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

go, stdout

Solution

I agree you should use the `fmt.Fprint` functions if you can manage it. However, if you don't control the code whose output you're capturing, you may not have that option.

Mostafa's answer works, but if you want to do it without a temporary file you can use os.Pipe. Here's an example that's equivalent to Mostafa's with some code inspired by Go's testing package.

package main

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

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

func main() {
    old := os.Stdout // keep backup of the real stdout
    r, w, _ := os.Pipe()
    os.Stdout = w

    print()

    outC := make(chan string)
    // copy the output in a separate goroutine so printing can't block indefinitely
    go func() {
        var buf bytes.Buffer
        io.Copy(&buf, r)
        outC <- buf.String()
    }()

    // back to normal state
    w.Close()
    os.Stdout = old // restoring the real stdout
    out := <-outC

    // reading our temp stdout
    fmt.Println("previous output:")
    fmt.Print(out)
}

Problem

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 StringIO object result = sys.stdout.getvalue() sys.stdout = realout ``` Can you do this in Go?

Original source