In Go, how do I capture stdout of a function into a string?

Question

In Python, for example, I can do the following:

realout = sys.stdout
sys.stdout = StringIO.StringIO()
some_function() # prints to stdout get captured in the          


        

Answer 1

I don't recommend this, but you can achieve it with altering os.Stdout. Since this variable is of type os.File, your temporary output should also be a file.

package main

import (
    "fmt"
    "io/ioutil"
    "os"
    "path/filepath"
)

func print() {
    fmt.Println("output")
}

func main() {
    // setting stdout to a file
    fname := filepath.Join(os.TempDir(), "stdout")
    fmt.Println("stdout is now set to", fname)
    old := os.Stdout // keep backup of the real stdout
    temp, _ := os.Create(fname) // create temp file
    os.Stdout = temp

    print()

    // back to normal state
    temp.Close()
    os.Stdout = old // restoring the real stdout

    // reading our temp stdout
    fmt.Println("previous output:")
    out, _ := ioutil.ReadFile(fname)
    fmt.Print(string(out))
}

I don't recommend because this is too much hacking, and not very idiomatic in Go. I suggest passing an io.Writer to the functions and writing outputs to that. This is the better way to do almost the same thing.

package main

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

func print(w io.Writer) {
    fmt.Fprintln(w, "output")
}

func main() {
    fmt.Println("print with byes.Buffer:")
    var b bytes.Buffer
    print(&b)
    fmt.Print(b.String())

    fmt.Println("print with os.Stdout:")
    print(os.Stdout)
}