add car root command

cmd/car/car.go

@@ -135,6 +135,11 @@ func main1() int {
 					},
 				},
 			},
+			{
+				Name:   "root",
+				Usage:  "Get the root CID of a car",
+				Action: CarRoot,
+			},
 			{
 				Name:    "verify",
 				Aliases: []string{"v"},

cmd/car/root.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	carv2 "github.com/ipld/go-car/v2"
+	"github.com/urfave/cli/v2"
+)
+
+// CarRoot prints the root CID in a car
+func CarRoot(c *cli.Context) (err error) {
+	inStream := os.Stdin
+	if c.Args().Len() >= 1 {
+		inStream, err = os.Open(c.Args().First())
+		if err != nil {
+			return err
+		}
+	}
+
+	rd, err := carv2.NewBlockReader(inStream)
+	if err != nil {
+		return err
+	}
+	for _, r := range rd.Roots {
+		fmt.Printf("%s\n", r.String())
+	}
+
+	return nil
+}

cmd/car/testdata/script/root.txt

@@ -0,0 +1,15 @@
+car root ${INPUTS}/sample-v1.car
+cmp stdout v1root.txt
+
+car root ${INPUTS}/sample-wrapped-v2.car
+cmp stdout v2root.txt
+
+stop stdin_test_needs_car_fix
+stdin ${INPUTS}/sample-wrapped-v2.car
+car root
+cmp stdout v2root.txt
+
+-- v1root.txt --
+bafy2bzaced4ueelaegfs5fqu4tzsh6ywbbpfk3cxppupmxfdhbpbhzawfw5oy
+-- v2root.txt --
+bafy2bzaced4ueelaegfs5fqu4tzsh6ywbbpfk3cxppupmxfdhbpbhzawfw5oy