How to View, Modify and Recreate initrd.img

by Sasikala on July 10, 2009

Question: How do I view, modify and recreate the new initrd.img on Ubuntu, Debian, CentOS, Fedora, Red-Hat, Arch Linux, or SUSE distributions?

1. How To View Content Of initrd.img file?

initrd.img is in gzip format.  So move initrd.img to initrd.gz as shown below.

# cp /tftpboot/el5/initrd.img  .

# ls
cdrom   initrd.img

# mv initrd.img initrd.gz

Unzip the initrd.gz file as shown below.

# gunzip initrd.gz

# ls
cdrom  initrd

After unziping the initrd.gz file, the initrd is further in cpio ‘newc’ format. So extract the files from initrd using cpio ‘newc’ format as shown below.
Note: info cpio will give more information about ‘newc’ format.

# mkdir tmp2

# cd tmp2/

# cpio -id < ../initrd
16524 blocks

Now you can view the content of initrd.img file

# ls
bin  dev  etc  init  modules proc  sbin  selinux  sys  tmp  var

2. How To Modify Content of Image and Recreate New Image?

After extracting the file as shown below, make appropriate modification to any of those files. Then pack the files back into the archive using the following commands. Pack the modified files back to cpio ‘newc’ format.

# find . | cpio --create --format='newc' > /tmp/newinitrd
16524 blocks

# ls /tmp/
cdrom  initrd  newinitrd  tmp2

# ls -l /tmp/newinitrd
-rw-r--r-- 1 root root 8460288 Jul  2 14:50 /tmp/newinitrd

Gzip the archive file.

# gzip newinitrd

# ls
cdrom  initrd  newinitrd.gz  tmp2

# ls -l newinitrd.gz
-rw-r--r--  1 root root 6649867 Jul  2 14:50 newinitrd.gz

Move file as an image file. You can use the newinitrd.img as your new boot image.

# mv newinitrd.gz newinitrd.img

# ls -l newinitrd.img
-rw-r--r-- 1 root root 6649867 Jul  2 14:50 newinitrd.img

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{ 19 comments… read them below or add one }

1 Lakshmanan July 10, 2009 at 3:06 am

Nice one…

2 runlevel0 July 10, 2009 at 8:23 am

Neat!

I have always done my initrd’s using Debian’s make-kpgk utils… but this hack is real neat!

Kudos for your site!

3 beparas July 10, 2009 at 11:15 pm

This is very useful.

4 Ramesh Natarajan July 14, 2009 at 10:13 pm

@Lakshmanan, @beparas, Thanks for your comments. I’m very glad that you found this helpful.

@runlevel0, Building kernel using make-kpkg by installing ‘kernel-package’ and ‘fakeroot’ packages definitely has it’s own advantages. Thanks for bringing make-kpkg utility to our attention.

5 lenny September 7, 2009 at 1:14 am

hello!
I want to know if I recreate new image file after modify something.using:
dd if=tmp2 of=newinitrd.img
I want to know It’s right or wrong?why?

6 Balakrishnan Mariyappan June 7, 2010 at 1:14 pm

very good article..
thanks.

7 greyzlii August 12, 2010 at 6:14 am

Thank you so much for this article !

8 Coogrrr October 8, 2010 at 9:02 am

Thank you soo much!!!

This was the ticket I could modify the grub and logon screens but the boot splash that was the hard one. Not any more! I have my own theme from end to end now!

YOU ROCK!!!!!

Coog

9 CharlesS January 20, 2011 at 11:50 am

Thanks for the article. I tend to have a use for this about once or twice a year.

The extraction can be merged into a single command, simplifying this a bit:

mkdir /tmp/initrd; cd /tmp/initrd
gunzip -c ../newinitrd.img

10 David November 18, 2011 at 1:54 pm

Hi fellas, just wanted to say I appreciate this post. I was able to prevent the loading of modules I didn’t want by changing the file under /conf in the archive. I found it easier to copy the thing to my home directory and then do all the stuff. About to see if I can use XZ to compress the image and make it take up less space ( I know it’s pointless but I don’t care :-) ). Thank you for the post. I now have more useful tools :)
-Dave

11 Petri Asikainen February 21, 2012 at 2:30 am

Thanks! This saved my day.
Just fixed one virtual machine with this information. Copied initrd from other virtual machine and then edited initrd to fit in vm configuration. Needed to edit run_all.sh and config/* scripts and replace rootdevice information and filesystem type and added correct fsck.

12 Deepak Mohanty March 16, 2012 at 2:41 am

Thanks. I wanted to inspect the contents of an initrd image and I found most of the articles on the web misleading / partially correct. Most of them either assume the image to be a gzipped image, or a cpio file or a plain image. The fact is that (at least on my Linux distro), it is a gzipped cpio archive. Your article was helpful.

13 Deepak Mohanty March 16, 2012 at 2:44 am

Another way I could mount the image is using sysfs

ex.: mount -t sysfs initrd-2.6.18-1.2798.fc6.img drtini

14 Deepak Mohanty March 19, 2012 at 10:57 am

A lot of information about initrd/initramfs is available at http://en.wikipedia.org/wiki/Initrd. This article explains the old initrd format (compressed image) and the newer initramfs format (cpio archive – optionally compressed).

15 Aniket July 24, 2012 at 7:30 pm

Nice one :)

16 Vipul September 3, 2013 at 1:59 am

Hello everybody,
can anyone tell me how to modify the boot image and resave the cpio file
for changing the plymouth theme (custom) in rhel6

17 Chris September 11, 2013 at 4:09 am

Modern linuxes (eg: CentOS 6.4) use a new compression format – here’s how I just added a new driver module:

[root@z600 ~]# mkdir tmo
[root@z600 ~]# cd tmo
[root@z600 tmo]# cat /media/LIVE/syslinux/initrd.img | xz –format=lzma –decompress –stdout | cpio -id
147921 blocks
[root@z600 tmo]# cp /media/LIVE/ocz10xx.ko modules/2.6.32-358.el6.x86_64/extra/
[root@z600 tmo]# find . | cpio –create –format=’newc’ | xz –format=lzma –compress –stdout >/tmp/newinitrd.img
153313 blocks
[root@z600 tmo]# ls -lsa /media/LIVE/syslinux/initrd.img /media/LIVE/ocz10xx.ko /tmp/newinitrd.img
2696 -rwxr-xr-x. 1 cnd cnd 2760604 Dec 11 2012 /media/LIVE/ocz10xx.ko
31732 -rwxr-xr-x. 1 cnd cnd 32491856 Sep 11 19:13 /media/LIVE/syslinux/initrd.img
32764 -rw-r–r–. 1 root root 33546904 Sep 11 20:05 /tmp/newinitrd.img
[root@z600 tmo]# cp /media/LIVE/syslinux/initrd.img /tmp/
[root@z600 tmo]# cp /tmp/newinitrd.img /media/LIVE/syslinux/initrd.img
cp: overwrite `/media/LIVE/syslinux/initrd.img’? y
[root@z600 tmo]# reboot

18 sanjeev September 16, 2013 at 5:55 am

I am not getting the dev directory after cpio option
# cpio -id < ../initrd
92370 blocks
when Iview the content of initrd.img file

# ls
bin dev(missing) etc init modules proc sbin selinux sys tmp var(missing)

also I am getting cpio : cannot symlink to Readonly File System

19 Vipul September 20, 2013 at 12:43 am

can anyone tell me how to modify the boot image and resave the cpio file
for changing the plymouth theme (custom) in rhel6..
thanks in advance

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