CODEWITHSUNDEEP
dockerdockerfile
Human
Human

Reputation: 866

How to use combination of ARG and ENV in Dockerfile to prevent unsupported modifier in substitution?

I am trying to use an example from Docker docs on ARG

FROM ubuntu                                                                                          
                                                                                                      
ARG TEST                                                                                             
ENV TEST=${TEST:foo}                                                                                 
                                                                                                      RUN echo $TEST

Docker docs says:

The variable expansion technique in this example allows you to pass arguments from the command line and persist them in the final image by leveraging the ENV instruction.

The build fails with the following error:

Sending build context to Docker daemon  2.048kB
Step 1/4 : FROM ubuntu
latest: Pulling from library/ubuntu
16ec32c2132b: Pull complete 
Digest: sha256:82becede498899ec668628e7cb0ad87b6e1c371cb8a1e597d83a47fac21d6af3
Status: Downloaded newer image for ubuntu:latest
 ---> 1318b700e415
Step 2/4 : ARG TEST
 ---> Running in 4130b437fd93
Removing intermediate container 4130b437fd93
 ---> a31b0f8a81ff
Step 3/4 : ENV TEST=${TEST:foo}
failed to process "${TEST:foo}": unsupported modifier (f) in substitution

Why is this not working? The error sounds like a complete misunderstanding between me and my docker engine.

OS: Ubuntu 20.04.3 LTS Engine: Docker version 20.10.8, build 3967b7d

Upvotes: 1

Views: 1245

Answers (1)

Arkadiusz Drabczyk
Arkadiusz Drabczyk

Reputation: 12498

You're missing -, it should be:

FROM ubuntu

ARG TEST
ENV TEST=${TEST:-foo}

Upvotes: 1

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