import sys

if len(sys.argv) > 1:
    with open(sys.argv[1]) as f:
        content = f.readlines()
    sys.stdout.writelines(content)
else:
    sys.exit('Usage: ./cat.py [FILE]')

--------------
import sys
import os

def listDirectory(d):
    fList = os.listdir(d)
    for f in fList:
        print 'd: ' + f

if len(sys.argv) > 1:
    if os.path.isdir(sys.argv[1]):
        listDirectory(sys.argv[1])
    else:
        print 'f: '+sys.argv[1]
else:
    sys.exit('something something try again')

-------------
import sys

if len(sys.argv) > 1:
    with open(sys.argv[1]) as f:
        content = f.readlines()
        sys.stdout.writelines(content)
else:
    sys.exit('this is madness')