Makefile which extracts an alpine initrd from the arm64 rpi tarball

Extract-Alpine-Rpi-initrd.md

Extract Alpine Linux ARM64 Raspberry Pi initrd

Want to see what's in the initrd without installing it?

This will extract it for you.

It also reminds me how to deal with cpio archives, which I can never remember.

Makefile

# Makefile which extracts an alpine initrd from the arm64 rpi tarball

rpi_alpine_tarball_basename := alpine-rpi-3.12.1-aarch64
rpi_alpine_extractdir := $(rpi_alpine_tarball_basename)
rpi_alpine_tarball := $(rpi_alpine_tarball_basename).tar.gz
rpi_alpine_tarball_uri_prefix := http://dl-cdn.alpinelinux.org/alpine/v3.12/releases/aarch64
rpi_alpine_tarball_uri := $(rpi_alpine_tarball_uri_prefix)/$(rpi_alpine_tarball)
initramfs_compressed := $(rpi_alpine_extractdir)/boot/initramfs-rpi4
initramfscpio := initramfs-rpi4.cpio
initramfsdir := initramfs
initramfs_extracted_init := $(initramfsdir)/init

.PHONY: cleanextracted

$(initramfs_extracted_init): $(initramfscpio) | $(initramfsdir)
	cd $(initramfsdir); cpio -vid < ../$(initramfscpio)
	# Touch it because it'll extract it with an old timestamp
	touch $(initramfs_extracted_init)

$(initramfsdir):
	mkdir $(initramfsdir)

$(initramfscpio): $(initramfs_compressed) | $(rpi_alpine_extractdir)
	zcat < $(initramfs_compressed) > initramfs-rpi4.cpio

$(initramfs_compressed): $(rpi_alpine_tarball) | $(rpi_alpine_extractdir)
	cd $(rpi_alpine_extractdir); tar -zxf ../$(rpi_alpine_tarball)
	# Touch it because it'll extract it with an old timestamp
	touch $(initramfs_compressed)

$(rpi_alpine_extractdir):
	mkdir $(rpi_alpine_extractdir)

$(rpi_alpine_tarball):
	curl -o $(rpi_alpine_tarball) $(rpi_alpine_tarball_uri)

cleanextracted:
	rm -rf $(rpi_alpine_extractdir) $(initramfscpio) $(initramfsdir)