Golang example of cmd stderr / stdout merge to a single channel

cmd.bat

@echo off
echo Stdout
echo Stderr 1>&2

main.go

package main

import (
	"bufio"
	"fmt"
	"io"
	"log"
	"os/exec"
)

func main() {
	cmd := exec.Command("cmd.bat")
	stderr, err := cmd.StderrPipe()
	if err != nil {
		log.Fatalf("could not get stderr pipe: %v", err)
	}
	stdout, err := cmd.StdoutPipe()
	if err != nil {
		log.Fatalf("could not get stdout pipe: %v", err)
	}
	go func() {
		merged := io.MultiReader(stderr, stdout)
		scanner := bufio.NewScanner(merged)
		for scanner.Scan() {
			msg := scanner.Text()
			fmt.Printf("msg: %s\n", msg)
		}
	}()
	if err := cmd.Run(); err != nil {
		log.Fatalf("could not run cmd: %v", err)
	}
	if err != nil {
		log.Fatalf("could not wait for cmd: %v", err)
	}
}

package main

import (
	"fmt"
	"os/exec"
)

func main() {

	cmd := exec.Command("/bin/bash", "-c", "ls /")
	res, err := cmd.CombinedOutput()
	if err != nil {
		fmt.Println(string(res))
		panic(err)
	}

	fmt.Println(string(res))
	cmd = exec.Command("/bin/bash", "-c", "ls /notfile")
	res, err = cmd.CombinedOutput()
	if err != nil {
		fmt.Println(string(res))
		panic(err)
	}
	fmt.Println(string(res))
}

the output:

bin
boot
dev
etc
go
home
lib
lib64
media
mnt
opt
proc
root
run
sbin
srv
sys
tmp
usr
var

ls: cannot access /notfile: No such file or directory

panic: exit status 2

goroutine 1 [running]:
main.main()
	/tmp/t.go:22 +0x3c7
exit status 2

Thanks for sharing this. Note that the implementation of io.MultiReader will not read from the second reader until the first reader is exhausted. As a result this approach is more of a concat than a merge.

https://go.dev/src/io/multi.go

To merge while maintaining write order, it’s possible to reassign.

cmdReader, err := cmd.StdoutPipe()
cmd.Stderr = cmd.Stdout

https://stackoverflow.com/a/35995372