Packing and unpacking initrd

Determine initrd format

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.

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

## ASCII SVR4 archive
# file initramfs.img                              
initramfs.img: ASCII cpio archive (SVR4 with no CRC)

Extracting initrd

Gzip

To unpack and extract initird.img using 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

ASCII cpio archive

To unpack initramfs using ASCII cpio archive that's generated by dracut, use skipcpio.

# /usr/lib/dracut/skipcpio initramfs.img | xz -d |cpio -id

Repacking initrd

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