Linux - Kernel Version

Source

# Extract

 gzip -dc ../initrd.gz | cpio -idmv --no-absolute-filenames

# Rebuild

 find . | cpio -o -H newc | gzip -9 > ../initrd.gz

XZ Base (Redhat)

# Extract

 xz -dc ../initrd.img | cpio -idmv --no-absolute-filenames

# Rebuild

 find . | cpio -o -H newc | xz -9 > ../initrd.img

Other Distros