Using the string form or sh -c breaks the ability to Ctrl-C out of docker run my-image. And in there you'll get variable expansion. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. To learn more, see our tips on writing great answers. But most apps take input/config through arguments instead of env vars, and that's where variable expansion is needed. Announcing the Stacks Editor Beta release! It falls back to sorting by highest score if no posts are trending. Drivetrain 1x12 or 2x10 for my MTB use case? The use cases I mention there have possible overlap with the issues with ENV. Transform characters of your choice into "Hello, world! Since there is no shell involved, that means: If you want your CMD to expand variables, you need to arrange for a shell. This example runs a node js app. The container is built from scratch, so it has a single binary and no shell. Why is a 220 resistor for this LED suggested if Ohm's law seems to say much less is required? Dockerfile CMD doesn't understand ENV variables. @beporter you need to add exec (which is a POSIX shell feature): The explanation is that the shell is responsible for expanding environment variables, not Docker. rev2022.8.2.42721. What type of variable do you mean: dockerfile variable or environmental variable (as in your system runtime)? I tried it a couple weeks ago and the env vars are always available within the app. NOTE: If I remove the CMD line from the Docker file and then run the Docker container, I am able to manually run Django-admin startproject $PROJECTNAME from inside the container and it will create the project Trending sort is based off of the default sorting method by highest score but it boosts votes that have happened recently, helping to surface more up-to-date answers. When I tested it I noticed that if I use exec.Command().Run() to run another Go binary it also gets access to the env vars. It doesn't work: docker run -P -i -t tomcat gives me: I can confirm this. Confirmed that the form CMD ["sh", "-c", "echo ${MY_HOME}"] works as expected. @akravetz I recall on DockerConf it was said multiple times avoid using docker-entrypoint.sh, they grow massively. It's a bit confusing considering numerous high profile projects on docker hub (memcached, postgres, redis, etc) use the docker-entrypoint.sh pattern. +1 many of us are using the ENTRYPOINT ["docker-entrypoint.sh"] CMD ["something", "${VARIABLE"] pattern where this fails. It for sure would have been a 3rd party binary I was trying to build into a container, so maybe it was just doing something odd and I didn't notice. The container is built from scratch, so it has a single binary and no shell. More like San Francis-go (Ep. Add note to docs about lack of shell processing in JSON form - Issue 5509, Add note to docs about lack of shell processing in JSON form, Add examples for JavaScript, Ruby, Groovy, Plain Java and Java Fat Jar, [clickhouse] fix error caused by non-expanding ENV inside JSON in Doc, Pass the number of R instances through args, Dockerfile CMD doesn't understand ARG variables, Illegal character error when using docker compose up, https://docs.docker.com/develop/develop-images/dockerfile_best-practices/, https://www.docker.com/products/resources/case-studies, Env variable interpolation within the CMD does require to use the sh , introduce docker multistage build to alpine image, https://docs.docker.com/engine/reference/builder/#cmd, Run kyuubi on kubernetes - make basic dockerfile and example, Don't hardcode the postgres container name for Docker, Refactor Dockerfile for multi-stage build. The explanation is that the shell is responsible for expanding environment variables, not Docker. What is the equivalent of the Run dialogue box in Windows for adding a printer? Anyone found a solution for that? You can do that like this: Or you can use a simple string instead of an execution list, which gets you a result largely identical to the previous example: If you want to use the value at runtime, set the ENV value in the Dockerfile. So the apps called from within the shell also has access to env vars. Hope the explanation is clear. How can I refill the toilet after the water has evaporated from disuse? However, it work when you enter through /bin/bash (docker run [image] /bin/bash) and type: I'll keep trying to work around this issue. Is any finite-dimensional algebra a sub-algebra of a finite-group algebra? Why can't I use Docker CMD multiple times to run multiple services? What is the difference between CMD and ENTRYPOINT in a Dockerfile? @holms thank you, this is good info. How to include files outside of Docker's build context? Also success stories: https://success.docker.com For python docker container, use this format: This way shell is still used to execute process. Successfully merging a pull request may close this issue. Have a question about this project? No tricks here! ENV var doesn't work using scratch image, any idea how to fix it? Using a Dockerfile with this. Announcing Design Accessibility Updates on SO, How to set environment variables in docker-compose, Docker can't find script living in home dir, Shell script failing on mkdir in Dockerfile. Why does the United States openly acknowledge targeted assassinations? (To the extent that they can exist in JavaScript). Well occasionally send you account related emails. to your account, I want to be able to say CMD [ "$CATALINA_HOME/bin/catalina.sh", "run"]. Is that a Go specific thing? Nothing in this answer should generally impact your ability to CTRL+C a container; if something isn't behaving as you expect, you might want to open a new question with details and we'll see if we can help you out. then Docker will execute the given command directly, without involving a shell. @rjshrjndrn of course [] make a difference. I've been trying to remember what I was getting stuck on and can't. In practice, how explicitly can we describe a Galois representation? I can immediately think of several scenarios in which this workaround is insufficient. Just write this in your docker file. It's basically the same as: Read this piece: https://docs.docker.com/engine/reference/builder/#cmd. I'm using tiny as my init daemon.. Difference between RUN and CMD in a Dockerfile. When you use the JSON syntax, you're explicitly requesting that your command bypass the shell and be execed directly. At the beginning of my Dockerfile using ENV. Wouldn't the env vars be available within the app, and so it would not need variable expansion during call time? How can I use a variable inside a Dockerfile CMD? The text was updated successfully, but these errors were encountered: Yes. The last line of docker-entrypoint.sh is typically exec $@ which will not expand variables. EDIT: Please see issue #34772, which I just created as a correlation for ARG directives. If you want to use it at build-time, then you should use ARG. Why doesn't the bootstrap diode in the half-bridge driver cause the high side to be driven at a lower voltage than the low side? Not sure if it's still relevant, but in order to use env vars in CMD I do this: Strip "$@" of "/bin/sh -c" in docker-entrypoint.sh: I think with and without [] makes some difference in CMD execution. Find centralized, trusted content and collaborate around the technologies you use most. Sign in By clicking Sign up for GitHub, you agree to our terms of service and What is the quickest workaround here? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Also I forgot to mention that if I remove the CMD line from the Dockerfile and then run the container, from inside the container I can run this command and it will create the project (meaning the ENV variable is valid). And it doesn't work on the build of the image. How can I use a variable inside a Dockerfile SHELL instruction? Another workaround is to move the command into the script. Assuming it's your own binary. Getting paid by mistake after leaving a company? Why classical mechanics is not able to explain the net magnetization in ferromagnets? What is the nature of a demiplane's walls? Lets say you want to start a java process inside a container: For the Java developers, following my solution below gonna work: if you tried to run your container with a Dockerfile like below. Try CMD echo ${MY_HOME} or CMD ["sh", "-c", "echo ${MY_HOME}"] and you should have more luck. ", reverse translation from amino acid string to DNA strings. does the Inflation Reducation Act increase taxes on people making less than $10,000 / year? In case this may be useful for anyone, I wrote a small Go script to achieve this. Using shell form has a main drawback of no running your software as PID 1. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. And based on @sfitts logic, as CMD is passed to the ENTRYPOINT as an argument, the following is possible: The "answer" as provided above is merely a workaround, punting the expansion of ENV variables to the shell. You signed in with another tab or window. From inside of a Docker container, how do I connect to the localhost of the machine? 468), Monitoring data quality with Bigeye(Ep. @andho Yes. Take a read here: https://docs.docker.com/engine/reference/builder/#cmd. Making statements based on opinion; back them up with references or personal experience. The current proposed work-around does not handle OS signal forwarding gracefully. Write the command without use the args array: ie CMD gunicorn --bind 0.0.0.0:$PORT wsgi:app. The below one is my final version of the Dockerfile and docker-entrypoint.sh files: and after you build your docker image and tried to run it, you will see the logs below that means it worked well: Thanks for contributing an answer to Stack Overflow! 469). Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Connect and share knowledge within a single location that is structured and easy to search. Does this JavaScript example create race conditions? How to mount a host directory in a Docker container. @ryanjaeb The apps will have access to the environment variables in the environment it's run within. I think this is a good example where shell expansion doesn't work. Also I'm reading a lot of Docker blog: https://blog.docker.com. Might be nice to add a note about this to the documentation. Already on GitHub? Make a tiny island robust to ecologic collapse. @akravetz same for me actually :) Docs has some best practises: https://docs.docker.com/develop/develop-images/dockerfile_best-practices/, Also there's case studies: https://www.docker.com/products/resources/case-studies Do the debris from the re-entry of Long March core stage ever reach the surface? Some more reading in Docker's issue tracker: If I use one of these tricks then I can't stop my container with CTRL+C anymore. When you use the JSON syntax, you're explicitly requesting that your command bypass the shell and be execed directly. I'm assuming the other form will as well. I'm curious how the app ends up with access to the expanded env vars if the shell is responsible for expanding them. Does Docker have any plan to "fix" or introduce this functionality in later versions of Docker? @ryanjaeb it will build the image correctly but print the error below during the run of container: instead, Java can read the command line parameters either through the command line or by _JAVA_OPTIONS environment variable. so, it means we can pass the desired command line parameters through _JAVA_OPTIONS without changing anything on Dockerfile as well as to allow it to be able to start as parent process of container for the valid docker signalization via exec "$@". Regarding entrypoint.sh I might be wrong but I clearly remember that some features appeared because of long entrypoint.sh, personally trying not to use them at all, because everything possible to do better most of the time, by using docker provided features. Asking for help, clarification, or responding to other answers. So I hope I'm good there. The shell is also an app, which is used to call other apps. San Francisco? privacy statement. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Meaning of 'glass that's with canary lined'? Wrapping with shell script works but does not forward OS signals to the process. Is there a place where best practices like that are documented for those of us who don't attend DockerConf? How can you use an ENV var in the CMD and still be able to interrupt the process? What are the possible attributes of aluminum-based blood? In a Dockerfile, How to update PATH environment variable? I think this is a good example where shell expansion doesn't work.