time, this command's output will note it. Unfortunately, it's been a while on the machine I'm using as a test case, and the default output (assuming no change) is nothing. On the bright side, we can be reasonably certain that no one's been goofing with the system clock :)
host # who -t
Enjoy the rest of your day, and have a great Memorial Day weekend ;)
Friday, May 23, 2008