cmd/env: Refactor mode checking

This commit moves the checking of the mode arguments of the env command
into an AfterParse function. It also adds tests for the error condition.

cmd/env.go

@@ -65,8 +65,8 @@ var errModesIncompatible = exit.Error{
 	ExitCode: exit.ExitCommandArguments,
 }
 
-func (e _env) Run(context ggman.Context) error {
-	// check that at most one mode was provided
+func (e _env) AfterParse() error {
+	// check that at most one mode was given
 	count := 0
 	if e.Describe {
 		count++
@@ -80,7 +80,10 @@ func (e _env) Run(context ggman.Context) error {
 	if count > 1 {
 		return errModesIncompatible
 	}
+	return nil
+}
 
+func (e _env) Run(context ggman.Context) error {
 	variables, err := e.variables()
 	if err != nil {
 		return err

cmd/env_test.go

@@ -85,6 +85,33 @@ func TestCommandEnv(t *testing.T) {
 			"${GGROOT}\n",
 			"",
 		},
+		{
+			"more than one mode",
+			"",
+			[]string{"env", "--list", "--raw"},
+
+			4,
+			"",
+			"At most one of `--raw`, `--list` and `--describe` may be given\n",
+		},
+		{
+			"more than one mode (2)",
+			"",
+			[]string{"env", "--list", "--describe"},
+
+			4,
+			"",
+			"At most one of `--raw`, `--list` and `--describe` may be given\n",
+		},
+		{
+			"more than one mode (3)",
+			"",
+			[]string{"env", "--raw", "--describe"},
+
+			4,
+			"",
+			"At most one of `--raw`, `--list` and `--describe` may be given\n",
+		},
 	}
 
 	for _, tt := range tests {