How to extract an initrd image

Well, I had this often repeated problem of searching for how to extract an initrd image only to find that it is basically as cpio+gzip archive but, over a period of time forgetting it so that I come back searching for this again:-) Finally, I made up my mind to put in what all it takes to extract an initrd image in Linux and re-packing it after customization (not covered here, as customization depends on the need), here.

Unpacking the initrd image:

1. Create an empty directory /tmp/initrd (this is an example; you are free to use any other directory name) and cd into it, using the following command

mkdir -p /tmp/initrd; cd /tmp/initrd;

(Replace /tmp/initrd with your directory name, if using a different directory name)

2. Extract the initrd image using the following command

gunzip absolute-path-to-initrd-image | cpio -i —make-directories

Now, the contents of the initrd image will be there in /tmp/initrd (or whatever directory you have used). Go ahead and do the necessary changes and you may follow the steps below to re-pack it into an initrd image again.

Packing the (custom) initrd image:

1. Get into the directory where you have extracted the initrd image (/tmp/initrd in this example), using the command

cd /tmp/initrd

(Replace /tmp/initrd with your directory name, if using a different directory name)

2. Pack the initrd image by using the command

find ./ | cpio -H newc -o > /tmp/initrd.cpio && gzip /tmp/initrd.cpio && mv /tmp/initrd.cpio.gz /tmp/initrd.img

(Replace /tmp/initrd with your directory name, if using a different directory name)

This will get you the (re)packed initrd image as /tmp/initrd.img (in this example). Now your custom initrd image is ready.