Conditional CMD in Docker

Question

I'd like to run CMD ["npm", "run", "dev"] if a dev script is present in package.json, otherwise CMD ["npm", "start"]

Is there an idiomatic docker way for doing this? I suppose my CMD could be a bash script which checks for the presence of 'npm run dev'. Is there a cleaner way to do it than that?

Answer 1

You could dynamically create a bash script containing appropriate command:

RUN bash -c "if npm run | grep -q dev ; then echo npm run dev > run.sh; else echo npm start > run.sh; fi; chmod 777 run.sh;"
CMD ./run.sh

• if npm run | grep -q dev will check if dev script is present in package.json. Running npm run with no arguments will list all runable scripts and grep -q dev will return true only if dev is among them.

• echo command > run.sh will create a file with provided command

• chmod 777 run.sh will make the run.sh executable

• CMD ./run.sh will execute created file

Answer 2

The Dockerfile syntax does not cover that such use case. You have to rely on scripts to achieve that.