Shell – Why do some commands ‘hang’ the terminal until they’ve finished

command linejobsshell

Sometimes you run a program from the terminal, say, lxpanel^†. The terminal won't drop you back to the prompt, it'll hang. You can press Ctrl+C to get back to the prompt, but that will kill lxpanel. However, pressing Alt+F2 (which pops up a window to take a command) and running lxpanel works gracefully.

Why is this? What is different between running a command from the terminal and from the 'run' window that appears when you press Alt+F2?

^{_{^† lxpanel here was just used as an example. I have experienced this with multiple programs}}

Best Answer

By default the terminal will run the program in the foreground, so you won't end up back at the shell until the program has finished. This is useful for programs that read from stdin and/or write to stdout -- you generally don't want many of them running at once. If you want a program to run in the background, you can start it like this:

$ lxpanel &

Or if it's already running, you can suspend it with Ctrl+Z and then run bg to move it into the background. Either way you will end up with a new shell prompt, but the program is still running and its output will appear in the terminal (so it can suddenly show up while you're in the middle of typing)

Some programs (typically daemons) will fork a separate process when they start, and then let the main process immediately exit. This lets the program keep running without blocking your shell

Potential solution #1

Use the timeout command:

$ date
Mon May  6 07:35:07 EDT 2013
$ timeout 5 sleep 100
$ date
Mon May  6 07:35:14 EDT 2013

You can put a guard into the timeout command as well to kill the process if it hasn't stopped after some period of time too.

$ date
Mon May  6 07:40:40 EDT 2013
$ timeout -k 20 5 sleep 100
$ date
Mon May  6 07:40:48 EDT 2013

This will wait up to 20 seconds after the process sleep 100 should've stopped, if it's still running, then timeout will send it a kill signal.

Potential solution #2

An alternative way, though more risky method would be as follows:

./myProgram &
sleep 1
kill $! 2>/dev/null && echo "myProgram didn't finish"

Found this technique on Stack Overflow in a question titled: Limiting the time a program runs in Linux. Specifically this answer.

NOTE: Per a comment left by @mattdm the above method can be risky given it makes the assumption that there hasn't been any new processes started since your process. So no new PIDs have been assigned. Given this, this approach should probably not be used but is here only as a reference for a general approach to the problem. The timeout method is the better option of the 2.

Best Answer

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