Wait for arbitrary process and get its exit code in Linux

Question

Is there a way to wait until a process finishes if I'm not the one who started it?

e.g. if I ran "ps -ef" and pick any PID (assuming I have rights to access process information) - is there a way I can wait until the PID completes and get its exit code?

Answer 1

You could use strace, which tracks signals and system calls. The following command waits until a program is done, then prints its exit code:

$ strace -e none -e exit_group -p $PID       # process calls exit(1)
Process 23541 attached - interrupt to quit
exit_group(1)                          = ?
Process 23541 detached

$ strace -e none -e exit_group -p $PID       # ^C at the keyboard
Process 22979 attached - interrupt to quit
--- SIGINT (Interrupt) @ 0 (0) ---
Process 22979 detached

$ strace -e none -e exit_group -p $PID       # kill -9 $PID
Process 22983 attached - interrupt to quit
+++ killed by SIGKILL +++

Signals from ^Z, fg and kill -USR1 get printed too. Either way, you'll need to use sed if you want to use the exit code in a shell script.

If that's too much shell code, you can use a program I hacked together in C a while back. It uses ptrace() to catch signals and exit codes of pids. (It has rough edges and may not work in all situations.)

I hope that helps!

Answer 2

is there a way I can wait until the PID completes and get its exit code

Yes, if the process is not being ptraced by somebody else, you can PTRACE_ATTACH to it, and get notified about various events (e.g. signals received), and about its exit.

Beware, this is quite complicated to handle properly.