Packing and unpacking initrd

To modify initird.img or other similar images, first determine the compression method used by the initrd file. Below, we see that it's using gzip which is typical.

# file initrd.img
initramfs.gz: gzip compressed data, last modified: Thu Jul 22 04:16:52 2021, max compression, from Unix, original size 46489600

To unpack and extract initird.img, compressed with gzip:

## Go to an empty directory before extracting the image.
# mkdir -p /mnt/initrd_extract && cd /mnt/initrd_extract
# gzip -dc < /path/to/initrd.img | cpio -i

To re-pack files back into initrd.img after making your changes:

# Go to the directory containing the contents of the image.
# cd /mnt/initrd_extract
# find . | cpio -o -c | gzip -9 > /path/to/initrd.img