simple cross-platform wrapper for lsof/netstat to tell what processes are listening for network connections

## listening
#!/usr/bin/env python3

import argparse
import platform
import subprocess
import sys


def eprint(*args, **kwargs):
    print(*args, file=sys.stderr, **kwargs)


def str2bool(v):
    if v.lower() in ('yes', 'true', 't', 'y', '1'):
        return True
    elif v.lower() in ('no', 'false', 'f', 'n', '0'):
        return False
    else:
        raise argparse.ArgumentTypeError('Boolean value expected.')


def cmd_macos(port=None, transport=None, ipv=None):
    return "lsof -nP -i{ipv:s}{transport:s}{port:s}".format(
        ipv="{:d}".format(ipv) if ipv else "",
        transport=transport.upper() if transport else "",
        port=":{:d}".format(port) if port else ""
    )


def cmd_linux(port=None, transport=None, ipv=None):
    if ipv == 4:
        ipv_part = " | grep --color=never -v -e \"tcp6\" -e \"udp6\""
    elif ipv == 6:
        ipv_part = " | grep --color=never -e \"tcp6\" -e \"udp6\""
    else:
        ipv_part = ""
    return "netstat -lnp{transport:s}".format(
        ipv=ipv_part,
        transport=transport[0].lower() if transport else "tu",
        port=" | grep --color=never \":{:d}\"".format(port) if port else ""
    )


if __name__ == "__main__":
    parser = argparse.ArgumentParser(description='List processes listening for network connections. (Wraps lsof on macOS; netstat on Linux.)')
    parser.add_argument('-p', '--port', type=int, default=None, help='Filter to processes listening on the given port. Default: none (all).')
    parser.add_argument('-t', '--transport', type=str, default=None, choices=['tcp','udp'], help='Specifies TCP or UDP. Default: none (both).')
    parser.add_argument('-i', '--ip', type=int, default=None, choices=[4, 6], help='Specifies IPv4 or IPv6. Default: none (both).')
    parser.add_argument('-d', '--debug', type=str2bool, default='false', help='Whether to print the lsof/netstat command to be executed to stderr. Default: False.')
    args = parser.parse_args()

    if "darwin" in platform.system().lower():
        cmd = cmd_macos(
            port=args.port,
            transport=args.transport,
            ipv=args.ip
        )
    elif "linux" in platform.system().lower():
        cmd = cmd_linux(
            port=args.port,
            transport=args.transport,
            ipv=args.ip
        )
    else:
        eprint("Platform {:s} is unsupported.".format(platform.system()))
        sys.exit(1)

    if args.debug:
        eprint("+ {:s}".format(cmd))
        eprint("")
    p = subprocess.Popen(cmd, shell=True, stdout=sys.stdout, stderr=sys.stderr)
    sys.exit(p.wait())