watch w/o buffering

main.go

package main

import (
	"context"
	"flag"
	"fmt"
	"os"
	"os/exec"
	"os/signal"
	"time"
)

func main() {
	period := flag.Duration("n", time.Second*2, "time between command restarts")
	flag.Parse()

	args := flag.Args()
	if len(args) < 1 {
		fmt.Fprintln(os.Stderr, "no command specified")
		os.Exit(1)
	}

	ctx, cancel := context.WithCancel(context.Background())

	signals := make(chan os.Signal)
	signal.Notify(signals, os.Interrupt)

	go func() {
		<-signals
		fmt.Fprintln(os.Stderr, "received signal")
		cancel()
	}()

	for {
		cmd := exec.CommandContext(ctx, args[0], args[1:]...)
		cmd.Stdout = os.Stdout
		cmd.Stderr = os.Stderr

		doneC := make(chan struct{})

		err := cmd.Start()
		if err != nil {
			fmt.Fprintf(os.Stderr, "failed to start command: %v\n", err)
		} else {
			go func() {
				err := cmd.Wait()
				if err != nil {
					fmt.Fprintf(os.Stderr, "command error: %v\n", err)
				}
				close(doneC)
			}()

			select {
			case <-doneC:
			case <-ctx.Done():
				return
			}
		}

		if *period == 0 {
			select {
			case <-ctx.Done():
				return
			default:
			}
			continue
		}

		sleepC := time.After(*period)

		select {
		case <-sleepC:
		case <-ctx.Done():
			return
		}
	}
}