Modify raw image file in Linux
Raw img VS ISO img
Similarity
- They are both byte by byte copy of block device.
Difference
-
ISO images have no partition table. They are read only.
-
Raw images have partition tables and they are read/write.
Modify Raw Images
In this section we talk about how to modify raw images.
Mount the image
First we have to mount the image to poke around its content. In a GUI you might just double-click the image file. This would normally mount the image in read-only mode which means you can check its content, but there’s not much more you can do about the content.
Instead you should mount it as read/write mode, which is the default mode of mount. But before any mount command, you should figure out which partition you want to mount since there are several partitions in that image. To do that you can use the fdisk command
fdisk -l raw_img_file.img
A sample output could look like
raw_img_file.img: 128 MiB, 134217728 bytes, 262144 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xf110ee87
Device Boot Start End Sectors Size Id Type
raw_img_file.img1 2048 100351 98304 48M 83 Linux
raw_img_file.img2 100352 253951 153600 75M 83 Linux
raw_img_file.img3 253952 262143 8192 4M 83 Linux
As we can see, firstly, it says that each sector is 512 bytes long (Sector size (logical/physical): 512 bytes / 512 bytes ), then that there’s a mountable partition that starts on sector 2048. If you want to mount that partition, you’ll have to pass that information along
mount -o loop,offset=1048576 raw_img_file.img /path/to/img/mount/point
Please check the mannual page of mount for details about these parameters.
Dismount the image
After done modifying the content, dismount the image with
umount /path/to/img/mount/point
And then you can copy it to a USB flash drive with dd
dd bs=1M if=raw_img_file.img of=/dev/sdX
where sdX is wherever your system maps your USB driver.
Modify ISO Images
For an ISO image, you have to copy its content out, edit as you wish, then re-burn the whole image.
Reference
https://www.linux.com/news/how-modify-raw-disk-image-your-custom-linux-distro/
Additional information
Add space to the image
This section is just copied from a web post. I haven’t tried it yet since the tool it uses works for ext2/3/4 file systems, not the one my image file is using.
You might find there’s not many space left on the img file for your modification
df -h /path/to/img/mount/point
Filesystem Size Used Avail Use% Mounted on
/dev/loop0 3.1G 2.7G 265M 92%
You have to make the mounted partition bigger and this is where things get sticky.
Use parted to check some basic info about this image
# parted ArduinoOS_Redux.x86_64-0.0.XX.raw
GNU Parted 3.2
Using /home/[your user]/ArduinoOS_Redux.x86_64-0.0.XX.raw
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print free
Model: (file)
Disk /home/[your user]/ArduinoOS_Redux.x86_64-0.0.XX.raw: 3427MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
32.3kB 1049kB 1016kB Free Space
1 1049kB 3427MB 3426MB primary ext3 boot, type=83
The ~1MB free space at the beginning is actually the partition info. We have to add space at the end.
dd if=/dev/zero bs=1M count=512 >> ArduinoOS_Redux.x86_64-0.0.XX.raw
Then again in parted
# parted ArduinoOS_Redux.x86_64-0.0.XX.raw
GNU Parted 3.2
Using /home/[your user]/ArduinoOS_Redux.x86_64-0.0.XX.raw
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print free
Model: (file)
Disk /home/[your user]/ArduinoOS_Redux.x86_64-0.0.XX.raw: 3964MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
32.3kB 1049kB 1016kB Free Space
1 1049kB 3427MB 3426MB primary ext3 boot, type=83
3427MB 3964MB 537MB Free Space
(parted) resizepart 1 3964MB
(parted) print free
Model: (file)
Disk /home/[your user]/ArduinoOS_Redux.x86_64-0.0.XX.raw: 3964MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:
Number Start End Size Type File system Flags
32.3kB 1049kB 1016kB Free Space
1 1049kB 3964MB 3963MB primary ext3 boot, type=83
parted‘s resizepart instruction takes two parameters: the partition to resize — in this case 1 — and the new end of the partition. As you saw two listings up, the empty space you created ends at the 3964MB mark, so you use that with resizepart.
But, resizepart works fine with real disks, for raw image, this is not enough. Therefore comes the kpartx tool. It take a raw image of a disc and create a “virtual device” in /dev/mapper that can then be manipulated as a real device with real partitions:
# kpartx -a ArduinoOS_Redux.x86_64-0.0.XX.raw
The -a option adds an entry, so if you now look under /dev/mapper:
$ ls /dev/mapper/
control loop0p1
We first check the virtual partition:
# e2fsck -f -y -v -C 0 /dev/mapper/loop0p1
e2fsck 1.42.13 (17-May-2015)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
86010 inodes used (41.10%, out of 209248)
5413 non-contiguous files (6.3%)
83 non-contiguous directories (0.1%)
# of inodes with ind/dind/tind blocks: 5404/51/0
726741 blocks used (86.89%, out of 836352)
0 bad blocks
1 large file
69360 regular files
8272 directories
7 character device files
4 block device files
0 fifos
7797 links
8358 symbolic links (8166 fast symbolic links)
0 sockets
------------
93798 files
Then we try to resizing the file system
# resize2fs -p /dev/mapper/loop0p1
resize2fs 1.42.13 (17-May-2015)
Resizing the filesystem on /dev/mapper/loop0p1 to 967424 (4k) blocks.
Begin pass 1 (max = 4)
Extending the inode table XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
The filesystem on /dev/mapper/loop0p1 is now 967424 (4k) blocks long.
We can now delete the virtual device
# kpartx -d ArduinoOS_Redux.x86_64-0.0.XX.raw
