hashicorp/nomad · error

cmd is required, but was empty

Error message

cmd is required, but was empty

What it means

The driver's legacy ExecTask path requires at least one command element; `cmd` is the full argv where cmd[0] is the binary and cmd[1:] its arguments. When an empty slice is passed, the driver rejects it before creating any Docker exec. This is defensive input validation because exec'ing with no binary is meaningless.

Source

Thrown at drivers/docker/driver.go:1920

	if err != nil {
		return fmt.Errorf("failed to parse signal: %v", err)
	}

	// TODO: review whether we can timeout in this and other Docker API
	// calls without breaking the expected client behavior.
	// see https://github.com/hashicorp/nomad/issues/9503
	_, err = h.dockerClient.ContainerKill(d.ctx, h.containerID, mclient.ContainerKillOptions{Signal: signal})
	return err
}

func (d *Driver) ExecTask(taskID string, cmd []string, timeout time.Duration) (*drivers.ExecTaskResult, error) {
	h, ok := d.tasks.Get(taskID)
	if !ok {
		return nil, drivers.ErrTaskNotFound
	}

	if len(cmd) == 0 {
		return nil, fmt.Errorf("cmd is required, but was empty")
	}

	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()

	return h.Exec(ctx, cmd[0], cmd[1:])
}

var _ drivers.ExecTaskStreamingDriver = (*Driver)(nil)

func (d *Driver) ExecTaskStreaming(ctx context.Context, taskID string, opts *drivers.ExecOptions) (*drivers.ExitResult, error) {
	defer opts.Stdout.Close()
	defer opts.Stderr.Close()

	done := make(chan interface{})
	defer close(done)

	h, ok := d.tasks.Get(taskID)

View on GitHub (pinned to 482b49bf1a)

Solutions

1. Pass a non-empty command, e.g. []string{"ls", "-la"}.
2. Validate job/task command fields before dispatching exec.
3. Guard the call site: return your own validation error if len(cmd) == 0.

Example fix

// before
driver.ExecTask(taskID, []string{}, 5*time.Second)
// after
driver.ExecTask(taskID, []string{"/bin/sh", "-c", "echo hi"}, 5*time.Second)

if len(cmd) == 0 { return errors.New("cmd must contain at least a binary") }

When it happens

Trigger: Calling driver.ExecTask(taskID, []string{}, timeout) or ExecTask(taskID, nil, timeout).

Common situations: Job spec with an empty `command` list; templating produced an empty argv; code path that split a command string yielding zero tokens.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• command is required but was empty
• command is not present
• does not match registry specification
• unknown task name %q
• must provide task name

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/0682b6709c2d657d. Report an issue: GitHub.