$() in a dockerfile

Question

On my dockerfile, I want to export a data from a json inside an env var. My issue is $(), don't work with jq How can I do-it?

MyDockerFile:

WORKDIR /myCurrentDir
RUN wget "http://stedolan.github.io/jq/download/linux64/jq"
RUN chmod 777 ./jq


RUN cat /myCurrentDir/build/BUILD_INFO.json | /myCurrentDir/jq '.version' 
#work: return v1.23


RUN $(cat /myCurrentDir/build/BUILD_INFO.json | /myCurrentDir/jq '.version')
#don't work: /bin/sh: /myCurrentDir/jq: not found

I have try with jq instead of /myCurrentDir/jq, and./jq, it still don't work. ls show that jq is installed in /myCurrentDir

Answer 1

First of all, you haven't specified base image, so your Dockerfile is not reproducible.

I've tried this one:

FROM alpine
WORKDIR /myCurrentDir
RUN wget "http://stedolan.github.io/jq/download/linux64/jq"
RUN chmod 777 ./jq

RUN mkdir build && echo -e "{\"version\":\"v1.23\"}" > /myCurrentDir/build/BUILD_INFO.json

RUN cat /myCurrentDir/build/BUILD_INFO.json | /myCurrentDir/jq ".version"

And even at this stage I've got /bin/sh: /myCurrentDir/jq: not found

First, I suggest you to install jq with the packet manager so that you don't have to worry about it's availability. And second, you could export your variable by specifying a file name in jq command. For example, Dockerfile:

FROM alpine
WORKDIR /myCurrentDir
RUN apk add --no-cache jq

RUN mkdir build && echo -e "{\"version\":\"v1.23\"}" > /myCurrentDir/build/BUILD_INFO.json

RUN VER=$(jq '.version' /myCurrentDir/build/BUILD_INFO.json) && echo $VER > temp

To test that:

$ docker build -t test .

$ docker run test cat temp
"v1.23"