Building your custom Raspbian image

I recently had to prepare a Raspberry Pi for a client, but it would take too long to send the Pi itself. I had to come with a way to prepare the Pi locally, create an image and send this image electronically.

The problem with creating an image of an SD card, is that it’s size is the size of the SD card. So if you take an image of 32 GB SD card, it has to be flashed back to a 32 GB SD card. That’s something you want to avoid.

In this blog post, I will explain how to prepare a Raspbian image on the Pi and turn it into a distributable image using Mac OS.

Photo by Hector Bermudez on Unsplash

First, start by removing the following line from the /boot/cmdline.txt file:

init=/usr/lib/raspi-config/init_resize.sh

This will prevent the Pi from expanding its file system upon boot. We need this removed, because we want to keep our partition as small as possible.

Next, boot from the SD card as you normally would. Install all the applications you want.

When your done, put your SD card back in to your Mac. Next, put the part your just removed from /boot/cmdline.txt back (or restore a backup if you took one).

Next, open the Terminal application and list your drives with

diskutil list

You will probably recognise your SD card as /dev/disk2:

/dev/disk2 (internal, physical):
   #:                       TYPE NAME    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme        *128.0 GB   disk2
   1:             Windows_FAT_32 boot    45.0 MB    disk2s1
   2:                      Linux         1.8 GB     disk2s2

Find the block size with diskutil info /dev/disk2:

$ diskutil info disk2
Device Identifier:        disk2
Device Node:              /dev/disk2
Whole:                    Yes
Part of Whole:            disk2
Device / Media Name:      SD Card Reader
Volume Name:              Not applicable (no file system)
Mounted:                  Not applicable (no file system)
File System:              None
Content (IOContent):      FDisk_partition_scheme
OS Can Be Installed:      No
Media Type:               Generic
Protocol:                 USB
SMART Status:             Not Supported
Disk Size:                128.0 GB (128043712512 Bytes) (exactly 250085376 512-Byte-Units)
Device Block Size:        512 Bytes
Read-Only Media:          No
Read-Only Volume:         Not applicable (no file system)
Device Location:          Internal
Removable Media:          Removable
Media Removal:            Software-Activated
Virtual:                  No

Next, find the used size of the SD card’s partitions with sudo fdisk /dev/disk2:

$ sudo fdisk /dev/disk2
Disk: /dev/disk2  geometry: 15567/255/63 [250085376 sectors]
Signature: 0xAA55
         Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [start -       size]
-------------------------------------------------------------------
 1: 0C    0 130   3 -    5 249  31 [ 8192 -      87851] Win95 FAT32L
 2: 83    6  30  25 -  219  68  41 [98304 -    3424256] Linux files*
 3: 00    0   0   0 -    0   0   0 [    0 -          0] unused      
 4: 00    0   0   0 -    0   0   0 [    0 -          0] unused

I our case, the last partition starts at block 98304 and is 3424256 blocks big. That means the last used block of our SD card is 3522560 (98304+3424256).

To copy only the used part of our SD card to a file, use dd:

sudo dd if=/dev/disk2 of=image.img bs=512 count=3522561

Note that we added 1 to the last used block to be on the safe side. Set bs to the block size you found earlier.

You should now have a small image (image.img) which you can further zip:

zip -r image.zip image.img

In my case, this resulted in a file of 483 MB, similar to the size of original Raspbian image.

That’s it. Now you can easily flash the image to any SD card larger than 483 MB (which equals 99% of the SD cards in the world I guess).