In the Debian example, the wiki introduced NetInstall. Later I find out the mk_mmc.sh script shown there to be useful for (1) duplicating and (2) restoring working Linux backup to new or corrupted MMC/SD card. I break down that Robert C. Nelson’s mk_mmc.sh script at GitHub to small routines for setting up the card.

Typical boot-rootfs partitions of SD card to boot ARM-Linux boards

It’s an elegant script automating the process of preparing SD card for embedded Linux, but requires large downloading on the run (mostly root file system and then U-Boot binaries and loader config). Duplicating/restoring means we already have those and probably only need to do elementary process rather than full card preparation. Below are some processes that can be repeatedly used.

Checking Tool Versions

The tools to work with are common in Linux distribution (I used Ubuntu) and require root privilege. The script checked for fdisk and parted installed. My versions are

$ fdisk -v
fdisk (util-linux-ng 2.17.2)
$ parted -v
parted (GNU parted) 2.2

Originally mk_mmc.sh will exit If “GNU Fdisk” found instead. Also for the above parted version, “--align cylinder” argument will be used.

Creating Two Partitions

Setting up two partitions which are the boot image (FAT 16) and root file system (ext4). The SD card plugged in through USB card reader found as /dev/sdb in this example (we’re formatting be sure of this!). If you plug MMC card to laptop slot, it may be found as /dev/mmcblk0.

1. Format card into a single disk

$ parted --script /dev/sdb mklabel msdos
$ fdisk -l /dev/sdb

2. Create the boot partition (allocating 64MB)

$ fdisk /dev/sdb << END
n
p
1
1
+64M
t
e
p
w
END

then make sure it’s done.

$ sync

3. Flag it as boot

$ parted --script /dev/sdb set 1 boot on

4. Create the root file system by first determining space left after the first partition

$ END_BOOT=$(LC_ALL=C parted -s /dev/sdb unit mb print free | grep primary | awk '{print $3}' | cut -d "M" -f1)
$ echo $END_BOOT 
69.7
$ END_DEVICE=$(LC_ALL=C parted -s /dev/sdb unit mb print free | grep Free | tail -n 1 | awk '{print $2}' | cut -d "M" -f1)
$ echo $END_DEVICE 
3951

In this case 4GB SD card is used and the above calculation will stretch space left from 69.7 to 3951 of as rootfs partition.

$ parted --script --align cylinder /dev/sdb mkpart primary ext4 $END_BOOT $END_DEVICE

5. Format boot and rootfs according to each file system type with those naming also. They’re already accessible as /dev/sdb1 and /dev/sdb2 (previously didn’t exist) or relevant /dev/mmcblk0p(some index).

$ mkfs.vfat -F 16 /dev/sdb1 -n boot
$ mkfs.ext4 /dev/sdb2 -L rootfs

6. Finally we have something like the output of parted below

$ LC_ALL=C parted -s /dev/sdb unit mb print free 
Model: HUAWEI MMC Storage (scsi)
Disk /dev/sdb: 4023MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
 
Number  Start   End     Size    Type     File system  Flags
 1      0.03MB  70.9MB  70.8MB  primary  fat16        boot, lba
        70.9MB  4023MB  3953MB           Free Space

(yes I used Huawei modem as the microSD card reader)

Restoring Linux

For the boot partition content we can simply copy-paste the backup files, while for rootfs normally compressed file (preserving path information) is used for backup. Restore this by mounting rootfs and then unpacking the file (again /dev/sdb2 in this case).

[root@host]$ mkdir /media/rootfs
[root@host]$ mount -t ext4 /dev/sdb2 /media/rootfs
[root@host]$ pv armel-rootfs-201110131018.tar | sudo tar --numeric-owner --preserve-permissions -xf - -C /media/rootfs

Below is an example of how to backup rootfs from the mounted SD card (/media/rootfs).

[root@host]$ cd /media/rootfs
[root@host]$ /media/rootfs# tar --numeric-owner --preserve-permissions -cvf /home/arif/Documents/arm-linux/ubuntu/rootstock-rootfs/armel-rootfs-201202141421.tar ./*

(Check my other post related to creating rootfs)

Creating Swap

mk_mmc.sh also offers optional swap creation. It is a “file” inside rootfs partition created by

[root@host]$ dd if=/dev/zero of=/media/rootfs/SWAP.swap bs=1M count=250
[root@host]$ mkswap /media/rootfs/SWAP.swap

(the above will create 262MB swap inside rootfs SD card currently mounted to /media/rootfs of the laptop)

It will be mounted as swap by the ARM-Linux board via fstab setting configured below.

[root@host]$ echo "/SWAP.swap  none  swap  sw  0 0" >> /media/rootfs/etc/fstab

Synchronize and safely remove the card

[root@host]$ sync
[root@host]$ umount /media/rootfs