Add error for unknown option

command.go

@@ -68,12 +68,12 @@ func (c *Command) Execute(args []string) {
 		args = args[1:]
 	}
 
-	parser := opts.NewParser(args, c.opts)
+	parser := opts.NewParser(args, c.opts, false)
 	restArgs, err := parser.Parse()
 	if err != nil {
-		fmt.Printf("Option error: %s\n", err.Error())
+		fmt.Println(err.Error())
 		c.ShowHelp()
-		return
+		os.Exit(1)
 	}
 
 	if len(restArgs) > 0 {

opts/parser.go

@@ -2,10 +2,12 @@ package opts
 
 import (
 	"errors"
+	"fmt"
 	"strings"
 )
 
 var (
+	ErrUnknownOption      = errors.New("unknown option")
 	ErrInvalidShortOption = errors.New("invalid short option")
 	ErrCannotChainOption  = errors.New("cannot chain option as it takes an argument")
 )
@@ -14,13 +16,17 @@ type Parser struct {
 	opts Set
 	args []string
 	curr int
+
+	ignoreUnknown bool
 }
 
-func NewParser(args []string, opts []Option) *Parser {
+func NewParser(args []string, opts []Option, ignoreUnknown bool) *Parser {
 	return &Parser{
 		opts: append(opts, globalOpts...),
 		args: args,
 		curr: -1,
+
+		ignoreUnknown: ignoreUnknown,
 	}
 }
 
@@ -93,11 +99,17 @@ func (p *Parser) parseLongOption(longName string) error {
 	}
 
 	opt, ok := p.opts.GetByLongName(longName)
-	if ok {
+	if !ok {
+		if !p.ignoreUnknown {
+			return fmt.Errorf("%w: %s", ErrUnknownOption, "--"+longName)
+		}
+
+		return nil // Ignore unknown option. Continue parsing.
+	}
+
 	if err := opt.Parse(value); err != nil {
 		return err
 	}
-	}
 
 	return nil
 }
@@ -109,18 +121,31 @@ func (p *Parser) parseShortOption(shortNames string) error {
 		value := p.value()
 
 		opt, ok := p.opts.GetByShortName(shortNames)
-		if ok {
+		if !ok {
+			if !p.ignoreUnknown {
+				return fmt.Errorf("%w: %s", ErrUnknownOption, "-"+shortNames)
+			}
+
+			return nil // Ignore unknown option. Continue parsing.
+		}
+
 		if err := opt.Parse(value); err != nil {
 			return err
 		}
-		}
 	} else {
 		for j := 0; j < len(shortNames); j++ {
 			shortName := shortNames[j]
 			value := ""
 
 			opt, ok := p.opts.GetByShortName(string(shortName))
-			if ok {
+			if !ok {
+				if !p.ignoreUnknown {
+					return fmt.Errorf("%w: %s", ErrUnknownOption, "-"+string(shortName))
+				}
+
+				continue // Ignore unknown option. Continue parsing.
+			}
+
 			if opt.TakesArg() {
 				if j > 0 {
 					return ErrCannotChainOption
@@ -139,7 +164,6 @@ func (p *Parser) parseShortOption(shortNames string) error {
 			}
 		}
 	}
-	}
 
 	return nil
 }