python -c "import sys, string, random;c = string.ascii_letters + string.digits + '_-(){}[].~+=%$#?';print ''.join(random.choice(c) for x in xrange(18 if len(sys.argv) == 1 else int(sys.argv[1])))"

# Long version:
import sys, string, random
c = string.ascii_letters + string.digits + '_-(){}[].~+=%$#?'
print ''.join(random.choice(c) for x in xrange(18 if len(sys.argv) == 1 else int(sys.argv[1])))