watch.go

package main

import (
	"fmt"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"time"
)

func usage() {
	fmt.Fprintf(os.Stderr, "usage: go-watch [dir] [cmd]\n")
	os.Exit(2)
}

func main() {
	if len(os.Args) < 3 {
		usage()
	}
	runCmd := func() error {
		cmd := exec.Command(os.Args[2], os.Args[3:]...)
		cmd.Stdout = os.Stdout
		cmd.Stdout = os.Stderr
		return cmd.Run()
	}
	if err := runCmd(); err != nil {
		log.Fatal(err)
	}

	dir := os.Args[1]

	lastMod := time.Now()
	lastExec := time.Now()
	wf := func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}
		mt := info.ModTime()
		if mt.After(lastMod) {
			lastMod = mt
		}
		return nil
	}
	for {
		time.Sleep(1 * time.Second)
		if err := filepath.Walk(dir, wf); err != nil {
			log.Fatal(err)
		}
		if lastExec.Before(lastMod) {
			runCmd()
			lastExec = lastMod
		}
	}
}