docker - how to pass environment variable to dockerfile

Question

how to pass kaps-service as a environment variable to my dockerfile

file.txt:- Match $Match name prod

dockerfile:- FROM image ENV Match=kaps-service COPY file.txt /etc/

the requirement is I want to replace the $Match value in file.txt from dockerfile or can I pass this value while starting the docker container ? is there a way to do it ?

Answer 1

From Dockerfile you can use something like:

FROM image

ARG Match="kaps-service"

COPY file.txt /etc/

RUN sed -i 's/$Match/'$Match'/g' /etc/file.txt

The last line will replace all occurrences of $Match with "kaps-service". This will happen during build step, and you can modify Match argument with the --build-arg flag

In case you want the replacement to be in run step (docker run) you can take the RUN command and put it in some script that will execute this command before your application. It can be something like:

FROM image

WORKDIR /app

ENV Match="kaps-service"

COPY file.txt /etc/
COPY run .

CMD "./run"

where run is:

#!/bin/bash

sed -i 's/$Match/'$Match'/g' /etc/file.txt
# run you app

The change will happen when you starts a container with docker run and you can modify Match variable add -e Match=something to the command.