initial commit

Author: Minori Tokuda

go.mod

@@ -0,0 +1,3 @@
+module github.com/minoritea/docker-list-stages
+
+go 1.16

main.go

@@ -0,0 +1,55 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"regexp"
+)
+
+func main() {
+	err := run()
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func run() error {
+	dockerfile := "Dockerfile"
+	if len(os.Args) >= 2 {
+		dockerfile = os.Args[1]
+	}
+
+	f, err := os.Open(dockerfile)
+	if err != nil {
+		return fmt.Errorf("cannot open Dockerfile(%s): %w)", dockerfile, err)
+	}
+	defer f.Close()
+
+	stages, err := parse(f)
+	if err != nil {
+		return fmt.Errorf("Failed to parse Dockerfile(%s): %w)", dockerfile, err)
+	}
+
+	for _, stage := range stages {
+		fmt.Println(stage)
+	}
+	return nil
+}
+
+var matcher = regexp.MustCompile(`^(FROM|from) \w+ (AS|as) (\w+)$`)
+
+func parse(r io.Reader) ([]string, error) {
+	scanner := bufio.NewScanner(r)
+	var result []string
+	for scanner.Scan() {
+		line := scanner.Bytes()
+		m := matcher.FindAllSubmatch(line, -1)
+		if len(m) > 0 && len(m[0]) > 3 {
+			result = append(result, string(m[0][3]))
+		}
+	}
+	return result, scanner.Err()
+}