switched: A tool to recognize state changes in a command-line pipe.

switched.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import os
import re
import sys

if sys.stdin.isatty():
    print("""Outputs a line if the input changed.

Usage: {scriptname} [<switch regex>] [<custom message prefix>] [-print0]

Example:
    ping google.com | {scriptname} 'time=\d+' 'Internet is ' | xargs -0 -n 1 notify-send
    # Notifies you with a Desktop notification
    # when pings start failing or succeeding.

    echo -e "hello\n\n\nx" | ./switch.py "" "Empty region is "
    # Outputs a message if for every switch in empty to non-empty lines
    # and vice versa.


Makes only sense as part of a pipe, hence if stdin is connected to a TTY
this message will be output.

The start state is OFF.

switch regex: Determines if input changes (match vs non-match).
    Default: non-whitespace.

custom message prefix: On flipping the message is output that
    made the switch change. If this prefix is given, it will be output instead
    and the word “ON” and “OFF” will be appended.

-print0: Terminates lines with 0, for usage with xargs. See `man find`.

""".format(scriptname=sys.argv[0]))
    sys.exit(0)

regex = sys.argv[1] if len(sys.argv) > 1 and sys.argv[1] else r"\S+"
messageprefix = sys.argv[2] if len(sys.argv) > 2 else None
pattern = re.compile(regex)
# Possible clash with a message prefix or regex … but don't care.
end = "\x00" if '-print0' in sys.argv else os.linesep


def print_on_change(line, state):
    message = line if not messageprefix else (messageprefix + state)
    print(message, end=end)
    sys.stdout.flush()


def main():
    is_on = False
    while True:
        rawline = sys.stdin.readline()
        if rawline == "":
            sys.exit(0)
        line = rawline.strip()
        match = pattern.match(line)
        if bool(match) ^ bool(is_on):
            is_on = not is_on
            print_on_change(line, "ON" if is_on else "OFF")

try:
    main()
except KeyboardInterrupt:
    pass