fleetctl: Add the args check for some subcommand

Some subcommand like destroy/load/start/status/stop/submit/unload,
need to give prompt if no args for them.
Add the same prompt "One unit file must be provided" as the "cat"
subcommand is doing:
[root@localhost fleet-0.10.1]# fleetctl cat
One unit file must be provided

Author: wuqixuan

Diff for: fleetctl/destroy.go

@@ -29,6 +29,10 @@ Destroyed units are impossible to start unless re-submitted.`,
 }
 
 func runDestroyUnits(args []string) (exit int) {
+	if len(args) == 0 {
+		stderr("One unit file must be provided.")
+		return 1
+	}
 	for _, v := range args {
 		name := unitNameMangle(v)
 		err := cAPI.DestroyUnit(name)

Diff for: fleetctl/load.go

@@ -46,6 +46,10 @@ func init() {
 }
 
 func runLoadUnits(args []string) (exit int) {
+	if len(args) == 0 {
+		stderr("One unit file must be provided.")
+		return 1
+	}
 	if err := lazyCreateUnits(args); err != nil {
 		stderr("Error creating units: %v", err)
 		return 1

Diff for: fleetctl/start.go

@@ -54,6 +54,10 @@ func init() {
 }
 
 func runStartUnit(args []string) (exit int) {
+	if len(args) == 0 {
+		stderr("One unit file must be provided.")
+		return 1
+	}
 	if err := lazyCreateUnits(args); err != nil {
 		stderr("Error creating units: %v", err)
 		return 1

Diff for: fleetctl/status.go

@@ -44,6 +44,10 @@ func init() {
 }
 
 func runStatusUnits(args []string) (exit int) {
+	if len(args) == 0 {
+		stderr("One unit file must be provided.")
+		return 1
+	}
 	units, err := cAPI.Units()
 	if err != nil {
 		stderr("Error retrieving unit: %v", err)

Diff for: fleetctl/stop.go

@@ -52,6 +52,10 @@ func init() {
 }
 
 func runStopUnit(args []string) (exit int) {
+	if len(args) == 0 {
+		stderr("One unit file must be provided.")
+		return 1
+	}
 	units, err := findUnits(args)
 	if err != nil {
 		stderr("%v", err)

Diff for: fleetctl/submit.go

@@ -36,6 +36,10 @@ func init() {
 }
 
 func runSubmitUnits(args []string) (exit int) {
+	if len(args) == 0 {
+		stderr("One unit file must be provided.")
+		return 1
+	}
 	if err := lazyCreateUnits(args); err != nil {
 		stderr("Error creating units: %v", err)
 		exit = 1

Diff for: fleetctl/unload.go

@@ -36,6 +36,10 @@ func init() {
 }
 
 func runUnloadUnit(args []string) (exit int) {
+	if len(args) == 0 {
+		stderr("One unit file must be provided.")
+		return 1
+	}
 	units, err := findUnits(args)
 	if err != nil {
 		stderr("%v", err)