Indent output of commands

go/archive/archive.go

package main
import (
	"bytes"
	"crypto/rand"
	"encoding/json"
	"flag"
	"fmt"
	"io"
	"net/url"
	"os"
	"os/exec"
	}
	cmd := exec.Command("prince", "--javascript", "--raster-output", fmt.Sprintf(".archive/%x-%%02d.png", buf), u.String())
	cmd.Stderr = os.Stderr
	cmd.Stdout = os.Stdout
	cmd.Stderr = prefixWriter("    | ", os.Stderr)
	cmd.Stdout = prefixWriter("    | ", os.Stdout)
	err = cmd.Run()
	if err != nil {
		exit("prince", err)
	fmt.Fprintf(os.Stderr, "Error: %s: %s\n", msg, err)
	os.Exit(1)
}
type prefixLineWriter struct {
	prefix      string
	needsPrefix bool
	w io.Writer
}
func prefixWriter(prefix string, w io.Writer) io.Writer {
	return &prefixLineWriter{
		prefix:      prefix,
		needsPrefix: true,
		w: w,
	}
}
func (p *prefixLineWriter) Write(b []byte) (n int, err error) {
	if p.needsPrefix {
		p.w.Write([]byte(p.prefix))
		p.needsPrefix = false
	}
	n = 0
	for {
		i := bytes.IndexByte(b, '\n')
		if i == -1 {
			nn, err := p.w.Write(b)
			return n + nn, err
		}
		if i+1 == len(b) {
			p.needsPrefix = true
			nn, err := p.w.Write(b)
			return n + nn, err
		}
		nn, err := p.w.Write(b[:i+1])
		if err != nil {
			return n + nn, err
		}
		n += nn
		p.w.Write([]byte(p.prefix))
		b = b[i+1:]
	}
}