My python script (python 3.4.3) calls a bash script via subprocess:
import subprocess as sp
res = sp.check_output(\"bashscript\", shell=True)
check_call()
returns as soon as /bin/sh
process exits without waiting for descendant processes (assuming shell=True
as in your case).
check_output()
waits until all output is read. If ssh
inherits the pipe then check_output()
will wait until it exits (until it closes its inherited pipe ends).
check_call()
code example:
#!/usr/bin/env python
import subprocess
import sys
import time
start = time.time()
cmd = sys.executable + " -c 'import time; time.sleep(2)' &"
subprocess.check_call(cmd, shell=True)
assert (time.time() - start) < 1
The output is not read; check_call()
returns immediately without waiting for the grandchild background python process.
check_call()
is just Popen().wait()
. Popen()
starts the external process and returns immediately without waiting for it to exit. .wait()
collects the exit status for the process -- it doesn't wait for other (grandchildren) processes.
If the output is read (it is redirected and the grandchild python process inherits the stdout pipe):
start = time.time()
subprocess.check_output(cmd, shell=True)
assert (time.time() - start) > 2
then it waits until the background python process that inherited the pipe exits.
check_output()
calls Popen().communicate()
, to get the output. .communicate()
calls .wait()
internally i.e., check_output()
also waits for the shell to exit and check_output()
waits for EOF.
If the grandchild doesn't inherit the pipe then check_output()
doesn't wait for it:
start = time.time()
cmd = sys.executable + " -c 'import time; time.sleep(2)' >/dev/null &"
subprocess.check_output(cmd, shell=True)
assert (time.time() - start) < 1
Grandchild's output is redirected to /dev/null
i.e., it doesn't inherit the parent's pipe and therefore check_output()
may exit without waiting for it.
Note: &
at the end which puts the grandchild python process into background. It won't work on Windows where shell=True
starts cmd.exe
by default.