CODEWITHSUNDEEP
dockerdockerfile
xsrf
xsrf

Reputation: 664

Select ENV/ARG in Dockerfile depending on build ARG

I currently have two Dockerfiles that are identical except for some ENV vars at the beginning that have different values. I want to combine them into one Dockerfile and select the ENV vars depending on one build-arg / ARG instead.

I tried something like this with $target being either WIN or UNIX:

FROM alpine
ARG target
ARG VAR1_WIN=Value4Win
ARG VAR1_UNIX=Value4Unix
ARG VAR1=VAR1_$target
ARG VAR1=${!VAR1}
RUN echo $VAR1

But it throws an error: failed to process "${!VAR1}": missing ':' in substitution I tried a lot but I'm unable to double expand $VAR1. How do I do this correctly? Thx.

Upvotes: 1

Views: 1438

Answers (3)

BMitch
BMitch

Reputation: 263469

For the conditional syntax, there is a pattern you can use with a multi-stage build:

# ARG defined before the first FROM can be used in FROM lines
ARG target

# first base image for WIN target
FROM alpine as base-WIN
# switching to ENV since ARG doesn't pass through to the next stage
ENV VAR1=Value4Win

# second base image for UNIX target
FROM alpine as base-UNIX
ENV VAR1=Value4Unix

# select one of the above images based on the value of target
FROM base-${target} as release
RUN echo $VAR1

Upvotes: 3

yyyy
yyyy

Reputation: 672

The double expand syntax ${!VAR} only works in bash, while Dockerfile is not parsed by shell. According to the docker manual, Dockerfile does not support double expand.

Note that alpine use ash instead of bash, so it does not support ${!VAR} either. You have to use eval + echo. Try RUN VAR1="$(eval "echo \$$VAR1")"; echo $VAR1.

Upvotes: 0

Perplexabot
Perplexabot

Reputation: 1989

If you can't pass Value4Win or Value4Unix directly via --build-arg, here is one way:

FROM alpine

ARG target
ARG VAR1=Value4${target}
RUN echo $VAR1

Doing a docker build --build-arg target=WIN . gives:

Step 4/4 : RUN echo $VAR1
 ---> Running in 6e94bc28d459
Value4WIN

Upvotes: -1

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