gistfile1.py

#!/usr/bin/python

import sys
import shutil

def copyr(f,t,fnd,repl):
    """ A simple string replace in files. You may wish to extend it for regex."""
    while f:
        line = f.readline()
        if len(line) == 0:
            break
        t.write(line.replace(fnd, repl))

def main(fl, fnd, repl):
    with open(fl)as pas:
        with open('tmp', 'w+') as tmp:
            copyr(pas, tmp, fnd, repl)

    shutil.copy('tmp', fl)

if __name__ == '__main__':
    arg = sys.argv[1:]
    main(fl=arg[0], fnd=arg[1], repl=arg[2])
# do ./reader.py ./aTextFile.txt expression_to_find expression_to_replace