func (c *MigrateCommand) Init(args []string) error {
	if len(args) == 0 {
		return errors.New("missing operation")
	}
	c.operation, args = args[0], args[1:]

	switch c.operation {
	case "export":
		if len(args) == 0 {
			return errors.New("missing model uuid")
		}
		c.modelUUID, args = args[0], args[1:]

	case "import":
		if len(args) == 0 {
			return errors.New("missing yaml filename")
		}
		c.filename, args = args[0], args[1:]
	default:
		return errors.Errorf("unknown operation %q", c.operation)
	}

	if !names.IsValidMachine(c.machineId) {
		return errors.Errorf("%q is not a valid machine id", c.machineId)
	}
	c.machineTag = names.NewMachineTag(c.machineId)
	return cmd.CheckEmpty(args)
}

func (s *machineSuite) TestMachineIdFormats(c *gc.C) {
	for i, test := range machineIdTests {
		c.Logf("test %d: %q", i, test.pattern)
		c.Assert(names.IsValidMachine(test.pattern), gc.Equals, test.valid)
		c.Assert(names.IsContainerMachine(test.pattern), gc.Equals, test.container)
	}
}

// AddUnits starts n units of the given service and allocates machines
// to them as necessary.
func AddUnits(st *state.State, svc *state.Service, n int, machineIdSpec string) ([]*state.Unit, error) {
	if machineIdSpec != "" && n != 1 {
		return nil, errors.Errorf("cannot add multiple units of service %q to a single machine", svc.Name())
	}
	var placement []*instance.Placement
	if machineIdSpec != "" {
		mid := machineIdSpec
		scope := instance.MachineScope
		var containerType instance.ContainerType
		specParts := strings.SplitN(machineIdSpec, ":", 2)
		if len(specParts) > 1 {
			firstPart := specParts[0]
			var err error
			if containerType, err = instance.ParseContainerType(firstPart); err == nil {
				mid = specParts[1]
				scope = string(containerType)
			}
		}
		if !names.IsValidMachine(mid) {
			return nil, fmt.Errorf("invalid force machine id %q", mid)
		}
		placement = []*instance.Placement{
			{
				Scope:     scope,
				Directive: mid,
			},
		}
	}
	return AddUnitsWithPlacement(st, svc, n, placement)
}

// PrivateAddress implements the server side of Client.PrivateAddress.
func (c *Client) PrivateAddress(p params.PrivateAddress) (results params.PrivateAddressResults, err error) {
	switch {
	case names.IsValidMachine(p.Target):
		machine, err := c.api.stateAccessor.Machine(p.Target)
		if err != nil {
			return results, err
		}
		addr, err := machine.PrivateAddress()
		if err != nil {
			return results, errors.Annotatef(err, "error fetching address for machine %q", machine)
		}
		return params.PrivateAddressResults{PrivateAddress: addr.Value}, nil

	case names.IsValidUnit(p.Target):
		unit, err := c.api.stateAccessor.Unit(p.Target)
		if err != nil {
			return results, err
		}
		addr, err := unit.PrivateAddress()
		if err != nil {
			return results, errors.Annotatef(err, "error fetching address for unit %q", unit)
		}
		return params.PrivateAddressResults{PrivateAddress: addr.Value}, nil
	}
	return results, fmt.Errorf("unknown unit or machine %q", p.Target)

}

func (c *SSHCommon) hostFromTarget(target string) (string, error) {
	// If the target is neither a machine nor a unit,
	// assume it's a hostname and try it directly.
	if !names.IsValidMachine(target) && !names.IsValidUnit(target) {
		return target, nil
	}
	// A target may not initially have an address (e.g. the
	// address updater hasn't yet run), so we must do this in
	// a loop.
	if _, err := c.ensureAPIClient(); err != nil {
		return "", err
	}
	var err error
	for a := sshHostFromTargetAttemptStrategy.Start(); a.Next(); {
		var addr string
		if c.proxy {
			addr, err = c.apiClient.PrivateAddress(target)
		} else {
			addr, err = c.apiClient.PublicAddress(target)
		}
		if err == nil {
			return addr, nil
		}
	}
	return "", err
}

// PrivateAddress implements the server side of Client.PrivateAddress.
func (c *Client) PrivateAddress(p params.PrivateAddress) (results params.PrivateAddressResults, err error) {
	switch {
	case names.IsValidMachine(p.Target):
		machine, err := c.api.state.Machine(p.Target)
		if err != nil {
			return results, err
		}
		addr := network.SelectInternalAddress(machine.Addresses(), false)
		if addr == "" {
			return results, fmt.Errorf("machine %q has no internal address", machine)
		}
		return params.PrivateAddressResults{PrivateAddress: addr}, nil

	case names.IsValidUnit(p.Target):
		unit, err := c.api.state.Unit(p.Target)
		if err != nil {
			return results, err
		}
		addr, ok := unit.PrivateAddress()
		if !ok {
			return results, fmt.Errorf("unit %q has no internal address", unit)
		}
		return params.PrivateAddressResults{PrivateAddress: addr}, nil
	}
	return results, fmt.Errorf("unknown unit or machine %q", p.Target)
}

// addServiceUnits adds a given number of units to a service.
func addServiceUnits(state *state.State, args params.AddServiceUnits) ([]*state.Unit, error) {
	service, err := state.Service(args.ServiceName)
	if err != nil {
		return nil, err
	}
	if args.NumUnits < 1 {
		return nil, fmt.Errorf("must add at least one unit")
	}

	// New API uses placement directives.
	if len(args.Placement) > 0 {
		return jjj.AddUnitsWithPlacement(state, service, args.NumUnits, args.Placement)
	}

	// Otherwise we use the older machine spec.
	if args.NumUnits > 1 && args.ToMachineSpec != "" {
		return nil, fmt.Errorf("cannot use NumUnits with ToMachineSpec")
	}

	if args.ToMachineSpec != "" && names.IsValidMachine(args.ToMachineSpec) {
		_, err = state.Machine(args.ToMachineSpec)
		if err != nil {
			return nil, errors.Annotatef(err, `cannot add units for service "%v" to machine %v`, args.ServiceName, args.ToMachineSpec)
		}
	}
	return jjj.AddUnits(state, service, args.NumUnits, args.ToMachineSpec)
}

// makePlacement makes a placement directive for the given machineIdSpec.
func makePlacement(machineIdSpec string) ([]*instance.Placement, error) {
	if machineIdSpec == "" {
		return nil, nil
	}
	mid := machineIdSpec
	scope := instance.MachineScope
	var containerType instance.ContainerType
	specParts := strings.SplitN(machineIdSpec, ":", 2)
	if len(specParts) > 1 {
		firstPart := specParts[0]
		var err error
		if containerType, err = instance.ParseContainerType(firstPart); err == nil {
			mid = specParts[1]
			scope = string(containerType)
		}
	}
	if !names.IsValidMachine(mid) {
		return nil, errors.Errorf("invalid force machine id %q", mid)
	}
	return []*instance.Placement{
		{
			Scope:     scope,
			Directive: mid,
		},
	}, nil
}

// ParseFilesystemAttachmentId parses a string as a filesystem attachment ID,
// returning the machine and filesystem components.
func ParseFilesystemAttachmentId(id string) (names.MachineTag, names.FilesystemTag, error) {
	fields := strings.SplitN(id, ":", 2)
	if len(fields) != 2 || !names.IsValidMachine(fields[0]) || !names.IsValidFilesystem(fields[1]) {
		return names.MachineTag{}, names.FilesystemTag{}, errors.Errorf("invalid filesystem attachment ID %q", id)
	}
	machineTag := names.NewMachineTag(fields[0])
	filesystemTag := names.NewFilesystemTag(fields[1])
	return machineTag, filesystemTag, nil
}

// ParseVolumeAttachmentId parses a string as a volume attachment ID,
// returning the machine and volume components.
func ParseVolumeAttachmentId(id string) (names.MachineTag, names.VolumeTag, error) {
	fields := strings.SplitN(id, ":", 2)
	if len(fields) != 2 || !names.IsValidMachine(fields[0]) || !names.IsValidVolume(fields[1]) {
		return names.MachineTag{}, names.VolumeTag{}, errors.Errorf("invalid volume attachment ID %q", id)
	}
	machineTag := names.NewMachineTag(fields[0])
	volumeTag := names.NewVolumeTag(fields[1])
	return machineTag, volumeTag, nil
}

func (c *removeCommand) Init(args []string) error {
	if len(args) == 0 {
		return fmt.Errorf("no machines specified")
	}
	for _, id := range args {
		if !names.IsValidMachine(id) {
			return fmt.Errorf("invalid machine id %q", id)
		}
	}
	c.MachineIds = args
	return nil
}

// Init initializes the command for running.
func (a *MachineAgent) Init(args []string) error {
	if !names.IsValidMachine(a.MachineId) {
		return fmt.Errorf("--machine-id option must be set, and expects a non-negative integer")
	}
	if err := a.AgentConf.CheckArgs(args); err != nil {
		return err
	}
	a.runner = newRunner(isFatal, moreImportant)
	a.workersStarted = make(chan struct{})
	a.upgradeWorkerContext = NewUpgradeWorkerContext()
	return nil
}

func (c *FixitCommand) Init(args []string) error {
	if len(args) == 0 {
		return errors.New("missing machine-id")
	}
	var machineId string
	machineId, args = args[0], args[1:]

	if !names.IsValidMachine(machineId) {
		return errors.Errorf("%q is not a valid machine id", machineId)
	}
	c.machineTag = names.NewMachineTag(machineId)
	return cmd.CheckEmpty(args)
}

func (c *RetryProvisioningCommand) Init(args []string) error {
	if len(args) == 0 {
		return fmt.Errorf("no machine specified")
	}
	c.Machines = make([]names.MachineTag, len(args))
	for i, arg := range args {
		if !names.IsValidMachine(arg) {
			return fmt.Errorf("invalid machine %q", arg)
		}
		c.Machines[i] = names.NewMachineTag(arg)
	}
	return nil
}

// ParsePlacement attempts to parse the specified string and create a
// corresponding Placement structure.
//
// If the placement directive is non-empty and missing a scope,
// ErrPlacementScopeMissing will be returned as well as a Placement
// with an empty Scope field.
func ParsePlacement(directive string) (*Placement, error) {
	if directive == "" {
		return nil, nil
	}
	if colon := strings.IndexRune(directive, ':'); colon != -1 {
		scope, directive := directive[:colon], directive[colon+1:]
		if scope == "" {
			return nil, ErrPlacementScopeMissing
		}
		// Sanity check: machine/container scopes require a machine ID as the value.
		if (scope == MachineScope || isContainerType(scope)) && !names.IsValidMachine(directive) {
			return nil, fmt.Errorf("invalid value %q for %q scope: expected machine-id", directive, scope)
		}
		return &Placement{Scope: scope, Directive: directive}, nil
	}
	if names.IsValidMachine(directive) {
		return &Placement{Scope: MachineScope, Directive: directive}, nil
	}
	if isContainerType(directive) {
		return &Placement{Scope: directive}, nil
	}
	return nil, ErrPlacementScopeMissing
}

func matchMachineId(m *state.Machine, patterns []string) (bool, bool, error) {
	var anyValid bool
	for _, p := range patterns {
		if !names.IsValidMachine(p) {
			continue
		}
		anyValid = true
		if m.Id() == p || strings.HasPrefix(m.Id(), p+"/") {
			// Pattern matches the machine, or container's
			// host machine.
			return true, true, nil
		}
	}
	return false, anyValid, nil
}

// NewAPI returns a new API client for the Singular facade. It exposes methods
// for claiming and observing administration responsibility for the apiCaller's
// model, on behalf of the supplied controller machine.
func NewAPI(apiCaller base.APICaller, controllerTag names.MachineTag) (*API, error) {
	controllerId := controllerTag.Id()
	if !names.IsValidMachine(controllerId) {
		return nil, errors.NotValidf("controller tag")
	}
	modelTag, err := apiCaller.ModelTag()
	if err != nil {
		return nil, errors.Trace(err)
	}
	facadeCaller := base.NewFacadeCaller(apiCaller, "Singular")
	return &API{
		modelTag:      modelTag,
		controllerTag: controllerTag,
		facadeCaller:  facadeCaller,
	}, nil
}

func (c *retryProvisioningCommand) Init(args []string) error {
	if len(args) == 0 {
		return errors.Errorf("no machine specified")
	}
	c.Machines = make([]names.MachineTag, len(args))
	for i, arg := range args {
		if !names.IsValidMachine(arg) {
			return errors.Errorf("invalid machine %q", arg)
		}
		if names.IsContainerMachine(arg) {
			return errors.Errorf("invalid machine %q retry-provisioning does not support containers", arg)
		}
		c.Machines[i] = names.NewMachineTag(arg)
	}
	return nil
}

func (c *RunCommand) Init(args []string) error {
	if len(args) == 0 {
		return fmt.Errorf("no commands specified")
	}
	c.commands, args = args[0], args[1:]

	if c.all {
		if len(c.machines) != 0 {
			return fmt.Errorf("You cannot specify --all and individual machines")
		}
		if len(c.services) != 0 {
			return fmt.Errorf("You cannot specify --all and individual services")
		}
		if len(c.units) != 0 {
			return fmt.Errorf("You cannot specify --all and individual units")
		}
	} else {
		if len(c.machines) == 0 && len(c.services) == 0 && len(c.units) == 0 {
			return fmt.Errorf("You must specify a target, either through --all, --machine, --service or --unit")
		}
	}

	var nameErrors []string
	for _, machineId := range c.machines {
		if !names.IsValidMachine(machineId) {
			nameErrors = append(nameErrors, fmt.Sprintf("  %q is not a valid machine id", machineId))
		}
	}
	for _, service := range c.services {
		if !names.IsValidService(service) {
			nameErrors = append(nameErrors, fmt.Sprintf("  %q is not a valid service name", service))
		}
	}
	for _, unit := range c.units {
		if !names.IsValidUnit(unit) {
			nameErrors = append(nameErrors, fmt.Sprintf("  %q is not a valid unit name", unit))
		}
	}
	if len(nameErrors) > 0 {
		return fmt.Errorf("The following run targets are not valid:\n%s",
			strings.Join(nameErrors, "\n"))
	}

	return cmd.CheckEmpty(args)
}

// addServiceUnits adds a given number of units to a service.
func addServiceUnits(state *state.State, args params.AddServiceUnits) ([]*state.Unit, error) {
	service, err := state.Service(args.ServiceName)
	if err != nil {
		return nil, err
	}
	if args.NumUnits < 1 {
		return nil, fmt.Errorf("must add at least one unit")
	}
	if args.NumUnits > 1 && args.ToMachineSpec != "" {
		return nil, fmt.Errorf("cannot use NumUnits with ToMachineSpec")
	}

	if args.ToMachineSpec != "" && names.IsValidMachine(args.ToMachineSpec) {
		_, err = state.Machine(args.ToMachineSpec)
		if err != nil {
			return nil, errors.Annotatef(err, `cannot add units for service "%v" to machine %v`, args.ServiceName, args.ToMachineSpec)
		}
	}
	return jjj.AddUnits(state, service, args.NumUnits, args.ToMachineSpec)
}