put-lines

#!/usr/bin/python3

import sys, os, subprocess, argparse

def put_lines(fname, lines, action):
    if not os.path.isfile(fname):
        return
    
    with open(fname) as fh:
        file = fh.read().split("\n")
        all_lines = []

        if action == 'prepend':
            lines.extend(file)
            all_lines = lines
        else:
            file.extend(lines)
            all_lines = file

    with open(fname, 'w') as fh:
        fh.write(("\n").join(all_lines))
        return True

def cmdline():
    parser = argparse.ArgumentParser(description='Append or prepend lines to a file')
    add_arg = parser.add_argument

    add_arg('-i', '--stdin', action='store_true', help='use stdin')
    add_arg('-o', '--output', nargs=1, help='file to put lines in', required=True)
    add_arg('-l', '--lines', nargs='+', help='lines to add to file. cannot be used with --stdin')
    add_arg('-a', '--append', action='store_true', help='append lines?')
    add_arg('-p', '--prepend', action='store_true', help='prepend lines?')

    args = parser.parse_args()
    lines = []

    if args.stdin:
        lines = sys.stdin.read().strip().split("\n")
    else:
        lines = args.lines

    outfile = args.output
    if not lines or len(lines) == 0:
        raise ValueError("expected at least one line, got none")

    outfile = outfile[0]
    if args.append:
        return (outfile, lines, 'append')
    else:
        return (outfile, lines, 'prepend')


def main():
    put_lines(*cmdline())

main()