diff --git a/main.go b/main.go
index b2ef240..60360e8 100644
--- a/main.go
+++ b/main.go
@@ -114,35 +114,75 @@ func main() {
 		os.Exit(1)
 	}
 
+	// Get non-flag command-line args
+	patterns := flag.Args()
+
 	// convert -skip flags to -ignore equivalents
 	for _, s := range skipExtensionFlags {
 		ignorePatterns = append(ignorePatterns, fmt.Sprintf("**/*.%s", s))
 	}
+
+	// create logger to print updates to stdout
+	logger := log.Default()
+
+	// real main
+	err := Run(
+		ignorePatterns,
+		spdx,
+		*holder,
+		*license,
+		*licensef,
+		*year,
+		*verbose,
+		*checkonly,
+		patterns,
+		logger,
+	)
+
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Run executes addLicense with supplied variables
+func Run(
+	ignorePatterns stringSlice,
+	spdx spdxFlag,
+	holder string,
+	license string,
+	licensef string,
+	year string,
+	verbose bool,
+	checkonly bool,
+	patterns []string,
+	logger *log.Logger,
+) error {
+
 	// verify that all ignorePatterns are valid
 	for _, p := range ignorePatterns {
 		if !doublestar.ValidatePattern(p) {
-			log.Fatalf("-ignore pattern %q is not valid", p)
+			return fmt.Errorf("-ignore pattern %q is not valid", p)
 		}
 	}
 
 	// map legacy license values
-	if t, ok := legacyLicenseTypes[*license]; ok {
-		*license = t
+	if t, ok := legacyLicenseTypes[license]; ok {
+		license = t
 	}
 
 	data := licenseData{
-		Year:   *year,
-		Holder: *holder,
-		SPDXID: *license,
+		Year:   year,
+		Holder: holder,
+		SPDXID: license,
 	}
 
-	tpl, err := fetchTemplate(*license, *licensef, spdx)
+	tpl, err := fetchTemplate(license, licensef, spdx)
 	if err != nil {
-		log.Fatal(err)
+		return err
 	}
 	t, err := template.New("").Parse(tpl)
 	if err != nil {
-		log.Fatal(err)
+		return err
 	}
 
 	// process at most 1000 files in parallel
@@ -153,11 +193,11 @@ func main() {
 		for f := range ch {
 			f := f // https://golang.org/doc/faq#closures_and_goroutines
 			wg.Go(func() error {
-				if *checkonly {
+				if checkonly {
 					// Check if file extension is known
 					lic, err := licenseHeader(f.path, t, data)
 					if err != nil {
-						log.Printf("%s: %v", f.path, err)
+						logger.Printf("%s: %v", f.path, err)
 						return err
 					}
 					if lic == nil { // Unknown fileExtension
@@ -166,21 +206,21 @@ func main() {
 					// Check if file has a license
 					hasLicense, err := fileHasLicense(f.path)
 					if err != nil {
-						log.Printf("%s: %v", f.path, err)
+						logger.Printf("%s: %v", f.path, err)
 						return err
 					}
 					if !hasLicense {
-						fmt.Printf("%s\n", f.path)
+						logger.Printf("%s\n", f.path)
 						return errors.New("missing license header")
 					}
 				} else {
 					modified, err := addLicense(f.path, f.mode, t, data)
 					if err != nil {
-						log.Printf("%s: %v", f.path, err)
+						logger.Printf("%s: %v", f.path, err)
 						return err
 					}
-					if *verbose && modified {
-						log.Printf("%s modified", f.path)
+					if verbose && modified {
+						logger.Printf("%s modified", f.path)
 					}
 				}
 				return nil
@@ -193,13 +233,15 @@ func main() {
 		}
 	}()
 
-	for _, d := range flag.Args() {
-		if err := walk(ch, d); err != nil {
-			log.Fatal(err)
+	for _, d := range patterns {
+		if err := walk(ch, d, logger); err != nil {
+			return err
 		}
 	}
 	close(ch)
 	<-done
+
+	return nil
 }
 
 type file struct {
@@ -207,17 +249,17 @@ type file struct {
 	mode os.FileMode
 }
 
-func walk(ch chan<- *file, start string) error {
+func walk(ch chan<- *file, start string, logger *log.Logger) error {
 	return filepath.Walk(start, func(path string, fi os.FileInfo, err error) error {
 		if err != nil {
-			log.Printf("%s error: %v", path, err)
+			logger.Printf("%s error: %v", path, err)
 			return nil
 		}
 		if fi.IsDir() {
 			return nil
 		}
 		if fileMatches(path, ignorePatterns) {
-			log.Printf("skipping: %s", path)
+			logger.Printf("skipping: %s", path)
 			return nil
 		}
 		ch <- &file{path, fi.Mode()}