improve/document mount_offset_tool

README.md

@@ -38,6 +38,7 @@ With this image, you can run a custom script on first boot and:
 1. Source image is obtained from the official Raspberry Pi [download page](https://www.raspberrypi.org/downloads/raspbian/).
 1. Be sure to verify the SHA hash!
 1. Mount the second partition of the source image - the `mount` command will require an `--offset` flag, [as described here](https://raspberrypi.stackexchange.com/questions/13137/how-can-i-mount-a-raspberry-pi-linux-distro-image).
+   * Note: the [mount_offset_tool](mount_offset_tool/) helps calculate the `--offset` on linux.
 1. Install [firstboot.service](firstboot.service) in `/mnt/lib/systemd/system/firstboot.service`
 1. Enable firstboot.service for systemd: `cd /mnt/etc/systemd/system/multi-user.target.wants && ln -s /lib/systemd/system/firstboot.service .`
 1. Unmount the second partition of the source image.

mount_offset_tool/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"log"
 	"os"
 	"os/exec"
@@ -10,10 +11,10 @@ import (
 )
 
 func main() {
-	if err := doit(os.Args[1]); err != nil {
+	if err := getDevices(os.Args[1]); err != nil {
 		log.Fatal(err)
 	}
-	log.Printf("OK!")
+	// log.Printf("OK!")
 }
 
 type DevInfo struct {
@@ -22,7 +23,7 @@ type DevInfo struct {
 	Sectors int
 }
 
-func doit(img string) error {
+func getDevices(img string) error {
 	raw, err := exec.Command("/sbin/fdisk", "-l", "-o", "Device,Start,Sectors", img).Output()
 	if err != nil {
 		return err
@@ -54,14 +55,12 @@ func doit(img string) error {
 		}
 
 	}
-	log.Printf("unitSz = %+v\n", unitSz)
-	log.Printf("devices = %+v\n", devices)
+	// log.Printf("unitSz = %+v\n", unitSz)
+	// log.Printf("devices = %+v\n", devices)
 	// calculate and print mount cmd for img2
 	for _, d := range devices {
-		if strings.HasSuffix(d.Device, ".img2") {
-			offset := unitSz * d.Start
-			log.Printf("img2 mount cmd:\nmount -v -o offset=%d,loop %s /mnt\n", offset, img)
-		}
+		offset := unitSz * d.Start
+		fmt.Printf("%s mount command:\n  mount -v -o offset=%d,loop %s /mnt\n", d.Device, offset, img)
 	}
 
 	return nil