python implementation of nl command(no options).

nl.py

#! /usr/bin/env python                                                                                                                       

import sys

DEFAULT_INDENT = 4

def main():
    argc = len(sys.argv)
    f = None # File IO                                                                                                                       

    if argc == 1:
        print "type text below. enter Ctrl-D to exit."
        f = sys.stdin
    elif argc == 2:
        try:
            f = open(sys.argv[1], "rU")
        except IOError:
            sys.exit("nl.py: %s: No such file or directory" % (sys.argv[1]))
    else:
        sys.exit("usage: python nl.py [file]")

    i = 1
    readlines = f.readlines()
    for line in readlines:
        if len(line.strip()) <= 0:
            print ""
        else:
            num = len(str(len(readlines))) - len(str(i)) + DEFAULT_INDENT
            print "%s%d  %s" % (" " * num, i, line.rstrip())
            i += 1

if __name__ == "__main__":
    main()