stderr stdout demo

0 instructions.sh

go build -o main -tags main && go build -o main2 -tags main2

# run main2 by itself
./main2

# exits with a non-zero exit code and gives:
#
# 1st line to stdout
# 2nd line to stderr
# 3rd line to stdout

# run main which calls main2
./main

# exits with a non-zero exit code and gives"
#
# we got an error running main2: exit status 1
# Stdout was:
# 1st line to stdout
# 3rd line to stdout
# Stderr was:
# 2nd line to stderr

# i.e. we lose the interleaving of stderr and stdout - this is, more often than not, useful/
# critical context when we have a non-zero exit status

main.go

// +build main

package main

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

func main() {
	stdout := new(bytes.Buffer)
	stderr := new(bytes.Buffer)

	cmd := exec.Command("./main2")
	cmd.Stdout = stdout
	cmd.Stderr = stderr

	if err := cmd.Run(); err != nil {
		fmt.Fprintf(os.Stderr, "we got an error running main2: %v\n", err)
		fmt.Fprintf(os.Stderr, "Stdout was:\n%s", stdout.Bytes())
		fmt.Fprintf(os.Stderr, "Stderr was:\n%s", stderr.Bytes())
		os.Exit(1)
	}
}

main2.go

// +build main2

package main

import (
	"fmt"
	"os"
)

func main() {
	fmt.Fprintln(os.Stdout, "1st line to stdout")
	fmt.Fprintln(os.Stderr, "2nd line to stderr")
	fmt.Fprintln(os.Stdout, "3rd line to stdout")

	os.Exit(1)
}