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I have some disk image, taken with dd if=/dev/somedevice of=filename.img. I was able to shrink them following this tutorial.

Now I would like to script all the procedure, and I managed to perform almost everything, apart the fdisk resize part. I'm trying to resize the partition with this command

echo " , +7506944K," | sfdisk -N 2 /dev/loop14

But independently from the size I use I get an error:

/dev/loop14p2: Failed to resize partition #2.

How can i script the redefinition of the end of a partition? Why is my command failing, can I get some more information somehow?

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  • The pipe is an automatic way to enter what you would have otherwise typed. Would running sfdisk -N 2 /dev/loop14 and then typing from the keyboard <space>,<space>+7506944K, work? Commented May 5, 2022 at 15:00
  • Could you update with the partition table for /dev/loop14, and how will look after. Commented May 5, 2022 at 15:36
  • @EduardoTrápani, I made some test using your method, and understood that the problem is the + sign. If I simply enter the size as 7506944K, without the plus it works. This obviously works also using the pipe Commented May 6, 2022 at 7:13

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I understood what was wrong:

  • First, sfdisk accept the size of the partition, not the increment, so the + sign is wrong. One difference from fdisk is that the end is the sector number from the beginning of the partition, not from the beginning of the device.
  • Then the unit cannot be other than sectors.

So in my case, given the sector size of 512 bytes and a requested final size of approximately 7Gb , I had to launch the command as:

sudo sh -c 'echo " ,14596416" | sfdisk -N 2 /dev/loop14'
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  • Units can be other than sectors, like size, see the reduce example here. I don't know if you define the end sector or sectors from the start, I guess the latter. Commented May 6, 2022 at 12:38
  • @thanasisp have a look at the man page (man7.org/linux/man-pages/man8/sfdisk.8.html), the unit option is deprecated and states that only sector is accepted. I don't think it's clear, if is just the unit option that is deprecated or the whole usage of the size inside the commands. Anyway I was not able to resize correctly until i moved to the sector size. For the end sector yes, it's the number of sectors from the start (i'll modify the answer) Commented May 6, 2022 at 12:46
  • If that worked for you, then it seems good (also there was a trailing comma into the question, I am sure you tried some size without it). Commented May 6, 2022 at 13:07

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