run.go

package main

import (
	"bytes"
	"fmt"
	"io"
	"os"
	"os/exec"
)

func main() {
	if len(os.Args) < 2 {
		fmt.Printf("Usage: %s <cmd> [<args>]*\n", os.Args[0])
		os.Exit(1)
	}

	cmdName := os.Args[1]
	var cmdArgs []string
	if len(os.Args) > 2 {
		cmdArgs = os.Args[2:]
	}
	cmd := exec.Command(cmdName, cmdArgs...)
	cmd.Stdout = newIndentWriter(">>> ", os.Stdout)
	cmd.Stderr = newIndentWriter(">>> ", os.Stderr)
	err := cmd.Run()
	fmt.Println()
	if err != nil {
		fmt.Println("Error:", err)
	}
}

type indentWriter struct {
	prefix      []byte
	writer      io.Writer
	needsPrefix bool // true when next write needs to write the prefix first
}

func newIndentWriter(prefix string, writer io.Writer) *indentWriter {
	return &indentWriter{
		prefix:      []byte(prefix),
		writer:      writer,
		needsPrefix: true,
	}
}

func (iw *indentWriter) Write(data []byte) (int, error) {
	if iw.needsPrefix {
		_, err := iw.writer.Write(iw.prefix)
		if err != nil {
			return 0, err // no bytes from data written yet
		}
		iw.needsPrefix = false
	}

	newLine := bytes.IndexByte(data, '\n')
	if newLine == -1 {
		return iw.writer.Write(data)
	}

	n, err := iw.writer.Write(data[0 : newLine+1])
	if err != nil {
		return n, err
	}

	iw.needsPrefix = true
	nn, err := iw.Write(data[newLine+1:])
	return n + nn, err
}