Modifying an existing initrd

19/01/2020 - Ted

It is sometimes useful to change an initrd to make additional programs available during initramfs shell.

To get to initramfs shell, add break=point to the kernel boot options, where point is one of top, modules, premount, mount, mountroot, bottom, init.

Extracting the contents of the initfs into a working directory:

mkdir /tmp/initrd-root
cd /tmp/initrd-root
gunzip --stdout /boot/initrd.img-4.9.0-11-amd64 | cpio -imd --quiet

Modify the file system created however you see fit, and then re-create it.
eg. cp /sbin/resize2fs sbin/

cd /tmp/initid-root
find . | cpio  -H newc -o | gzip -9 -n > ../initrd.img-4.9.0-11-amd64

The new initrd needs to be put back in place. Keep a backup of the original just in case it's all up shit creek now!
cp -piv ../initrd.img-4.9.0-11-amd64 /boot/initrd.img-4.9.0-11-amd64

Don't forget to update-grub so that the new initrd is included.

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