Golang testing — gotest.tools icmd

Published on Sep 18, 2018 by Vincent Demeester.


Let’s continue the gotest.tools serie, this time with the icmd package.

Package icmd executes binaries and provides convenient assertions for testing the results.

After file-system operations (seen in fs), another common use-case in tests is to execute a command. The reasons can be you’re testing the cli you’re currently writing or you need to setup something using a command line. A classic execution in a test might lookup like the following.

cmd := exec.Command("echo", "foo")
cmd.Stout = &stdout
cmd.Env = env
if err := cmd.Run(); err != nil {
if string(stdout) != "foo" {
        t.Fatalf("expected: foo, got %s", string(stdout))

The package icmd is there to ease your pain (as usual 😉) — we used the name icmd instead of cmd because it’s a pretty common identifier in Go source code, thus would be really easy to shadow — and have some really weird problems going on.

The usual icmd workflow is the following:

  1. Describe the command you want to execute using : type Cmd, function Command and CmdOp operators.
  2. Run it using : function RunCmd or RunCommand (that does 1. for you). You can also use StartCmd and WaitOnCmd if you want more control on the execution workflow.
  3. Check the result using the Assert, Equal or Compare methods attached to the Result struct that the command execution return.

Create and run a command

Let’s first dig how to create commands. In this part, the assumption here is that the command is successful, so we’ll have .Assert(t, icmd.Success) for now — we’ll learn more about Assert in the next section 👼.

The simplest way to create and run a command is using RunCommand, it has the same signature as os/exec.Command. A simple command execution goes as below.

icmd.RunCommand("echo", "foo").Assert(t, icmd.Sucess)

Sometimes, you need to customize the command a bit more, like adding some environment variable. In those case, you are going to use RunCmd, it takes a Cmd and operators. Let’s look at those functions.

func RunCmd(cmd Cmd, cmdOperators ...CmdOp) *Result

func Command(command string, args ...string) Cmd

type Cmd struct {
        Command []string
        Timeout time.Duration
        Stdin   io.Reader
        Stdout  io.Writer
        Dir     string
        Env     []string

As we’ve seen multiple times before, it uses the powerful functional arguments. At the time I wrote this post, the icmd package doesn’t contains too much CmdOp 1, so I’ll propose two version for each example : one with CmdOpt present in this PR and one without them.

// With
icmd.RunCmd(icmd.Command("sh", "-c", "echo $FOO"),
        icmd.WithEnv("FOO=bar", "BAR=baz"), icmd.Dir("/tmp"),
).Assert(t, icmd.Success)

// Without
        Command: []string{"sh", "-c", "echo $FOO"},
        Env: []string{"FOO=bar", "BAR=baz"},
        Dir: "/tmp",
        Timeout: 10*time.Second,
}).Assert(t, icmd.Success)

As usual, the intent is clear, it’s simple to read and composable (with CmdOp’s).


Let’s dig into the assertion part of icmd. Running a command returns a struct Result. It has the following methods :

  • Assert compares the Result against the Expected struct, and fails the test if any of the expectations are not met.
  • Compare compares the result to Expected and return an error if they do not match.
  • Equal compares the result to Expected. If the result doesn’t match expected returns a formatted failure message with the command, stdout, stderr, exit code, and any failed expectations. It returns an assert.Comparison struct, that can be used by other gotest.tools.
  • Combined returns the stdout and stderr combined into a single string.
  • Stderr returns the stderr of the process as a string.
  • Stdout returns the stdout of the process as a string.

When you have a result, you, most likely want to do two things :

  • assert that the command succeed or failed with some specific values (exit code, stderr, stdout)
  • use the output — most likely stdout but maybe stderr — in the rest of the test.

As seen above, asserting the command result is using the Expected struct.

type Expected struct {
        ExitCode int    // the exit code the command returned
        Timeout  bool   // did it timeout ?
        Error    string // error returned by the execution (os/exe)
        Out      string // content of stdout
        Err      string // content of stderr

Success is a constant that defines a success — it’s an exit code of 0, didn’t timeout, no error. There is also the None constant, that should be used for Out or Err, to specify that we don’t want any content for those standard outputs.

icmd.RunCmd(icmd.Command("cat", "/does/not/exist")).Assert(t, icmd.Expected{
        ExitCode: 1,
        Err:      "cat: /does/not/exist: No such file or directory",

// In case of success, we may want to do something with the result
result := icmd.RunCommand("cat", "/does/exist")
result.Assert(t, icmd.Success)
// Read the output line by line
scanner := bufio.NewScanner(strings.NewReader(result.Stdout()))
for scanner.Scan() {
        // Do something with it

If the Result doesn’t map the Expected, a test failure will happen with a useful message that will contains the executed command and what differs between the result and the expectation.

result := icmd.RunCommand(…)
result.Assert(t, icmd.Expected{
                ExitCode: 101,
                Out:      "Something else",
                Err:      None,
// Command:  binary arg1
// ExitCode: 99 (timeout)
// Error:    exit code 99
// Stdout:   the output
// Stderr:   the stderr
// Failures:
// ExitCode was 99 expected 101
// Expected command to finish, but it hit the timeout
// Expected stdout to contain "Something else"
// Expected stderr to contain "[NOTHING]"

Finally, we listed Equal above, that returns a Comparison struct. This means we can use it easily with the assert package. As written in a previous post (about the assert package), I tend prefer to use cmp.Comparison. Let’s convert the above examples using assert.

result := icmd.RunCmd(icmd.Command("cat", "/does/not/exist"))
assert.Check(t, result.Equal(icmd.Expected{
        ExitCode: 1,
        Err:      "cat: /does/not/exist: No such file or directory",

// In case of success, we may want to do something with the result
result := icmd.RunCommand("cat", "/does/exist")
assert.Assert(t, result.Equal(icmd.Success))
// Read the output line by line
scanner := bufio.NewScanner(strings.NewReader(result.Stdout()))
for scanner.Scan() {
        // Do something with it


… that’s a wrap. The icmd package allows to easily run command and describe what result are expected of the execution, with the least noise possible. We use this package heavily on several docker/* projects (the engine, the cli)…



The icmd package is one of the oldest gotest.tools package, that comes from the docker/docker initially. We introduced these CmdOp but implementations were in docker/docker at first and we never really updated them.