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:

 1package main
 2
 3import (
 4    "bufio"
 5    "os/exec"
 6
 7    "github.com/pieterclaerhout/go-log"
 8)
 9
10func main() {
11
12    // Print the log timestamps
13    log.PrintTimestamp = true
14
15    // The command you want to run along with the argument
16    cmd := exec.Command("brew", "info", "golang")
17
18    // Get a pipe to read from standard out
19    r, _ := cmd.StdoutPipe()
20
21    // Use the same pipe for standard error
22    cmd.Stderr = cmd.Stdout
23
24    // Make a new channel which will be used to ensure we get all output
25    done := make(chan struct{})
26
27    // Create a scanner which scans r in a line-by-line fashion
28    scanner := bufio.NewScanner(r)
29
30    // Use the scanner to scan the output line by line and log it
31    // It's running in a goroutine so that it doesn't block
32    go func() {
33
34        // Read line by line and process it
35        for scanner.Scan() {
36            line := scanner.Text()
37            log.Info(line)
38        }
39
40        // We're all done, unblock the channel
41        done <- struct{}{}
42
43    }()
44
45    // Start the command and check for errors
46    err := cmd.Start()
47    log.CheckError(err)
48
49    // Wait for all output to be processed
50    <-done
51
52    // Wait for the command to finish
53    err = cmd.Wait()
54    log.CheckError(err)
55
56}

The full source code can be found here.