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Ubuntu – How to calculate and set PATH inside Dockerfile, once and forever?

This is my Dockerfile (I need to generate the path in runtime, I don’t know it upfront):

FROM ubuntu:22.04
RUN year=$(date +%Y) 
  && echo "export PATH=${PATH}:/usr/local/${year}" >> /root/.bashrc

Then, I build the image and run a container:

$ docker build . -t tmp
$ docker run --rm tmp env

The PATH doesn’t include /usr/local/2024.

AFAIU, it’s not possible to use ENV for this, since the PATH is not static, but generated when the image is being built.

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2

Answers


  1. 0

    Firstly, understand that Docker’s ENV instruction doesn’t support shell command execution directly. So, you can’t use $(date +%Y) directly within an ENV instruction to dynamically generate part of your path. However, you can cleverly work around this limitation using a multi-stage build process. Here’s how you can do it effectively:

    Multi-Stage Build Approach: Utilize a two-stage build process. In the first stage, generate the dynamic part of your PATH and output it to a file. In the second stage, copy this file over and use it to set the PATH environment variable correctly.

    Copy code
# Stage 1: Generate the dynamic PATH component
FROM ubuntu:22.04 AS builder
RUN echo "PATH=/usr/local/$(date +%Y):${PATH}" > /path.env

# Stage 2: Build the final image and set the PATH
FROM ubuntu:22.04
COPY --from=builder /path.env /etc/environment
# Ensure the PATH is sourced for both interactive and non-interactive shells
RUN echo 'source /etc/environment' >> /etc/bash.bashrc && 
    echo 'source /etc/environment' >> /etc/profile

    This approach ensures that your dynamic PATH is respected in both interactive and non-interactive shell sessions, aligning with Docker’s best practices for setting environment variables that need to be evaluated or generated at build time.

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  2. 0

    When you run a non-interactive shell, bash doesn’t read your .bashrc file. Compare:

    $ docker run --rm pathtest env | grep PATH
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

    With:

    $ docker run -it --rm pathtest
root@6e9bfd8a4e04:/# env | grep PATH
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/local/2024

    Note that in the second case, in which we start an interactive shell, PATH includes /usr/local/2024.

    I think the best way of handling this would be to set PATH in an ENTRYPOINT script. E.g.:

    FROM ubuntu:22.04

ENTRYPOINT ["sh", "-c", "export PATH="${PATH}:/usr/local/$(date +%Y)" && exec "$@"", "--"]
CMD ["bash"]

    This will set PATH regardless of what you pass as the command, so your test now works as expected:

    $ docker run --rm pathtest env | grep PATH
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/local/2024
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