Compress / Decompress initrd files

To Decompress:

zcat initrd.gz > initrd
mount -t ext2 -o loop,rw initrd /tmp/

After the modification of the folder structure, to compress again:
gzip -c initrd > initrd.gz

Hex Edit using VI

Here are some few commands to edit an hexadecimal file using vi:

vi file.hex
#now editing in vi
[esc] :%!xxd
#now editing file.hex in hex
#find some innocuous string or rcsid
#change the values on the hex side
[esc] :%!xxd -r
[esc] :wq!
#Now have a look on the file just created
hexdump file.hex