script for watching content and executing arbitrary commands when changes are detected

script for executing arbitrary commands based on inotify events

#!/bin/bash

usage() {
    echo "Usage: $0 --watch <path_pattern> --cmd <command>"
    echo "  Monitors files/directories matching <path_pattern> for close_write events"
    echo "  and executes <command> upon detection."
    echo "  The placeholder {} in <command> will be replaced with the full path of the changed file."
    echo "Examples:"
    echo "  $0 --watch 'src/**/*.js' --cmd 'echo \"File {} changed\" && eslint {}'"
    echo "  $0 --watch '/config/app.conf' --cmd '/bin/bash /opt/scripts/reload_service.sh {}'"
    exit 1
}

# Parse command line options
WATCH_PATTERN=""
COMMAND_TO_RUN=""
while [[ $# -gt 0 ]]; do
    key="$1"
    case $key in
        --watch)
            WATCH_PATTERN="$2"
            shift; shift
            ;;
        --cmd)
            COMMAND_TO_RUN="$2"
            shift; shift
            ;;
        --help|-h)
            usage
            ;;
        *)
            echo "Unknown option: $1"
            usage
            ;;
    esac
done

if [ -z "$WATCH_PATTERN" ] || [ -z "$COMMAND_TO_RUN" ]; then
    echo "Error: --watch and --cmd are required."
    usage
fi

# Define function to monitor and execute command for a given path.
watch_and_execute() {
    local watch_path="$1"
    local command_template="$2"

    echo "Watching: $watch_path"
    echo "Command: $command_template"
    # Ensure the directory exists before watching
    if [ ! -e "$watch_path" ]; then
        echo "Error: Watch path '$watch_path' does not exist."
        return 1
    fi

    inotifywait -m -r -e close_write --format '%w%f' "${watch_path}" | while read file
    do
        echo "Detected change in: $file"
        # Replace placeholder {} with the filename
        local cmd_instance="${command_template//\{\}/$file}"
        echo "Executing: $cmd_instance"
        # Execute the command. Consider error handling or background execution if needed.
        eval "$cmd_instance"
    done
}

# Expand watch pattern supporting globs.
# Use eval to handle potential tilde expansion in the pattern
eval "watch_matches=( $WATCH_PATTERN )"
if [ ${#watch_matches[@]} -eq 0 ]; then
    echo "Error: No matches found for watch pattern '$WATCH_PATTERN'."
    exit 1
fi

# Launch each watch_and_execute function asynchronously.
for path_to_watch in "${watch_matches[@]}"; do
    # Check if the matched path exists before launching the watcher
    if [ -e "$path_to_watch" ]; then
        watch_and_execute "$path_to_watch" "$COMMAND_TO_RUN" &
    else
         echo "Warning: Matched path '$path_to_watch' does not exist. Skipping."
    fi
done

# Wait for all background processes to finish (they won't, as inotifywait runs indefinitely)
# This keeps the main script alive.
wait