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,
 	}
 }
 
@@ -93,10 +99,16 @@ func (p *Parser) parseLongOption(longName string) error {
 	}
 
 	opt, ok := p.opts.GetByLongName(longName)
-	if ok {
-		if err := opt.Parse(value); err != nil {
-			return err
+	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
@@ -109,10 +121,16 @@ func (p *Parser) parseShortOption(shortNames string) error {
 		value := p.value()
 
 		opt, ok := p.opts.GetByShortName(shortNames)
-		if ok {
-			if err := opt.Parse(value); err != nil {
-				return err
+		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++ {
@@ -120,23 +138,29 @@ func (p *Parser) parseShortOption(shortNames string) error {
 			value := ""
 
 			opt, ok := p.opts.GetByShortName(string(shortName))
-			if ok {
-				if opt.TakesArg() {
-					if j > 0 {
-						return ErrCannotChainOption
-					}
-
-					value = shortNames[1:]
-					value = strings.TrimPrefix(value, "=")
-					if len(value) == 0 {
-						value = p.value()
-					}
-					j = len(shortNames)
+			if !ok {
+				if !p.ignoreUnknown {
+					return fmt.Errorf("%w: %s", ErrUnknownOption, "-"+string(shortName))
 				}
 
-				if err := opt.Parse(value); err != nil {
-					return err
+				continue // Ignore unknown option. Continue parsing.
+			}
+
+			if opt.TakesArg() {
+				if j > 0 {
+					return ErrCannotChainOption
 				}
+
+				value = shortNames[1:]
+				value = strings.TrimPrefix(value, "=")
+				if len(value) == 0 {
+					value = p.value()
+				}
+				j = len(shortNames)
+			}
+
+			if err := opt.Parse(value); err != nil {
+				return err
 			}
 		}
 	}