Case and If in Dockerfile

dockerfile case if targetarch targetplatform

There is many tips building docker image, one of it is Case and If. Some of you may already familiar with this, this the condition we set to build image. Most of it is to download different binary url, like this:

Using CASE:

RUN case ${TARGETPLATFORM} in \
         "linux/amd64")  QBIT_ARCH=x86_64  ;; \
         "linux/arm64")  QBIT_ARCH=aarch64  ;; \
    esac \ 
    && curl -L https://github.com/userdocs/qbittorrent-nox-static/releases/download/$QBVERSION/$QBIT_ARCH-qbittorrent-nox -o qbittorrent-nox && \
    mv ./qbittorrent-nox $HOME/.local/bin/qbittorrent-nox;

Using IF:

RUN if [ "${TARGETARCH}" = "amd64" ]; then \
        wget https://github.com/userdocs/qbittorrent-nox-static/releases/download/$QBVERSION/x86_64-cmake-qbittorrent-nox && \
        mv ./x86_64-cmake-qbittorrent-nox /home/$CONTAINERUSER/.local/bin/qbittorrent-nox; \
    fi;

RUN if [ "${TARGETARCH}" = "arm64" ]; then \
        wget https://github.com/userdocs/qbittorrent-nox-static/releases/download/$QBVERSION/aarch64-cmake-qbittorrent-nox && \
        mv ./aarch64-cmake-qbittorrent-nox /home/$CONTAINERUSER/.local/bin/qbittorrent-nox; \
    fi;

The both sample it used to download binary for 2 different architecture and pack it to docker image, which one to use? It matter of preferences, some people like case otherhand if also likeable. But as we noticed about $TARGETARCH and $TARGETPLATFORM, what is it? Well it's a default parameter that passed when using docker buildx. On docker buildx we already know that we need --platform=linux/amd64,linux/arm64 or so depends you want. The difference is the result we are looking,

When using $TARGETPLATFORM

  • linux/amd64
  • linux/arm64
  • linux/arm

There is also linux/arm/v6, linux/i386 but pretty rare nowadays as the application moving forward to 64bit instructions, the only most requested 32bit is linux/arm because of raspberry pi, surprisingly it still promote 32bit.

When using $TARGETARCH

  • amd64
  • arm64
  • arm

With that differences, you can't make a compare $TARGETARCH=linux/amd64 and vice versa, they got their own results.

The last part is how do we catch it? We modify our Dockerfile by adding ARG. If we dont adding ARG it wont catch it even we passed with --platform on building.

FROM debian:latest
ARG TARGETARCH
ARG TARGETPLATFORM
RUN this
RUN that
RUN if $TARGETARCH = bla
RUN case $TARGETPLATFORM = bla

sources:

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