add environment variable support 🎉

app.go

@@ -228,7 +228,7 @@ func (a *App) Run(arguments []string) (err error) {
 		return nil
 	}
 
-	cerr := checkRequiredFlags(a.Flags, set)
+	cerr := checkRequiredFlags(a.Flags, context)
 	if cerr != nil {
 		ShowAppHelp(context)
 		return cerr
@@ -358,7 +358,7 @@ func (a *App) RunAsSubcommand(ctx *Context) (err error) {
 		}
 	}
 
-	cerr := checkRequiredFlags(a.Flags, set)
+	cerr := checkRequiredFlags(a.Flags, context)
 	if cerr != nil {
 		ShowSubcommandHelp(context)
 		return cerr

command.go

@@ -135,7 +135,7 @@ func (c Command) Run(ctx *Context) (err error) {
 		return nil
 	}
 
-	cerr := checkRequiredFlags(c.Flags, set)
+	cerr := checkRequiredFlags(c.Flags, context)
 	if cerr != nil {
 		ShowCommandHelp(context, c.Name)
 		return cerr

context.go

@@ -309,17 +309,12 @@ func (e *errRequiredFlags) getMissingFlags() []string {
 	return e.missingFlags
 }
 
-func checkRequiredFlags(flags []Flag, set *flag.FlagSet) requiredFlagsErr {
+func checkRequiredFlags(flags []Flag, context *Context) requiredFlagsErr {
-	visited := make(map[string]bool)
-	set.Visit(func(f *flag.Flag) {
-		visited[f.Name] = true
-	})
-
 	var missingFlags []string
 	for _, f := range flags {
 		if rf, ok := f.(RequiredFlag); ok && rf.IsRequired() {
 			key := strings.Split(f.GetName(), ",")[0]
-			if !visited[key] {
+			if !context.IsSet(key) {
 				missingFlags = append(missingFlags, key)
 			}
 		}

context_test.go

@@ -495,9 +495,11 @@ func TestCheckRequiredFlags(t *testing.T) {
 				flags.Apply(set)
 			}
 			set.Parse(test.parseInput)
+			ctx := &Context{}
+			context := NewContext(ctx.App, set, ctx)
 
 			// logic under test
-			err := checkRequiredFlags(test.flags, set)
+			err := checkRequiredFlags(test.flags, context)
 
 			// assertions
 			if test.expectedAnError && err == nil {