No tricks here! Well occasionally send you account related emails. I can immediately think of several scenarios in which this workaround is insufficient. Make a tiny island robust to ecologic collapse. 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. I'm using tiny as my init daemon.. Might be nice to add a note about this to the documentation. Why can't I use Docker CMD multiple times to run multiple services? 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? How can you use an ENV var in the CMD and still be able to interrupt the process? @andho Yes. Also I'm reading a lot of Docker blog: https://blog.docker.com. More like San Francis-go (Ep. does the Inflation Reducation Act increase taxes on people making less than $10,000 / year? privacy statement. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. What is the difference between CMD and ENTRYPOINT in a Dockerfile? Is that a Go specific thing? When you use the JSON syntax, you're explicitly requesting that your command bypass the shell and be execed directly. Difference between RUN and CMD in a Dockerfile. Lets say you want to start a java process inside a container: For the Java developers, following my solution below gonna work: if you tried to run your container with a Dockerfile like below. From inside of a Docker container, how do I connect to the localhost of the machine? The container is built from scratch, so it has a single binary and no shell. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. How can I use a variable inside a Dockerfile SHELL instruction? At the beginning of my Dockerfile using ENV. 469). Not sure if it's still relevant, but in order to use env vars in CMD I do this: Strip "$@" of "/bin/sh -c" in docker-entrypoint.sh: I think with and without [] makes some difference in CMD execution. 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! How can I refill the toilet after the water has evaporated from disuse? It's basically the same as: Read this piece: https://docs.docker.com/engine/reference/builder/#cmd. rev2022.8.2.42721. In practice, how explicitly can we describe a Galois representation? What is the quickest workaround here? Why classical mechanics is not able to explain the net magnetization in ferromagnets? Hope the explanation is clear. Since there is no shell involved, that means: If you want your CMD to expand variables, you need to arrange for a shell. 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. Does this JavaScript example create race conditions? It's a bit confusing considering numerous high profile projects on docker hub (memcached, postgres, redis, etc) use the docker-entrypoint.sh pattern. The text was updated successfully, but these errors were encountered: Yes. Connect and share knowledge within a single location that is structured and easy to search. 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. 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. Sign in I think this is a good example where shell expansion doesn't work. 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. Wrapping with shell script works but does not forward OS signals to the process. In case this may be useful for anyone, I wrote a small Go script to achieve this. @ryanjaeb The apps will have access to the environment variables in the environment it's run within. By clicking Sign up for GitHub, you agree to our terms of service and You signed in with another tab or window. How to mount a host directory in a Docker container. @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. @akravetz I recall on DockerConf it was said multiple times avoid using docker-entrypoint.sh, they grow massively. To learn more, see our tips on writing great answers. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. San Francisco? Have a question about this project? The last line of docker-entrypoint.sh is typically exec $@ which will not expand variables. Why is a 220 resistor for this LED suggested if Ohm's law seems to say much less is required? Transform characters of your choice into "Hello, world! 468), Monitoring data quality with Bigeye(Ep. Using shell form has a main drawback of no running your software as PID 1. The use cases I mention there have possible overlap with the issues with ENV. The current proposed work-around does not handle OS signal forwarding gracefully. 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. Confirmed that the form CMD ["sh", "-c", "echo ${MY_HOME}"] works as expected. And it doesn't work on the build of the image. I think this is a good example where shell expansion doesn't work. It doesn't work: docker run -P -i -t tomcat gives me: I can confirm this. Announcing the Stacks Editor Beta release! What is the nature of a demiplane's walls? Asking for help, clarification, or responding to other answers. This example runs a node js app. Successfully merging a pull request may close this issue. Why does the United States openly acknowledge targeted assassinations? ENV var doesn't work using scratch image, any idea how to fix it? Take a read here: https://docs.docker.com/engine/reference/builder/#cmd. Write the command without use the args array: ie CMD gunicorn --bind 0.0.0.0:$PORT wsgi:app. Is there a place where best practices like that are documented for those of us who don't attend DockerConf? What is the equivalent of the Run dialogue box in Windows for adding a printer? to your account, I want to be able to say CMD [ "$CATALINA_HOME/bin/catalina.sh", "run"]. Just write this in your docker file. I've been trying to remember what I was getting stuck on and can't. The explanation is that the shell is responsible for expanding environment variables, not Docker. And in there you'll get variable expansion. Another workaround is to move the command into the script. 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. 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). @ryanjaeb The container is built from scratch, so it has a single binary and no shell. Does Docker have any plan to "fix" or introduce this functionality in later versions of Docker? I'm curious how the app ends up with access to the expanded env vars if the shell is responsible for expanding them. In a Dockerfile, How to update PATH environment variable? For python docker container, use this format: This way shell is still used to execute process. EDIT: Please see issue #34772, which I just created as a correlation for ARG directives. I tried it a couple weeks ago and the env vars are always available within the app. So I hope I'm good there. Drivetrain 1x12 or 2x10 for my MTB use case? Already on GitHub? When you use the JSON syntax, you're explicitly requesting that your command bypass the shell and be execed directly. @holms thank you, this is good info. Find centralized, trusted content and collaborate around the technologies you use most. Using a Dockerfile with this. So the apps called from within the shell also has access to env vars. 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. Using the string form or sh -c breaks the ability to Ctrl-C out of docker run my-image. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Wouldn't the env vars be available within the app, and so it would not need variable expansion during call time? 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 $PROJECTNAME from inside the container and it will create the project Trending sort is based off of the default sorting method by highest score but it boosts votes that have happened recently, helping to surface more up-to-date answers. Also success stories: https://success.docker.com I'm assuming the other form will as well. But most apps take input/config through arguments instead of env vars, and that's where variable expansion is needed. Making statements based on opinion; back them up with references or personal experience. +1 many of us are using the ENTRYPOINT ["docker-entrypoint.sh"] CMD ["something", "${VARIABLE"] pattern where this fails. How can I use a variable inside a Dockerfile CMD? ", reverse translation from amino acid string to DNA strings. Is any finite-dimensional algebra a sub-algebra of a finite-group algebra? It falls back to sorting by highest score if no posts are trending. What type of variable do you mean: dockerfile variable or environmental variable (as in your system runtime)? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. then Docker will execute the given command directly, without involving a shell. Do the debris from the re-entry of Long March core stage ever reach the surface? Assuming it's your own binary. @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 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. 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. How to include files outside of Docker's build context? What are the possible attributes of aluminum-based blood? If you want to use it at build-time, then you should use ARG. The shell is also an app, which is used to call other apps. 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 "$@". Anyone found a solution for that? Meaning of 'glass that's with canary lined'? 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. Dockerfile CMD doesn't understand ENV variables. (To the extent that they can exist in JavaScript). Getting paid by mistake after leaving a company? Try CMD echo ${MY_HOME} or CMD ["sh", "-c", "echo ${MY_HOME}"] and you should have more luck. @rjshrjndrn of course [] make a difference.
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docker cmd environment variables