use strings.Join

context.go

@@ -308,16 +308,8 @@ func checkRequiredFlags(flags []Flag, set *flag.FlagSet) error {
 		return fmt.Errorf("Required flag %q not set", missingFlags[0])
 	}
 	if numberOfMissingFlags >= 2 {
-		var missingFlagsOutput string
-		for idx, f := range missingFlags {
-			// if not the last item, append with a ", "
-			if idx != numberOfMissingFlags-1 {
-				missingFlagsOutput = fmt.Sprintf("%s%s, ", missingFlagsOutput, f)
-			} else {
-				missingFlagsOutput = fmt.Sprintf("%s%s", missingFlagsOutput, f)
-			}
-		}
-		return fmt.Errorf("Required flags %q not set", missingFlagsOutput)
+		joinedMissingFlags := strings.Join(missingFlags, ", ")
+		return fmt.Errorf("Required flags %q not set", joinedMissingFlags)
 	}
 
 	return nil