watch-var-file.sh

#!/usr/bin/env bash

# Watches a file and prints when a specified variable value changes.
# We assume the file is a list of key=value pairs separated by newlines.

if [ $# -ne 2 ]; then
    echo "Usage: $0 FILE KEY"
    exit 1
fi

file="$1"
key="$2"

read_value() {
    grep --only-match --perl-regexp "${key}=\K.*" "${file}"
}

value="$(read_value)"

while true; do
    inotifywait --quiet --quiet "${file}"
    new_value=$(read_value)

    if [ "${new_value}" == "${value}" ]; then
        continue
    fi

    value="${new_value}"
    echo "${value}"
done