From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Udvare <audvare-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Subject: Re: Minimal initramfs Modification
Date: Thu, 23 Jul 2015 13:42:24 -0700
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On 23/07/15 12:08, Rick Tillery wrote:
> I need to make a small modification to an existing initramfs image
> file (to add driver and executable that need to run before main system
> drives are mounted), but I want to do so with minimal changes to the
> existing image, in order to avoid incompatibilities with any other
> customization that may have been added to the initramfs before.
> 
> dracut seems to create initramfs from scratch, using the currently
> running environment.  But I would prefer to unarchive the image, make
> my small modification, re-archive the tree, and then replace the
> existing initramfs file (backing up the old one) to be used on the
> next reboot.  The goal is as small a change as possible from what was
> there originally.
> 
> I tried using cpio, but it doesn't handle the device directories when
> unarchiving without generating errors.  And I'm not sure how to use
> this to reconstruct the archive, especially with the compression and
> early CPIO portions restored correctly.

I have had this on bookmark just in case I need it.

http://www.thegeekstuff.com/2009/07/how-to-view-modify-and-recreate-initrd-img/