End-to-End Pipeline

Tracing bitbake core-image-minimal from invocation to flashable image:

bitbake core-image-minimal
        │
        ├── PHASE 1: Parse conf/
        │         ├── bitbake.conf       (default vars, paths, tool flags)
        │         ├── layer.conf files   (each layer registers its BBFILES)
        │         └── local.conf         (MACHINE, DISTRO, user overrides)
        │
        ├── PHASE 2: Parse all .bb + .bbappend + .bbclass files
        │         └── Build per-recipe DataStore for every recipe in BBFILES
        │
        ├── PHASE 3: Resolve target "core-image-minimal"
        │         └── IMAGE_INSTALL → package list
        │              └── DEPENDS → build-time dep tree per package
        │                  └── RDEPENDS → runtime dep tree
        │
        ├── PHASE 4: Task Graph Construction
        │         └── For every recipe × task combination:
        │              do_fetch → do_unpack → do_patch → do_configure
        │              → do_compile → do_install → do_package
        │              → do_package_write_* → do_rootfs → do_image
        │
        ├── PHASE 5: Hash Computation + sstate Lookup
        │         ├── Cache hit  → do_setscene (restore in seconds)
        │         └── Cache miss → execute task, populate sstate-cache
        │
        └── PHASE 6: Assemble Output
                  ├── do_rootfs   → install packages into staged rootfs
                  ├── do_image    → write IMAGE_FSTYPES (ext4, squashfs, wic)
                  └── Output: tmp/deploy/images/<MACHINE>/

Per-Recipe Task Pipeline

For a typical C/C++ recipe (e.g., bash_5.2.bb):

Task	What It Does	Key Variables
do_fetch	Downloads source using the fetcher matching SRC_URI scheme	SRC_URI, SRCREV, DL_DIR
do_unpack	Extracts archive or checks out git repo into ${WORKDIR}	S (source dir)
do_patch	Applies .patch files from SRC_URI in order	PATCHTOOL
do_configure	Runs ./configure, cmake, meson, etc.	EXTRA_OECONF, EXTRA_OECMAKE
do_compile	Compiles the source	EXTRA_OEMAKE, PARALLEL_MAKE
do_install	Copies artifacts into ${D} (fake root staging dir)	D, bindir, libdir
do_package	Splits ${D} into sub-packages (main, -dev, -dbg, -doc)	PACKAGES, FILES
do_package_write_rpm	Writes .rpm files to tmp/deploy/rpm/	PACKAGE_CLASSES

Key Directory Variables

Variable	Typical Value	Purpose
WORKDIR	tmp/work/<tuple>/<pn>/<pv>-<pr>/	Recipe scratchpad; all build work happens here
S	${WORKDIR}/<pn>-<pv>/	Extracted source tree
B	${S}	Build directory (separate for out-of-tree builds)
D	${WORKDIR}/image	Destination / fake root filesystem
STAGING_DIR_TARGET	tmp/sysroots/<machine>/	Headers/libs from DEPENDS
DEPLOY_DIR_IMAGE	tmp/deploy/images/<machine>/	Final output images

do_rootfs and do_image

These tasks run in the image recipe after all packages are built:

do_rootfs: Installs packages listed in IMAGE_INSTALL into a staging rootfs using the target package manager. Runs ROOTFS_POSTPROCESS_COMMAND hooks (locale generation, ldconfig, etc.). Produces the populated rootfs directory.

do_image: For each filesystem type in IMAGE_FSTYPES, invokes the corresponding class:

IMAGE_FSTYPES = "ext4 squashfs wic"

• ext4 → calls mkfs.ext4 on the rootfs
• squashfs → calls mksquashfs
• wic → calls wic with a .wks kickstart file to assemble a partitioned disk image (boot + kernel + rootfs in correct layout for the board)

WIC / WKS Partition Layout

For boards that need a specific partition layout (e.g., SD card with FAT boot + ext4 rootfs):

# Example: my-board-sdcard.wks
part /boot --source bootimg-partition --ondisk mmcblk0 --fstype=vfat \
     --label boot --active --align 4 --size 64
part /     --source rootfs --ondisk mmcblk0 --fstype=ext4 \
     --label root --align 4 --size 2048

The WKS file is referenced via WKS_FILE = "my-board-sdcard.wks" in the machine conf. WIC replaces the older IMAGE_FSTYPES = "sdcard" approach and is the current standard for producing boot-ready images.

Multiconfig Builds

For CI pipelines that must produce images for multiple machines:

# conf/multiconfig/raspberrypi4.conf
MACHINE = "raspberrypi4-64"

# conf/multiconfig/qemux86.conf
MACHINE = "qemux86-64"

# Build both targets in one invocation, sharing sstate-cache
bitbake mc:raspberrypi4:core-image-minimal mc:qemux86:core-image-minimal

Examining the Task Graph

# Generate task-depends.dot and pn-buildlist
bitbake -g core-image-minimal

# Visualise
dot -Tsvg task-depends.dot -o task-deps.svg

# Check which packages will be included
cat pn-buildlist

The task-depends.dot file is the authoritative answer to "what will BitBake build and in what order" for a given target.

Contributors: Vasu Grover

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