016-CMD-and-Entrypoint.md

CMD and ENTRYPOINT

Containers are not meant to host operating systems. Thus when you launch a container of a Linux Image like Ubuntu, it's default command or CMD is bash. This can be seen from the dockerfile itself.

However if it doesn't detect any terminal, it just stops the process which also stops the container.

CMD

If you want to define a default command or instruction to run besides the bash when the container is ran, you can specify it in the dockerfile using the CMD keyword.

As an example, we can set the container to sleep for 60 seconds when it is ran by:

docker run ubuntu sleep 60

An easier way to do this is by including the command itself when creating the Dockerfile.

$ cat > dockerfile 

FROM ubuntu
CMD sleep 60

There are ways to specify a command in the dockerfile

CMD <command> <parameter1>
CMD ["<command>", "<parameter1>"]                   <<< JSON format

ENTRYPOINT

We can also use a parameter from the commandline itself. This can be done by using ENTRYPOINT in the dockerfile.

FROM ubuntu
ENTRYPOINT ["sleep"]

Now when you run the container, you'll just have to define the parameter.

$ docker run ubuntu-sleeper 60

Note that you'll get an error when you don't append a parameter in the docker run command because the ENTRYPOINT is expecting a parameter.

To include a default value in case user doesn't provide a parameter along with the docker run command, you can use CMD and ENTRYPOINT together

FROM ubuntu
ENTRYPOINT ["sleep"]
CMD ["60"]

Overriding ENTRYPOINT

You can also override the entrypoint during runtime by using the "--entrypoint" flag

docker run --entrypoint sleep2.0 ubuntu-sleeper 60

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