3cpio

3cpio is a tool to manage initramfs cpio files for the Linux kernel. The Linux kernel's initramfs buffer format is based around the newc or crc cpio formats. Multiple cpio archives can be concatenated and the last archive can be compressed. Different compression algorithms can be used depending on what support was compiled into the Linux kernel. 3cpio is tailored to initramfs cpio files and will not gain support for other cpio formats.

As of now, 3cpio supports examining and listing the content of the initramfs cpio.

Note: The Rust crate is named threecpio, because package names are not allowed to start with numbers.

Usage examples

Examine the content of the initramfs cpio on an Ubunu 24.04 system:

$ 3cpio --examine /boot/initrd.img
0	cpio
77312	cpio
7286272	cpio
85523968	zstd

This initramfs cpio consists of three uncompressed cpio archives followed by a Zstandard-compressed cpio archive.

List the content of the initramfs cpio on an Ubunu 24.04 system:

$ 3cpio --list /boot/initrd.img
.
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/AuthenticAMD.bin
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/.enuineIntel.align.0123456789abc
kernel/x86/microcode/GenuineIntel.bin
.
usr
usr/lib
usr/lib/firmware
usr/lib/firmware/3com
usr/lib/firmware/3com/typhoon.bin.zst
[...]

The first cpio contains only the AMD microcode. The second cpio contains only the Intel microcode. The third cpio contains firmware files and kernel modules.

Quick examples comparing

Runtime comparison measured with time over five runs on different initramfs cpios:

3cpio is 88 to 274 times faster than lsinitramfs.

Commands used:

3cpio -t /boot/$file | wc -l
time 3cpio -t /boot/$file > /dev/null
time lsinitramfs /boot/$file > /dev/null