wholistens

#!/usr/bin/env python
'''Find out who is listening on a port'''

from os import popen
from os.path import isdir
import re

is_int = re.compile("\d+").match

def find_pid(port):
    for line in popen("netstat -nlp 2>&1"):
        match = re.search(":(%s)\\s+" % port, line)
        if not match:
            continue

        pidname = line.split()[-1].strip()
        return pidname.split("/")[0]

    return None

def find_cmdline(pid):
    cmd = open("/proc/%s/cmdline" % pid, "rb").read()

    return " ".join(cmd.split(chr(0)))

def find_pwd(pid):
    data = open("/proc/%s/environ" % pid, "rb").read()
    for line in data.split(chr(0)):
        if line.startswith("PWD"):
            return line.split("=")[1]

    return None

def main(argv=None):
    if argv is None:
        import sys
        argv = sys.argv

    from optparse import OptionParser

    parser = OptionParser("usage: %prog PORT")
    opts, args = parser.parse_args(argv[1:])
    if len(args) != 1:
        parser.error("wrong number of arguments") # Will exit

    port = args[0]
    pid = find_pid(port)
    if not (pid and is_int(pid)):
        raise SystemExit(
            "error: can't find who listens on port %s"
            " [try again with sudo?] " % port)

    if not isdir("/proc/%s" % pid):
        raise SystemExit("error: can't find information on pid %s" % pid)

    pwd = find_pwd(pid) or "<unknown>"
    print "%s (pid=%s, pwd=%s)" % (find_cmdline(pid), pid, pwd)

if __name__ == "__main__":
    main()

Note: This does not work on OSX (no /proc and different netstat api)