makecore

makecore is a Bash-based build script that creates a tiny bootable Linux ISO for x86_64.

It automates:

• Linux kernel download/build
• Optional glibc build (for dynamic BusyBox)
• BusyBox build (static by default)
• Root filesystem + initramfs generation
• Optional ./overlay/ merge into the rootfs
• Bootable ISO creation with Syslinux/Isolinux
• SHA256 verification of source archives
• Build summary (stage times + artefact sizes)
• Optional QEMU run and headless smoke test
• Network (virtio-net + DHCP) and optional Dropbear SSH

Repository layout

Path	Role
script.sh	Main entry point
config	Build configuration (URLs + options)
checksums.sha256	SHA256 pins for source archives
kernel.config	Minimal kvm-oriented kernel config (Linux 6.12.x)
busybox.config	Minimal static BusyBox config (versioned)
overlay/	Optional rootfs overlay (OVERLAY_TYPE=folder)
sources/	Optional local source archives (offline mode)
ntmkstdlib.sh	Logging / error helpers

Prerequisites

• bash, make, gcc, tar, cpio, gzip, sed, find, wget, sha256sum

• One ISO tool: xorriso or mkisofs or genisoimage

• qemu-system-x86_64 (for run and test)

• Static C library for default static BusyBox: glibc-static (Fedora/RHEL) or libc6-dev with static libs (Debian/Ubuntu). Without it the build fails with a clear message; alternative: USE_GLIBC=true for a dynamic BusyBox. Notes:

• USE_LOCAL_SOURCE=true looks for archives under sources/ first.

• Exact basename from the URL is preferred; otherwise a component-*.tar.* match is used.

• If nothing is local, the script downloads from the configured URL.

• With VERIFY_CHECKSUMS=true, archives are checked against checksums.sha256.

Quick start

chmod +x script.sh
./script.sh all
./script.sh test    # headless QEMU smoke test

Outputs:

• ISO: ~/work/tmp/out/minidistro.iso (override with WORK_DIR / ISO_NAME)
• Summary: ~/work/tmp/out/build-summary.txt
• Workspace: ~/work/tmp by default

Interactive QEMU (serial console; quit with Ctrl-A then X):

./script.sh run

Commands

./script.sh all              # kernel → (glibc) → busybox → initramfs → ISO + summary
./script.sh kernel           # kernel only
./script.sh glibc            # glibc only (needs USE_GLIBC=true)
./script.sh busybox          # BusyBox only
./script.sh initramfs        # rootfs + initramfs only
./script.sh iso              # ISO stage (needs kernel + initramfs)
./script.sh dropbear         # Dropbear only (ENABLE_DROPBEAR=true)
./script.sh menuconfig       # interactive kernel config → kernel.config
./script.sh busybox-config   # interactive BusyBox config → busybox.config
./script.sh run              # QEMU BIOS + virtio-net (+ SSH hostfwd)
./script.sh run-uefi         # QEMU UEFI (OVMF)
./script.sh test             # BIOS smoke (MAKECORE_BOOT_OK)
./script.sh test-net         # BIOS + DHCP (MAKECORE_NET_OK)
./script.sh test-uefi        # UEFI smoke (OVMF)
./script.sh test-uefi-net    # UEFI + DHCP
./script.sh summary          # print last build sizes / stage times
./script.sh clean            # remove WORK_DIR
./script.sh help

Options:

./script.sh --force all       # ignore stamps, rebuild
./script.sh -f busybox        # force BusyBox rebuild only

Phase D: UEFI hybrid boot

The ISO is BIOS + UEFI hybrid:

Mode	Bootloader	How
BIOS	Isolinux	El Torito + isohybrid MBR
UEFI	GRUB BOOTX64.EFI	El Torito ESP (EFI/efiboot.img) + GPT basdat

Kernel has CONFIG_EFI + CONFIG_EFI_STUB (+ ACPI). The ESP embeds kernel+initrd so UEFI does not depend on ISO9660 path search.

./script.sh all
./script.sh test          # BIOS
./script.sh test-uefi     # OVMF UEFI
./script.sh run-uefi      # interactive UEFI

Host packages for UEFI build/test: xorriso, grub2-efi-x64-modules, grub2-tools, mtools, dosfstools, edk2-ovmf (or ovmf).

Write to USB (hybrid ISO)

# CAUTION: destroys all data on the target device
sudo dd if=~/work/tmp/out/minidistro.iso of=/dev/sdX bs=4M status=progress oflag=sync
# or:
sudo cp ~/work/tmp/out/minidistro.iso /dev/sdX && sync

Then boot the USB stick in either legacy BIOS or UEFI mode (disable Secure Boot if GRUB is unsigned — this image is not signed).

Variable	Default	Meaning
ENABLE_UEFI	true	Build hybrid BIOS+UEFI ISO
UEFI_ESP_SIZE_KB	auto (≥8MiB)	Minimum ESP size override
OVMF_CODE / OVMF_VARS	auto	QEMU firmware paths
TEST_UEFI_TIMEOUT	240	UEFI smoke timeout (s)

Phase C: network & services

Feature	How
Virtio-net in QEMU	./script.sh run / test attach -device virtio-net-pci + user net
DHCP	/init runs udhcpc → marker MAKECORE_NET_OK
Dropbear SSH	Static build; host keys baked at build; password from ROOT_PASSWORD
Overlay examples	overlay/etc/motd, issue, root/.profile, …

./script.sh all && ./script.sh test-net
./script.sh run
# other terminal:
ssh -p 2222 root@127.0.0.1   # password: makecore (default)

Variable	Default	Meaning
ENABLE_NETWORK	true	Bring up lo + first NIC
ENABLE_DHCP	true	udhcpc on that NIC
ENABLE_DROPBEAR	true	Build/install dropbear
ROOT_PASSWORD	makecore	root shadow password
QEMU_SSH_PORT	2222	Host port → guest :22
DROPBEAR_SOURCE_URL	dropbear-2025.88	Source tarball

Disable SSH to shrink the image: ENABLE_DROPBEAR=false then rebuild.

Phase B: minimalism

Versioned configs tuned for a small QEMU-friendly image:

Artefact	Phase A (defconfig-ish)	Phase B (minimal)
Kernel bzImage	~14M	~2.7M
BusyBox	~2.4M	~2.0M
Initramfs	~1.3M	~1.0M
ISO	~16M	~4.2M
Full build (16 cores)	~5+ min	~2.5 min

• kernel.config: allnoconfig base + initrd, serial, virtio, networking, ext4/squashfs/overlay; CONFIG_CC_OPTIMIZE_FOR_SIZE, no modules.
• busybox.config: static, ash, coreutils, mount, ip/udhcpc/wget, vi/awk — servers and niche applets off.
• USE_PREDEFINED_BUSYBOX_CONFIG=true by default.
• Iterate configs: ./script.sh menuconfig / ./script.sh busybox-config, then ./script.sh --force all && ./script.sh test.

Phase A: reliability features

Checksums

Pinned SHA256 digests live in checksums.sha256 (standard sha256sum format).

Variable	Default	Meaning
VERIFY_CHECKSUMS	true	Hash each archive after resolve/download
CHECKSUMS_REQUIRED	false	If true, fail when an archive has no entry
CHECKSUMS_FILE	checksums.sha256	Path to the pin file

Manual check:

cd sources && sha256sum -c ../checksums.sha256

Build summary

After all (and several single stages), the script prints and saves:

• Per-stage durations (kernel, glibc, busybox, initramfs, iso)
• Artefact sizes (kernel, busybox, initramfs, ISO)

File: $WORK_DIR/out/build-summary.txt — also via ./script.sh summary.

Smoke test

./script.sh test

Boots the ISO headless in QEMU (-display none -serial file:…), waits for the marker string emitted by /init:

MAKECORE_BOOT_OK

Variable	Default	Meaning
TEST_TIMEOUT	180	Seconds before failure
TEST_MARKER	MAKECORE_BOOT_OK	Success string on serial
QEMU_MEM	256	Guest RAM (MiB)

Logs: $WORK_DIR/test/serial.log, last-result.txt, qemu.stderr.
Uses KVM when /dev/kvm is available.

Configuration (config)

Variable	Meaning
KERNEL_SOURCE_URL	Linux kernel tarball URL
GLIBC_SOURCE_URL	glibc tarball URL
BUSYBOX_SOURCE_URL	BusyBox tarball URL
SYSLINUX_SOURCE_URL	Syslinux tarball URL
WORK_DIR	Build workspace (default ~/work/tmp)
ISO_NAME	Output ISO filename
USE_PREDEFINED_KERNEL_CONFIG	true → use kernel.config, else x86_64_defconfig
USE_PREDEFINED_BUSYBOX_CONFIG	true → use busybox.config if present
USE_GLIBC	Dynamic BusyBox + glibc runtime in rootfs
USE_LOCAL_SOURCE	Prefer ./sources/ archives
OVERLAY_TYPE	none or folder (copy ./overlay/)
JOB_FACTOR	jobs = cores * JOB_FACTOR
CFLAGS	BusyBox compile flags
KERNEL_CMDLINE	Appended by isolinux (default dual console)
QEMU_MEM	RAM in MiB for run / test
VERIFY_CHECKSUMS	Enable SHA256 checks
CHECKSUMS_REQUIRED	Fail on missing checksum entries
CHECKSUMS_FILE	Checksums path
TEST_TIMEOUT	Smoke test timeout (s)
TEST_MARKER	Smoke test success string

Incremental builds (stamps)

Each stage writes a stamp under $WORK_DIR/stamps/ that embeds a fingerprint of its inputs. Changing those inputs invalidates the stamp. Downstream stages are also invalidated when an upstream stage is rebuilt. Use --force to rebuild regardless.

Overlay

1. Put files under overlay/ mirroring the rootfs tree.
2. Set OVERLAY_TYPE=folder in config.
3. Rebuild initramfs / all.

Boot behaviour

The initramfs /init mounts proc, sys, dev, tmp, run, sets hostname, prints a banner, emits MAKECORE_BOOT_OK, then starts a shell.

Default kernel cmdline includes serial + VGA (console=tty0 console=ttyS0).

Troubleshooting

Problem	What to try
iso fails (missing kernel/initramfs)	Run ./script.sh all first
ISO tool missing	Install xorriso, mkisofs, or genisoimage
Download fails / offline	Put archives in sources/ or set USE_LOCAL_SOURCE=false
SHA256 mismatch	Re-download archive or update checksums.sha256
Smoke test timeout	Raise TEST_TIMEOUT; inspect $WORK_DIR/test/serial.log
Smoke test: QEMU dies early	Check qemu.stderr; ensure ISO boots under BIOS
Slow build	Raise JOB_FACTOR
Stamp seems stuck	./script.sh --force <stage> or ./script.sh clean