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Jamison Dance
Jamison Dance

Reputation: 20184

In golang how can I write the stdout of an exec.Cmd to a file?

I am trying to run a shell command, capture stdout and write that output to a file. However, I seem to be missing a few steps, as the file I am trying to write is empty when the program exists. How can I capture the stdout of the command and write that to a file?

package main

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

func main() {

    cmd := exec.Command("echo", "'WHAT THE HECK IS UP'")

    // open the out file for writing
    outfile, err := os.Create("./out.txt")
    if err != nil {
        panic(err)
    }
    defer outfile.Close()

    stdoutPipe, err := cmd.StdoutPipe()
    if err != nil {
        panic(err)
    }

    writer := bufio.NewWriter(outfile)

    err = cmd.Start()
    if err != nil {
        panic(err)
    }

    go io.Copy(writer, stdoutPipe)
    cmd.Wait()
}

Upvotes: 35

Views: 45291

Answers (3)

mutecipher
mutecipher

Reputation: 199

You can also use:

cmd.Stdout = os.Stdout

which will redirect all cmd output to the OS' standard output.

Upvotes: 19

Jamison Dance
Jamison Dance

Reputation: 20184

Thanks to KirkMcDonald on the #go-nuts irc channel, I solved this by assigning the output file to cmd.Stdout, which means that stdout writes directly to the file. It looks like this:

package main

import (
    "os"
    "os/exec"
)

func main() {

    cmd := exec.Command("echo", "'WHAT THE HECK IS UP'")

    // open the out file for writing
    outfile, err := os.Create("./out.txt")
    if err != nil {
        panic(err)
    }
    defer outfile.Close()
    cmd.Stdout = outfile

    err = cmd.Start(); if err != nil {
        panic(err)
    }
    cmd.Wait()
}

Upvotes: 55

Kluyg
Kluyg

Reputation: 5347

You need to flush the writer. Add the following:

    writer := bufio.NewWriter(outfile)
    defer writer.Flush()

Upvotes: 17

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