Python fork-exec problem, child process output goes to same place as parent process.

Question

Try the code below with: python fork.py and with: python fork.py 1 to see what it does.

#!/usr/bin/env python2

import os
import sys

child_exit_status = 0
if len(sys.argv) > 1:
    child_exit_status = int(sys.argv[1])

pid = os.fork()
if pid == 0:
    print "This is the child"
    if child_exit_status == 0:
        os.execl('/usr/bin/whoami')
    else:
        os._exit(child_exit_status)
else:
    print "This is the parent"
    (child_pid, child_status) = os.wait()
    print "Our child %s exited with status %s" % (child_pid, child_status)

Question: How come the child process can do 'print' and it still gets outputted to the same place as the parent process?

(Am using Python 2.6 on Ubuntu 10.04)

Answer 1

Because you haven't changed the destination of file descriptor 1, standard output, for the child.

Answer 2

Under linux, the child process inherits (almost) everything from the parent, including file descriptors. In your case, file descriptor 1 (stdout) and file descriptor 2 (stderr) are open to the same file as the parent.

See the man page for fork().

If you want the output of the child to go someplace else, you can open a new file(s) in the child.