I tried to extract the initrd casper/initrd of Ubuntu 18.10 and got an unexpected result. I did not see the root filesystem and files, but just a folder named kernel.

What I have done

Firstly I tried to know if I should decompress the initrd or just extract the archive directly, so I issued this command:

$ file initrd
initrd: ASCII cpio archive (SVR4 with no CRC)

What I got

According to the output, it should be an cpio archive and I used cpio to extract the archive.

$ cpio -id < initrd 
56 blocks
$ ls
initrd  kernel

If I went to have a look of the directory kernel, I got

kernel/
└── x86
    └── microcode
        └── AuthenticAMD.bin

2 directories, 1 file

What I expect

There should be files and folders like init, etc, usr, and so on. For example:

bin  conf  cryptroot  etc  init  lib  lib64  run  sbin  scripts  usr  var