Pausing Python subprocesses from keyboard input without killing the subprocess

Question

I'm working on a project to produce a shell in Python, and one important feature is the ability to pause and background a running subprocess. However the only methods I've found of pausing the subprocess appear to kill it instantly, so I can't resume it later. Our group has tried excepting KeyboardInterrupt:

try:
    process = subprocess.Popen(processName)
    process.communicate()
except KeyboardInterrupt:
    print "control character pressed"

and also using signals:

def signal_handler(signal,frame):
    print 'control character pressed'

signal.signal(signal.SIGINT, signal_handler)
process.communicate()

Another issue is that both of these only work when Ctrl-C is pressed, nothing else has any effect (I imagine this is why the subprocesses are being killed).

what do you mean by "pause". Is it "suspend" as with SIGSTOP/SIGCONT signals (see Jobs and sessions)? — jfs
– jfs, Commented Nov 11, 2013 at 20:16

Jay Bosamiya · Accepted Answer · 2015-06-15 20:04:22Z

1

The reason you have the process dying is because you are allowing the Ctrl+C to reach the subprocess. If you were to use the parameter preexec_fn = os.setpgrp, as part of the Popen call, then the the child is set to be in a different process group from the parent.

Ctrl+C sends a SIGINT to the complete process group, but since the child is in a different process group, it doesn't receive the SIGINT and thus doesn't die.

After that, the send_signal() function can be used to send a SIGSTOP to the child process whenever you want to pause it, and a SIGCONT to resume it.

answered Jun 15, 2015 at 20:04

Jay Bosamiya

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