Welcome to Part 4 of my series on my tool for backing up ZFS Snapshots to an external device. In this part, I am discussing how to exec a command and read its output.

To deal with external commands in Go, you use the os/exec package. The primary pieces of the package that I need for now are exec.Command() and CombinedOutput(). exec.Command() sets up the Command structure with the command and any arguments that I am passing to it.

var listCommand = exec.Command("zfs", "list", "-Hrt", "snapshot", "dpool")

That code creates a variable called listCommand, which is ready to run the command zfs with the arguments list, -Hrt, and snapshot as individual arguments.

var snapList, err = listCommand.CombinedOutput()

That line of code runs the command I previously prepared, puts both its Standard Output and Standard Error in a slice of bytes. If the command exited with an error code other than 0, CombinedOutput sets err to a non-nil value. snapList will have the Standard Error of the executed command, so printing snapList’s contents will be useful for debugging.

var snapScanner = bufio.NewScanner(bytes.NewReader(snapList))
	if err != nil {
		fmt.Println(listCommand)
		fmt.Println("Error trying to list snapshots:", err.Error())
		for snapScanner.Scan() {
			fmt.Println(snapScanner.Text())
		}
	}

I will need to use the more complicated IO redirection tools provided in the os/exec package for the zfs send and zfs receive commands. However, for a test run today, I can use a modification of the loop I used to print the output from zfs if it errored.

for snapScanner.Scan() {
		if snapshotLineRegex.MatchString(snapScanner.Text()) {
			var temp = strings.SplitN(snapScanner.Text(), "\t", 2)
			var snapshot = ParseSnapshot(temp[0])
			if snapshot != nil {
				fmt.Println("I found snapshot", snapshot.Name(), "at", snapshot.Path())
			}
		}
	}