usable netcat in 99 lines of python

Code:
import sys
import socket
import getopt
import threading
import subprocess

l = False
c = False
u = False
e = ""
t = ""
d = ""
p = 0

def RC(C):
    C = C.rstrip()
    try:
        O = subprocess.check_output(C, stderr=subprocess.STDOUT, shell=True)
    except:
        O = "exe fail\n"
    return O

def CH(CS):
    global u,e,c
    if len(d):
        fb = ""
        while True:
            da = CS.recv(1024)
            if not da:
                break
            else:
                fb += da
        try:
            file_descriptor = open(d, "wb")
            file_descriptor.write(fb)
            filedescriptor.close()
            CS.send("Pass %s\n" % d)
        except:
            CS.send("Fail %s\n" % d)
    if len(e):
        op=RC(e)
        CS.send(op)
        if c:
          while True:
          CS.send(": ")
          cb = ""
          while "\n" not in cb:
              cb += CS.recv(1024)
              r = RC(cb)
              CS.send(r)

def SL():
    global t,p
    if not len(t):
        t = "0.0.0.0"
        S = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        S.bind((t, p))
        S.listen(5)
        while True:
        client_socket, addr = S.accept()
        client_thread = threading.Thread(target=CH, args=(client_socket,))
        client_thread.start()

def sn(bu):
    cl = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
    try:
        cl.connect((t,p))
        if len(bu):
            cl.send(bu)
        while True:
            recv_len=1
            re = ""
            while recv_len:
                da       = cl.recv(4096)
                recv_len = len(da)
                re       += da
                if recv_len < 4096:
                    break
                    print re,
                    bu=raw_input("")
                    bu+="\n"
                    cl.send(bu)
    except:
        print "Excpt"
        cl.close()

def use():
    print "Use: nc.py -t target_host -p port"
    print "-l listen"
    print "-e exec=file"
    print "-c command"
    print "-u upload=path"
    sys.exit(0)

def main():
    global l,p,e,c,d,t
    if not len(sys.argv[1:]):
        use()
        try:
            opts, args=getopt.getopt(sys.argv[1:], "hle:t:p:cu:")
        except getopt.GetoptError as err:
            print str(err)
            use()
        for o, a in opts:
            if o in ("-h"):
                use()
            elif o in ("-l"):
                l = True
            elif o in ("-e"):
                e = a
            elif o in ("-c"):
                c = True
            elif o in ("-u"):
                d = a
            elif o in ("-t"):
                t = a
            elif o in ("-p"):
                p = int(a)
            else:
                assert False, "bad option"
            if not l and len(t) and p > 0:
                bu = sys.stdin.read()
                sn(bu)
                if l:
                    SL()
                main()