将stdout流写入文件

Question

I am running an external process via exec.Command() and I want the stdout from the command to be printed as well as written to file, in real time (similar to using tee from a command-line) .

I can achieve this with a scanner and a writer:

cmd := exec.Command("mycmd")
cmdStdOut, _ := cmd.StdoutPipe()

s := bufio.NewScanner(cmdStdOut)
f, _ := os.Create("stdout.log")
w := bufio.NewWriter(f)

go func() {
    for s.Scan(){
        t := s.Text()
        fmt.Println(t)
        fmt.Fprint(w, t)
        w.Flush()    
    }
}

Is there a more idiomatic way to do this that avoids clobbering Scan and Flush?

Answer 1

Assign a multiwriter to the commmand's stdout that writes to a file and to a pipe. You can then use the pipe's read end to follow the output.

This example behaves similar to the tee tool:

package main

import (
    "io"
    "os"
    "os/exec"
)

func main() {
    var f *os.File // e.g. os.Create, os.Open

    r, w := io.Pipe()
    defer w.Close()

    cmd := exec.Command("mycmd")
    cmd.Stdout = io.MultiWriter(w, f)

    // do something with the output while cmd is running by reading from r
    go io.Copy(os.Stdout, r) 

    cmd.Run()
}

Alternative with StdoutPipe:

package main

import (
    "io"
    "os"
    "os/exec"
)

func main() {
    var f *os.File

    cmd := exec.Command("date")
    stdout, _ := cmd.StdoutPipe()

    go io.Copy(io.MultiWriter(f, os.Stdout), stdout)

    cmd.Run()
}