I have a function like this:
package main
import (
"fmt"
)
// PrintSomething prints some thing
func PrintSomething() {
fmt.Println("print something")
}
func main() {
PrintSomething()
}
How do I wrap PrintSomething to another function call CaptureSomething to save the string "print something" to a variable and return it?
Create pipe and set stdout to the pipe writer. Start a goroutine to copy the pipe reader to a buffer. When done, close the pipe writer and wait for goroutine to complete reading. Return the buffer as a string.
// capture replaces os.Stdout with a writer that buffers any data written
// to os.Stdout. Call the returned function to cleanup and get the data
// as a string.
func capture() func() (string, error) {
r, w, err := os.Pipe()
if err != nil {
panic(err)
}
done := make(chan error, 1)
save := os.Stdout
os.Stdout = w
var buf strings.Builder
go func() {
_, err := io.Copy(&buf, r)
r.Close()
done <- err
}()
return func() (string, error) {
os.Stdout = save
w.Close()
err := <-done
return buf.String(), err
}
}
Use it like this:
done := capture()
fmt.Println("Hello, playground")
capturedOutput, err := done()
if err != nil {
// handle error
}
done transmit error instead of struct{} and would send on it the error returned from io.Copy or r.Close, if any, or just nil otherwise. Then I'd check what <-done returns. Well, in this case errors are unlikely but still it's better to blow up on an unexpected error than ignore it silently.Use one of these, whichever works for you:
package main
import (
"bytes"
"fmt"
"io"
"os"
"strings"
)
func PrintSomething() {
fmt.Println("print something")
}
func PrintSomethingBig() {
for i := 0; i < 100000; i++ {
fmt.Println("print something")
}
}
func PrintSomethingOut(out io.Writer) {
fmt.Fprintln(out, "print something to io.Writer")
}
func PrintSomethingString() string {
return fmt.Sprintln("print something into a string")
}
// not thread safe
// modified by [email protected] from original at http://craigwickesser.com/2015/01/capture-stdout-in-go/
func captureStdout(f func()) string {
old := os.Stdout
r, w, _ := os.Pipe()
os.Stdout = w
go func() {
f()
w.Close()
}()
buf := &bytes.Buffer{}
// Will complete when the goroutine calls w.Close()
io.Copy(buf, r)
// Clean up.
os.Stdout = old
r.Close()
return buf.String()
}
func main() {
str1 := &strings.Builder{}
str2 := PrintSomethingString()
PrintSomethingOut(str1)
PrintSomethingOut(os.Stdout)
str3 := captureStdout(PrintSomething)
str4 := captureStdout(PrintSomethingBig)
fmt.Println("string 1 is", str1)
fmt.Println("string 2 is", str2)
fmt.Println("string 3 is", str3)
fmt.Println("string 4 len", len(str4))
}
