Python subprocess.Popen带有外壳和管道的块

Question

Running the following command: yes | head -1 yes | head -1 , from the shell, outputs y . Using python's subprocess module to call it with a shell hangs indefinitely:

import subprocess

arg = "yes | head -1"
process = subprocess.Popen(arg,
    shell=True,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
)

print "Command started: %d" % process.pid
r = process.communicate()
print "Command ended: %s %s" % r

Killing the process exogenously using kill does not help, nor does making the child process it's session leader using preexec_fn=os.setsid .

What could be causing this behavior, and is there anyway I can prevent it?

I'm running python 2.7.3 and my /bin/sh is GNU bash, version 3.2.48(1)-release (x86_64-apple-darwin12)

Answer 1

So it turns out that this is caused due to a known issue with python not resetting signal handlers before exec in the subprocess module.

This is the same problem that causes 'yes' reporting error with subprocess communicate() , except with GNU yes , the EPIPE returned by a write causes the program to abort, whereas with BSD yes (which is used on OS X as far as I can tell), the return code of the write is not checked, so without a SIGPIPE , yes will not terminate.

It can be fixed by backporting the reset_signals code, as in the above linked question:

def restore_signals():
    signals = ('SIGPIPE', 'SIGXFZ', 'SIGXFSZ')
    for sig in signals:
        if hasattr(signal, sig):
            signal.signal(getattr(signal, sig), signal.SIG_DFL)

then setting preexec_fn=restore_signals in the Popen call.