Host-wide exclusive lock without any file use (instead of flock -x $0 $0 "$@")

only.py

#!/usr/bin/python
# a tool to allow only one instance of the command to be ran at once
# host-wide exclusive lock
# no lock- or pid- files, never-used-port is "locked" instead
# written after http://timkay.com/solo/ idea

THE_POINT_TO_LOCK = ("127.1.2.3", 9000)

import sys
import os
import socket
import errno

if len(sys.argv) < 2:
    sys.stderr.write('What to run?\n')
    sys.exit(1)

sock = socket.socket()

def abort(msg, s=sock):
    sys.stderr.write(msg)
    try:
        s.close()
    except:
        pass
    sys.exit(1)

try:
    sock.bind(THE_POINT_TO_LOCK)
except socket.error as why:
    if why.errno == errno.EADDRINUSE:
        abort('Command processor is busy.\n')
    abort('Cannot bind %r: %r\n' % (THE_POINT_TO_LOCK, why))

try:
    cmd = sys.argv[1]
    os.execv(cmd, sys.argv[1:])
except Exception as why:
    abort('Cannot exec(%r, %r): %r\n' % (cmd, sys.argv[1:], why))

abort('Could not exec %r\n' % (sys.argv[1:],))

# vim: set ft=python :EOF #

If you need a traditional solution, consider something like

#!/bin/bash

[ "$1" != LoCkEd ] && {
    flock -n -x -E 199 "$0" "$0" LoCkEd "$@"
    rc=$?
    [ $rc = 199 ] && { echo "Busy." >&2; exit 2; }
    exit $rc # done
}
shift # pop LoCkEd "arg"

echo "Working..."; sleep 30; echo "Done." # continue with regular payload...

# EOF #