refactor: use cobra to parse arguments

go.mod

@@ -5,5 +5,6 @@ go 1.16
 require (
 	github.com/asottile/dockerfile v3.1.0+incompatible
 	github.com/moby/buildkit v0.9.2 // indirect
+	github.com/spf13/cobra v1.0.0
 	github.com/stretchr/testify v1.7.0
 )

go.sum

@@ -646,6 +646,7 @@ github.com/imdario/mergo v0.3.8/go.mod h1:2EnlNZ0deacrJVfApfmtdGgDfMuh/nq6Ok1EcJ
 github.com/imdario/mergo v0.3.9/go.mod h1:2EnlNZ0deacrJVfApfmtdGgDfMuh/nq6Ok1EcJh5FfA=
 github.com/imdario/mergo v0.3.10/go.mod h1:jmQim1M+e3UYxmgPu/WyfjB3N3VflVyUjjjwH0dnCYA=
 github.com/imdario/mergo v0.3.11/go.mod h1:jmQim1M+e3UYxmgPu/WyfjB3N3VflVyUjjjwH0dnCYA=
+github.com/inconshreveable/mousetrap v1.0.0 h1:Z8tu5sraLXCXIcARxBp/8cbvlwVa7Z1NHg9XEKhtSvM=
 github.com/inconshreveable/mousetrap v1.0.0/go.mod h1:PxqpIevigyE2G7u3NXJIT2ANytuPF1OarO4DADm73n8=
 github.com/ishidawataru/sctp v0.0.0-20191218070446-00ab2ac2db07/go.mod h1:co9pwDoBCm1kGxawmb4sPq0cSIOOWNPT4KnHotMP1Zg=
 github.com/ishidawataru/sctp v0.0.0-20210226210310-f2269e66cdee/go.mod h1:co9pwDoBCm1kGxawmb4sPq0cSIOOWNPT4KnHotMP1Zg=
@@ -952,13 +953,15 @@ github.com/spf13/cast v1.3.1/go.mod h1:Qx5cxh0v+4UWYiBimWS+eyWzqEqokIECu5etghLkU
 github.com/spf13/cobra v0.0.2-0.20171109065643-2da4a54c5cee/go.mod h1:1l0Ry5zgKvJasoi3XT1TypsSe7PqH0Sj9dhYf7v3XqQ=
 github.com/spf13/cobra v0.0.3/go.mod h1:1l0Ry5zgKvJasoi3XT1TypsSe7PqH0Sj9dhYf7v3XqQ=
 github.com/spf13/cobra v0.0.5/go.mod h1:3K3wKZymM7VvHMDS9+Akkh4K60UwM26emMESw8tLCHU=
+github.com/spf13/cobra v1.0.0 h1:6m/oheQuQ13N9ks4hubMG6BnvwOeaJrqSPLahSnczz8=
 github.com/spf13/cobra v1.0.0/go.mod h1:/6GTrnGXV9HjY+aR4k0oJ5tcvakLuG6EuKReYlHNrgE=
 github.com/spf13/jwalterweatherman v1.0.0/go.mod h1:cQK4TGJAtQXfYWX+Ddv3mKDzgVb68N+wFjFa4jdeBTo=
 github.com/spf13/jwalterweatherman v1.1.0/go.mod h1:aNWZUN0dPAAO/Ljvb5BEdw96iTZ0EXowPYD95IqWIGo=
 github.com/spf13/pflag v0.0.0-20170130214245-9ff6c6923cff/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
 github.com/spf13/pflag v1.0.1-0.20171106142849-4c012f6dcd95/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
 github.com/spf13/pflag v1.0.1/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
 github.com/spf13/pflag v1.0.3/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
+github.com/spf13/pflag v1.0.5 h1:iy+VFUOCP1a+8yFto/drg2CJ5u0yRoB7fZw3DKv/JXA=
 github.com/spf13/pflag v1.0.5/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
 github.com/spf13/viper v1.3.2/go.mod h1:ZiWeW+zYFKm7srdB9IoDzzZXaJaI5eL9QjNiN/DMA2s=
 github.com/spf13/viper v1.4.0/go.mod h1:PTJ7Z/lr49W6bUbkmS1V3by4uWynFiR9p7+dSq/yZzE=

main.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"github.com/asottile/dockerfile"
+	"github.com/spf13/cobra"
 )
 
 const (
@@ -20,33 +21,44 @@ type Image struct {
 }
 
 func main() {
-	file, err := getInput(os.Args)
+	var rootCmd = &cobra.Command{
+		Use:   "dockerfile-image-tags",
+		Short: "List images & tags used in a Dockerfile.",
+		Args:  cobra.MaximumNArgs(1),
+		Run: func(cmd *cobra.Command, args []string) {
+			file, err := getInput(args)
+
+			if err != nil {
+				log.Fatalf("Could not read Dockerfile: %s", err)
+			}
 
-	if err != nil {
-		log.Fatalf("Could not read Dockerfile: %s", err)
-	}
+			parsed, err := dockerfile.ParseReader(file)
 
-	parsed, err := dockerfile.ParseReader(file)
+			if err != nil {
+				log.Fatalf("Could not parse Dockerfile: %s\n", err)
+			}
 
-	if err != nil {
-		log.Fatalf("Could not parse Dockerfile: %s\n", err)
-	}
+			images := getTags(parsed, UNKNOWN_MARKER)
 
-	images := getTags(parsed, UNKNOWN_MARKER)
+			val, err := json.Marshal(images)
 
-	val, err := json.Marshal(images)
+			if err != nil {
+				log.Fatalf("Could not serialize images as JSON: %s\n", err)
+			}
 
-	if err != nil {
-		log.Fatalf("Could not serialize images as JSON: %s\n", err)
+			fmt.Println(string(val))
+		},
 	}
 
-	fmt.Println(string(val))
+	if err := rootCmd.Execute(); err != nil {
+		log.Fatal(err)
+	}
 }
 
 // getInput opens the file named in `args` if present, `os.Stdin` if not.
 func getInput(args []string) (*os.File, error) {
-	if len(args) > 1 {
-		name := args[1]
+	if len(args) == 1 {
+		name := args[0]
 		f, err := os.Open(name)
 
 		if err != nil {