command/validate: respond to --help

Fixes #5072

This is very simple: we need to process CLI flags on validate in order
to catch and respond to `--help`.

command/validate.go

@@ -1,8 +1,10 @@
 package command
 
 import (
+	"flag"
 	"fmt"
 	"path/filepath"
+	"strings"
 
 	"github.com/hashicorp/terraform/config"
 )
@@ -14,14 +16,16 @@ type ValidateCommand struct {
 
 const defaultPath = "."
 
-func (c *ValidateCommand) Help() string {
-	return ""
-}
-
 func (c *ValidateCommand) Run(args []string) int {
 	args = c.Meta.process(args, false)
 	var dirPath string
 
+	cmdFlags := flag.NewFlagSet("validate", flag.ContinueOnError)
+	cmdFlags.Usage = func() { c.Ui.Error(c.Help()) }
+	if err := cmdFlags.Parse(args); err != nil {
+		return 1
+	}
+
 	if len(args) == 1 {
 		dirPath = args[0]
 	} else {
@@ -42,6 +46,25 @@ func (c *ValidateCommand) Synopsis() string {
 	return "Validates the Terraform files"
 }
 
+func (c *ValidateCommand) Help() string {
+	helpText := `
+Usage: terraform validate [options] [path]
+
+  Reads the Terraform files in the given path (directory) and
+  validates their syntax and basic semantics.
+
+  This is not a full validation that is normally done with
+  a plan or apply operation, but can be used to verify the basic
+  syntax and usage of Terraform configurations is correct.
+
+Options:
+
+  -no-color           If specified, output won't contain any color.
+
+`
+	return strings.TrimSpace(helpText)
+}
+
 func (c *ValidateCommand) validate(dir string) int {
 	cfg, err := config.LoadDir(dir)
 	if err != nil {