Docker CMD and Entrypoint #

Running Docker containers results in the container running and then exiting immediately.

Containers are not meant to persist; they finish a task, and exit.

CMD #

Use CMD to specify commands for containers in the Dockerfile.

The structure is CMD <command> <parameter>, such as CMD sleep 5.

Alternatively, it can be stated in an array form like CMD ["<command>", "<parameter>"], such as CMD ["sleep", "5"].

For instance, in a Dockerfile:

FROM Ubuntu

CMD sleep 5

^ This approach is hard-coded in the Dockerfile.

Entrypoint #

ENTRYPOINT is an alternative to CMD.

It differs from CMD in that it only requires the passing of a parameter in the invocation of a Docker run.

So the Dockerfile might look something like:

FROM Ubuntu

ENTRYPOINT ["<command>"]

And the run command would look something like this, where the parameter is implicitly passed to the command:

docker run <docker app> <parameter>

Where if the command is sleep, the following might be a specification to sleep for a certain number of seconds:

docker run <docker app> 10

If there’s ENTRYPOINT in the Dockerfile, but no specification of a parameter with the run, this will lead to an error by default. To handle this, use both ENTRYPOINT and CMD in the Dockerfile. This provides a default parameter, which would be overriden by any explicitly provided parameter in the run invocation.

FROM Ubuntu

ENTRYPOINT ["<command>"]

CMD ["<parameter>"]