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Context

I have a 16 GB SD card with a Linux based OS for a Raspberry Pi. Most of the space is empty.

I want to share the SD .img with other people but if I use the command

dd if=/dev/sdXX of=/home/user123/SD.img

it will create a 16 GB image. Too big.

Question

How can I re-size a 16GB SD card image into a smaller 4GB?

I have tried with GParted: it creates a partition with 4GB with no problem, however the whole .img of the SD card continues to be 16 GB with 12 GB of unallocated space.

GParted table

I have read the question and answer Cloning multiple partitions in Ubuntu, but I still cannot re-size the 16GB SD card into a 4GB one.

More info

~$ lsblk 
...

sdc      8:32   1  14,9G  0 disk 
├─sdc1   8:33   1   100M  0 part 
└─sdc2   8:34   1     4G  0 part 
~$ sudo fdisk -l /dev/sdc
Disk /dev/sdc: 14,9 GiB, 15931539456 bytes, 31116288 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: 0xf8a631ce

Device     Boot  Start     End Sectors  Size Id Type
/dev/sdc1  *      2048  206847  204800  100M  c W95 FAT32 (LBA)
/dev/sdc2       206848 8595455 8388608    4G 83 Linux

Any advice is appreciated!

Please note: as observed by Melebius in a comment, the right word to use is shrink:

You cannot resize an SD card as it is hardware with a given capacity that cannot be changed. You clearly want to shrink an SD card image.

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    have a look at resize2fs command – marosg Sep 16 '19 at 9:33
  • Possible duplicate of Cloning multiple partitions in Ubuntu – Melebius Sep 16 '19 at 9:50
  • @guiverc, thanks for pointing out it. It is true that the OS info is unnecessary for the question. – Leos313 Sep 16 '19 at 9:54
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    @Melebius, I think that cloning multiple partitions could be related with the problem of this question but, to be honest, the real problem is another one: to shrink an SD CARD (even if it has just one partition). Does it make sense? The key is the truncat command and the use of GParted. Of course we can use GParted to handle multiple partitions but the trick works also with just one. Moreover, the use of truncat is never mentioned in the other answers – Leos313 Sep 18 '19 at 17:02
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    @Melebius, thank you. However using the word "shrink" on google will guide you on many other solutions, all working perfectly. The problem, for me, was to find the right word to insert in the google search engine. I was looking for re-size that, clearly, was not the right word to use. I suggest keeping, in some way, the word 're-size' together with the right therminology. Do you agree? I have added a please note at the end of the question. – Leos313 Sep 19 '19 at 10:23
20

This article gives a solution that solves my problem. It is quite similar to the other one, but it better explains how to calculate and which meaning have the numbers and the partitions.

The key information was the use of the command truncate. Following the full solution in order to not lose the answer.

A preliminary step consists in cloning the SD card in your PC:

  1. use lsblk to see which devices are available and if their partitions are mounted

  2. unmount all partitions of the device you want to copy on your pc. For example:

    umount /dev/sdc1
    umount /dev/sdc2
    
  3. create a copy of the whole sd card with all the partitions unmounted

    dd if=/dev/sdc of=/path/to/file/myimage.img
    

Shrinking images on Linux

Context of the problem:

Having a myimage.img bigger then the hardware support (if it is smaller there should be no problem; however, using the same strategy, you can better fit the image in the hardware support).

The secret is to use standard Linux tools and instruments: GParted, fdisk and truncate.

Requirements:

  • A Linux PC
  • The .img you want to shrink (myimage.img in this example)

Creating loopback device:

GParted is an application typically used to manage partition tables and filesystems. In order to shrink the image, GParted is going to be used along the first part of the answer.

GParted operates on devices, not simple files like images. This is why we first need to create a device for the image. We do this using the loopback-functionality of Linux.

Let's enable enable the loopback:

sudo modprobe loop

Let's request a new (free) loopback device:

sudo losetup -f

The command returns the path to a free loopback device:

/dev/loop0

Let's create a device of the image:

sudo losetup /dev/loop0 myimage.img

The device /dev/loop0 represents myimage.img. We want to access the partitions that are on the image, so we need to ask the kernel to load those too:

sudo partprobe /dev/loop0

This should give us the device /dev/loop0p1, which represents the first partition in myimage.img. We do not need this device directly, but GParted requires it.

Resize partition using GParted:

Let's load the new device using GParted:

sudo gparted /dev/loop0

When the GParted application opens, it should appear a window similar to the following:

screenshot

Now notice a few things:

  • There is one partition.
  • The partition allocates the entire disk/device/image.
  • The partition is filled partly.

We want to resize this partition so that is fits its content, but not more than that.

Select the partition and click Resize/Move. A window similar to the following will pop up:

screenshot of dialog

Drag the right bar to the left as much as possible.

Note that sometimes GParted will need a few MB extra to place some filesystem-related data. You can press the up-arrow at the New size-box a few times to do so. For example, I pressed it 10 times (=10MiB) for FAT32 to work. For NTFS you might not need to at all.

Finally press Resize/Move. You will return to the GParted window. This time it will look similar to the following:

unallocated space on right

Notice that there is a part of the disk unallocated. This part of the disk will not be used by the partition, so we can shave this part off of the image later. GParted is a tool for disks, so it doesn't shrink images, only partitions, we have to do the shrinking of the image ourselves.

Press Apply in GParted. It will now move files and finally shrink the partition, so it can take a minute or two, but most of the time it finishes quickly. Afterwards close GParted.

Now we don't need the loopback-device anymore, so unload it:

sudo losetup -d /dev/loop0

Shaving the image:

Now that we have all the important data at the beginning of the image it is time to shave off that unallocated part. We will first need to know where our partition ends and where the unallocated part begins. We do this using fdisk:

fdisk -l myimage.img

Here we will see an output similar to the following:

Disk myimage.img: 6144 MB, 6144000000 bytes, 12000000 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
Disk identifier: 0x000ea37d

      Device Boot      Start         End      Blocks   Id  System
myimage.img1            2048     9181183     4589568    b  W95 FAT32

Note two things in the output:

  • The partition ends on block 9181183 (shown under End)
  • The block-size is 512 bytes (shown as sectors of 1 * 512)

We will use these numbers in the rest of the example. The block-size (512) is often the same, but the ending block (9181183) will differ for you. The numbers mean that the partition ends on byte 9181183*512 of the file. After that byte comes the unallocated-part. Only the first 9181183*512 bytes will be useful for our image.

Next we shrink the image-file to a size that can just contain the partition. For this we will use the truncate command (thanks uggla!). With the truncate command need to supply the size of the file in bytes. The last block was 9181183 and block-numbers start at 0. That means we need (9181183+1)*512 bytes. This is important, else the partition will not fit the image. So now we use truncate with the calculations:

truncate --size=$[(9181183+1)*512] myimage.img
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    The quality standards of Ask Ubuntu site implies a link only answer is not good enough. The answer should contain the essential instructions so that one can use them without referring to the original source. However, that doesn't mean one should not respect copyright laws. I suggest using your own words to describe what has to be done. – user68186 Sep 19 '19 at 11:34
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    I will rewrite the test to propose the same solution in other word. However I inserted the link and the name of the author in order to avoid every "misunderstanding". I am not the author – Leos313 Sep 19 '19 at 12:55
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    ...updating little by little :) – Leos313 Sep 19 '19 at 13:17
  • Does not really work fine, because following problem: Device Boot Start End Sectors Size Id Type xxx.img1 8192 93802 85611 41,8M c W95 FAT32 (LBA) xxx.img2 98304 30375935 30277632 14,4G 83 Linux – Micha93 Feb 10 at 13:10
  • I do not really understand. I suggest you to open a new question with the details of your error: which step, command used, the error appearing. The lines you have just shared are cryptical – Leos313 Feb 10 at 14:19
3

resize2fs can also be used to resize that.

sudo resize2fs -fp SD.img 4G

It also resizes the file itself!

2

You can make use of the options bs and count in the dd-command to limit the size of the output file.

Example:

dd if=sdx of=SD.img bs=1G count=4

would result in an outputfile with a size of 4 GiB.

Take a deep look into man dd.

You'd need to know how many bytes you have to copy so that all partitions are fully covered, so take a look with sudo fdisk -l /dev/sdx which sector is the last one you need.

The partitions need to be at the start of the disk (like in the picture you provided).

Disks with msdos-partition-table can be cloned easily this way, but if the disk uses GPT and is to be cloned to a disk with different size, the protective MBR needs to be adapted afterwords and the GPT-backup which resides at the very end of the disk needs to be recreated, this can be done with gdisk.


From your fdisk-output you can see that the last sector of the last partition is sector 8595455, that means you have to copy at least 8595455+1 sectors (first sector is 0). With a sector-size of 512 bytes this is equal to 4,400,873,472 bytes. bs multiplied with count have to be greater or equal than this.

Maybe this is still too big for a 4GB USB-stick, you can still reduce the size of sdc2, there's plenty of unused space in it.

For the current example you provided,

 dd if=/dev/sdc of=SD.img bs=10M count=420

will cover the partition-table, sdc1 and sdc2. Calculate:

10*1024*1024*420 = 4,404,019,200 > 4,400,873,472
3
  • testing the solution. One question: what do you mean with "You'd need to know how many bytes you have to copy so that all partitions are fully covered, so take a look with sudo fdisk -l /dev/sdx which sector is the last one you need. The partitions need to be at the start of the disk (like in the picture you provided)". Also I am updating the question with the output of the command you were speaking about. – Leos313 Sep 16 '19 at 10:14
  • You don't want to clone empty space, that's why the partitions should be at the start of the disk. And you don't want to clone a part of a partition (file-system), that's why you need to know on which sector the last partition on the disk ends. – mook765 Sep 16 '19 at 10:19
  • this was the solution I have founded, detailed with commands and images and it works. It is a similar answer but uses, also truncate and a loopback device. It works properly. softwarebakery.com//shrinking-images-on-linux Once also this answer is update I will vote up, it was the key to find the solution – Leos313 Sep 16 '19 at 10:35
2

The selected answer applies perfectly for dos disklabel type. For GPT type, one needs to consider adding 33 sectors as GPT stores a table also at the end of the disk.

So for GPT users, 'truncate' commands needs to look like this:

truncate --size=$[(End_of_last_partition+1+33)*512] myimage.img

This should produce a GPT error in fdisk -l. To fix this, run following:

gdisk myimage.img

Run command to verify the disk: v. You should see some errors found on the disk.

"In gdisk, you would type x to enter the experts' menu, then type e to move the backup partition table data to the new end of the disk, and then type w to write the changes to disk." [Source] how to truncate a disk image file of unused space without corrupting GPT partition table (end pointer) - Thanks!

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