Docker: How to set env variable in one RUN command and use it in next RUN command?

I'm trying to set a variable in a RUN command and use it in the next RUN command.

Depending on the TARGETARCH arg, I'm trying to set PROTOC_ARCH variable and use it in the next RUN command to download a architecture specific file. But the variable isn't passing through. If I print the variable, it's always empty.

FROM ubuntu

ARG TARGETARCH="arm64"

ENV PROTOC_ARCH=""

RUN if [ "$TARGETARCH" = "arm64" ]; then \
       PROTOC_ARCH="aarch_64"; \
    else \
       PROTOC_ARCH="x86_64"; \
    fi

RUN apt-get update && apt-get install -y wget

RUN echo "PROTOC_ARCH=$PROTOC_ARCH"

RUN wget https://github.com/protocolbuffers/protobuf/releases/download/v22.2/protoc-22.2-linux-$PROTOC_ARCH.zip

Can anyone please tell me what I'm doing wrong here ?

Thanks in advance.

You should not think of your Dockerfile in the same way we think about shell scripts, with state. However, you can move your heavy lifting into a shell script to access desired functionality. Example Dockerfile:

FROM ubuntu

ARG TARGETARCH="arm64"
ENV TARGETARCH=$TARGETARCH

COPY init.sh /
RUN chmod +x /init.sh

CMD /init.sh

And then your init.sh could look like this:

#!/bin/bash 

if [ "$TARGETARCH" = "arm64" ]; then
    PROTOC_ARCH="aarch_64"
else
    PROTOC_ARCH="x86_64"
fi

echo PROTOC_ARCH is $PROTOC_ARCH and TARGETARCH is $TARGETARCH

Now you can access the env vars you expect and do conditional downloads, etc.

This pattern is quite common. You will find a lot of docker images using:

ENTRYPOINT ["docker-entrypoint.sh"]

Each RUN statement is run in a separate shell. That means that any environment variables you set are lost when the RUN statement finishes.

What you can do is run all your commands in a single RUN statement, like this

FROM ubuntu

ARG TARGETARCH="arm64"

ENV PROTOC_ARCH=""

RUN if [ "$TARGETARCH" = "arm64" ]; then \
       PROTOC_ARCH="aarch_64"; \
    else \
       PROTOC_ARCH="x86_64"; \
    fi && \
    apt-get update && apt-get install -y wget && \
    echo "PROTOC_ARCH=$PROTOC_ARCH" && \
    wget https://github.com/protocolbuffers/protobuf/releases/download/v22.2/protoc-22.2-linux-$PROTOC_ARCH.zip

Then PROTOC_ARCH will be available in the wget command, since it's not lost until the complete RUN statement is finished.

One thing to note is that at run-time, PROTOC_ARCH will be blank, since it's the value set in the ENV statement that is put in the image. AFAIK, there's no way to conditionally set a variable in an ENV statement.