How to properly override the ENTRYPOINT using docker run

kyohei ito — Docker-4
Originally published on https://oprea.rocks/blog/how-to-properly-override-the-entrypoint-using-docker-run/

As we all know, Docker is an amazing piece of software. I don’t want to go over its benefits. That’s for another article, coming very soon. What I do want to share with you is the way to properly override a Docker image entrypoint when using docker run.

From the official documentation:

The ENTRYPOINT of an image is similar to a COMMAND because it specifies what executable to run when the container starts, but it is (purposely) more difficult to override

Keep in mind the “what executable to run” part!

Many people trying to override the ENTRYPOINT at runtime, using docker runwill make mistake of passing the executable's arguments directly after it, like so (happy horizontal scrolling):

docker run --entrypoint "/bin/ls -al /root" debian

What is the problem here? Well the documentation clearly states that the ENTRYPOINT only specifies the executable to run, when the container starts.

So they get an error like below

container_linux.go:247: starting container process caused "exec: \"/usr/bin/ls -al\": stat /usr/bin/ls -al: no such file or directory"
docker: Error response from daemon: oci runtime error: container_linux.go:247: starting container process caused "exec: \"/usr/bin/ls -al\": stat /usr/bin/ls -al: no such file or directory".
ERRO[0001] error getting events from daemon: net/http: request canceled

There is something a bit counter-intuitive here and if you take a good look at the example commands on the documentation page, you’ll see that the arguments are being passed after the image name.

docker run -it --entrypoint /usr/bin/redis-cli example/redis --help

This means that if we want to pass the -al flags to our ls command we should write the whole thing like this:

docker run --entrypoint "/bin/ls" debian -al /root

Hope this clears things out for you and saves you a ton of headaches.

Photo credits: kyohei ito — Docker-4

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