🐥 Reading Command output line by line

Sometimes, you want to execute a system command from within a Go app and process it's output line-by-line in a streaming fashion.

We of course want to avoid that we need to buffer all the output, wait for the command to finish and then process each line.

We also want to check in the end if the command ran succesfully or not and we also want to capture both standard out and standard error.

Well, here's an example that shows exactly how to do this:

package main

import (
  "bufio"
  "os/exec"

  "github.com/pieterclaerhout/go-log"
)

func main() {

  // Print the log timestamps
  log.PrintTimestamp = true

  // The command you want to run along with the argument
  cmd := exec.Command("brew", "info", "golang")

  // Get a pipe to read from standard out
  r, _ := cmd.StdoutPipe()

  // Use the same pipe for standard error
  cmd.Stderr = cmd.Stdout

  // Make a new channel which will be used to ensure we get all output
  done := make(chan struct{})

  // Create a scanner which scans r in a line-by-line fashion
  scanner := bufio.NewScanner(r)

  // Use the scanner to scan the output line by line and log it
  // It's running in a goroutine so that it doesn't block
  go func() {

    // Read line by line and process it
    for scanner.Scan() {
      line := scanner.Text()
      log.Info(line)
    }

    // We're all done, unblock the channel
    done <- struct{}{}

  }()

  // Start the command and check for errors
  err := cmd.Start()
  log.CheckError(err)

  // Wait for all output to be processed
  <-done

  // Wait for the command to finish
  err = cmd.Wait()
  log.CheckError(err)

}

The full source code can be found here.