How can a background shell script find out who the active, current user is?

Let me explain what I'm trying to do before I get to my question. I'm trying to create a shell script for my Mac, OS 10.5.4, that does the following:

-Runs every 20 seconds
-Checks to see if a user is logged in and active
-Captures a screenshot of that user's desktop and saves it to disk

I figure out how to make the script run every 20 seconds by using launchctl along with a plist file. The plist file uses the StartInterval key with a value of 20, thus invoking it every 20 seconds. I loaded the job as root with the command "sudo launchtl load $path_to_plist_file", then "sudo load $name_of_job".

I figured out how to capture a screenshot with a script. Since the script is running AS root but it is not grabbing a screenshot FOR root, according to the screencapture man page I have to use the command "sudo launchctl bsexec $PID screencapture..." I have tested this and it works, but it only works if I know the PID for the current, active user.

I do not know how to find the PID for the current, active user.

As I said above, I would like the script to see first of all IF there is a user logged in with their desktop open. Since I have Fast User Switching enabled, there can be multiple users logged in but not active and the monitor displaying the login window. Also, the script is running as root so I can't use "whoami" or "logname" because they won't tell me anything about which user is using their desktop.

Does anyone know how a script can check to see if a user is logged in with their desktop active on the monitor?

 

But if I'm running a shell script using launchctl, then it will not in fact open it as the current logged-in user, it will run it as root because I initiated it with the command "launchctl start $job_name".

But that gives me an idea; can I set launchctl to run the script as the current logged-in user?

Thanks for the idea.

 

I think what I'm trying to do may not be entirely possible. Here's a couple of blurbs from Apple's document "Supporting Fast User Switching" at http://developer.apple.com/documentation/MacOSX/Conceptual/BPMultipleUsers/Concepts/FastUserSwitching.html

...and finally, from "User Switch Notifications"

So I think what happens is loginwindow handles logging a user in, launching their dock and finder, etc. If a user switch occurs, an event is sent out to all apps in the login session that is deactivated and to the apps in the login session that is activated.

I think I'd have to create an actual application that starts up for each user when they login, accesses the Core Graphics Framework to get their username, and registers to receive the activation/deactivation events so it only generates screenshots while the user is active.

I am going to go another route based on what I've figured out. I am going to make my script run for each user when they login, then continue to run in the background no matter if they are active or not. I will make the script limit the number of screenshots it captures and I will make it store each user's screenshots in a separate folder.

If I am wrong about what I said above and it turns out there IS some way to do what I was trying to do, by all means let me know, but I think this will prove to be the easiest way to make my script work.

 

My bad. I see that your method does indeed work. It seems to use more system resources to launch the application; even though it is launching in the background I can tell *something* is going on, whereas with the script I couldn't tell that something was going on.

I think I am still going to try my second option, which is to use launchctl to run the script for each user upon login and continue to run until the user logs out. I'll post an update if/when I get this working.

My one concern with doing this via launchctl is that if the computer is put to sleep the launchctl events are queued up and run sequentially when the computer is turned back on. That could lead to a pretty severe performance hit if the computer is put to sleep for any period of time. I guess I'll just have to see what kind of impact it has.

 

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Wow, so helpful. I read that, I don't think you read my question. If you did, perhaps you know how to use "who" to do what I'm trying to do. "Who" doesn't tell you which user is active, just which users are logged in.