docker/cli · error

unknown option ' ' in

Error message

unknown option '%s' in '%s'

What it means

Returned by MountOpt.Set (opts/mount.go:140) when a mount option key matches none of the recognized cases. The key (lowercased) is switched against the full known set (type, source/src, target/dst/destination, readonly/ro, consistency, bind-propagation, bind-recursive, bind-create-src, volume-subpath, volume-nocopy, volume-label, volume-driver, volume-opt, image-subpath, tmpfs-size, tmpfs-mode); anything else falls to default and is rejected.

Solutions

1. Check the spelling against the supported mount options and use aliases (src for source, dst/destination for target).
2. Confirm the option exists in your CLI version — newer daemon options may not be parsed yet.
3. Remove the unsupported field; some options belong in the daemon config, not the mount spec.

Example fix

// before
--mount "type=bind,src=/data,dest=/data,read-only=true"

// after
--mount "type=bind,source=/data,target=/data,readonly"

var knownMountKeys = map[string]bool{
    "type": true, "source": true, "src": true, "target": true, "dst": true, "destination": true,
    "readonly": true, "ro": true, "consistency": true, "bind-propagation": true, "bind-recursive": true,
    "bind-create-src": true, "volume-subpath": true, "volume-nocopy": true, "volume-label": true,
    "volume-driver": true, "volume-opt": true, "image-subpath": true, "tmpfs-size": true, "tmpfs-mode": true,
}

func validateKnownKeys(spec string) error {
    r := csv.NewReader(strings.NewReader(spec))
    fields, err := r.Read()
    if err != nil {
        return err
    }
    for _, f := range fields {
        k, _, _ := strings.Cut(f, "=")
        if !knownMountKeys[strings.ToLower(k)] {
            return fmt.Errorf("unknown mount option %q", k)
        }
    }
    return nil
}

if err := m.Set(spec); err != nil {
    return fmt.Errorf("mount %q: %w", spec, err)
}

When it happens

Trigger: Passing a typo or unsupported option: `src`, `destintion`, `read-only`, `volume-copy`, `tmpfs-uid`, `propagation`, or an Engine version's option not implemented by this CLI version.

Common situations: Misspelling an option name, using a hyphen vs underscore mismatch, or referencing a daemon option that the CLI opts layer does not surface.

Related errors

• invalid value for ' ': value is empty
• invalid value for ' ' in ' ': value should not have…
• invalid field ' ' must be a key=value pair
• invalid value for : (must be "enabled", "disabled"…
• invalid value for

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/1cf678e518d54c68. Report an issue: GitHub.

Appendix: source

Thrown at opts/mount.go:140

		case "volume-opt":
			volumeDriver := ensureVolumeDriver(&mount)
			volumeDriver.Options = setValueOnMap(volumeDriver.Options, val)
		case "image-subpath":
			ensureImageOptions(&mount).Subpath = val
		case "tmpfs-size":
			sizeBytes, err := units.RAMInBytes(val)
			if err != nil {
				return fmt.Errorf("invalid value for %s: %s", key, val)
			}
			ensureTmpfsOptions(&mount).SizeBytes = sizeBytes
		case "tmpfs-mode":
			ui64, err := strconv.ParseUint(val, 8, 32)
			if err != nil {
				return fmt.Errorf("invalid value for %s: %s", key, val)
			}
			ensureTmpfsOptions(&mount).Mode = os.FileMode(ui64)
		default:
			return fmt.Errorf("unknown option '%s' in '%s'", key, field)
		}
	}

	if err := validateMountOptions(&mount); err != nil {
		return err
	}

	m.values = append(m.values, mount)
	return nil
}

// Type returns the type of this option
func (*MountOpt) Type() string {
	return "mount"
}

// String returns a string repr of this option
func (m *MountOpt) String() string {

View on GitHub (pinned to 4f84911bfe)