30 min·Intermediate

Build a small container image

Interpret a deliberately small Containerfile language into a root filesystem and launch metadata.

Turn a specification into files

A builder interprets instructions and produces a root filesystem plus configuration. Our FROM names one predefined BusyBox base. COPY adds a regular context file, and RUN executes a build command through chroot.

Keep build-time and run-time state distinct

ENV records KEY=VALUE lines. WORKDIR creates an absolute image directory and records it. RUN uses the environment and current directory immediately. CMD records a shell command for a later runner; building must not execute it.

FROM minimal-rootfs
COPY hello /hello
ENV GREETING=hello
WORKDIR /app
RUN echo built > /built
CMD /hello

Your change

Finish the instruction dispatcher. The helpers validate COPY paths, install the base, and execute build commands. This intentionally limited format supports one FROM, regular-file COPY, shell-form RUN/CMD, one KEY=VALUE per ENV, and absolute WORKDIR; JSON syntax, quoting rules, multistage builds, and layer caching are outside this language.

Your goal

Implement COPY, RUN, ENV, WORKDIR, and CMD using the supplied filesystem helpers and a predefined FROM base.

What the tests observe

Two distinct real rootfs builds
Build-time environment and directory
Run metadata preserved
COPY context boundary

Need a nudge?

Hint 1

Use the supplied init_image, copy_file, and run_build helpers; finish the case branches.

Hint 2

ENV appends a validated KEY=VALUE line. WORKDIR creates the image directory and writes its absolute path to image/workdir.

Hint 3

CMD writes its shell command to image/cmd; do not execute it during the build. RUN calls run_build after earlier state changes.

What happens inside Linux

You now own a small image format: filesystem bytes plus execution metadata. A production builder adds content-addressed layers, reproducibility, cache keys, isolation for RUN, and a much larger instruction grammar; this builder makes those boundaries explicit.

Optional account sync. Exercises work without an account.

Containers