add get block to car cli (#230)

add `get block` to car cli for extracting an individual block by CID from a car

v2/cmd/car/car.go

@@ -13,6 +13,30 @@ func main() {
 		Name:  "car",
 		Usage: "Utility for working with car files",
 		Commands: []*cli.Command{
+			{
+				Name:   "detach-index",
+				Usage:  "Detach an index to a detached file",
+				Action: DetachCar,
+			},
+			{
+				Name:    "filter",
+				Aliases: []string{"f"},
+				Usage:   "Filter the CIDs in a car",
+				Action:  FilterCar,
+				Flags: []cli.Flag{
+					&cli.StringFlag{
+						Name:      "cid-file",
+						Usage:     "A file to read CIDs from",
+						TakesFile: true,
+					},
+				},
+			},
+			{
+				Name:    "get-block",
+				Aliases: []string{"gb"},
+				Usage:   "Get a block out of a car",
+				Action:  GetCarBlock,
+			},
 			{
 				Name:    "index",
 				Aliases: []string{"i"},
@@ -27,30 +51,12 @@ func main() {
 					},
 				},
 			},
-			{
-				Name:   "detach-index",
-				Usage:  "Detach an index to a detached file",
-				Action: DetachCar,
-			},
 			{
 				Name:    "list",
 				Aliases: []string{"l"},
 				Usage:   "List the CIDs in a car",
 				Action:  ListCar,
 			},
-			{
-				Name:    "filter",
-				Aliases: []string{"f"},
-				Usage:   "Filter the CIDs in a car",
-				Action:  FilterCar,
-				Flags: []cli.Flag{
-					&cli.StringFlag{
-						Name:      "cid-file",
-						Usage:     "A file to read CIDs from",
-						TakesFile: true,
-					},
-				},
-			},
 		},
 	}
 

v2/cmd/car/get.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/ipfs/go-cid"
+	"github.com/ipld/go-car/v2/blockstore"
+	"github.com/urfave/cli/v2"
+)
+
+// GetCarBlock is a command to get a block out of a car
+func GetCarBlock(c *cli.Context) error {
+	if c.Args().Len() < 2 {
+		return fmt.Errorf("usage: car get-block <file.car> <block cid> [output file]")
+	}
+
+	bs, err := blockstore.OpenReadOnly(c.Args().Get(0))
+	if err != nil {
+		return err
+	}
+
+	// string to CID
+	blkCid, err := cid.Parse(c.Args().Get(1))
+	if err != nil {
+		return err
+	}
+
+	blk, err := bs.Get(blkCid)
+	if err != nil {
+		return err
+	}
+
+	outStream := os.Stdout
+	if c.Args().Len() >= 3 {
+		outStream, err = os.Create(c.Args().Get(2))
+		if err != nil {
+			return err
+		}
+		defer outStream.Close()
+	}
+
+	_, err = outStream.Write(blk.RawData())
+	return err
+}