CMD in Dockerfile

Defines the program that will be run within the container when it starts.

For nginx image is the:

CMD ["nginx"] # nginx command

For the mysql image is the:

CMD ["mysqld"] # mysql command

If this process finishes or crashes - the container exits as well.
There are different ways to specify the CMD:

• simple shell form:

  CMD sleep 5

• JSON array format:

  CMD ["sleep", "5"]

To override this command for a running container pass the option [COMMAND] to the docker executable:

docker run ubuntu [COMMAND]

for example:

docker run ubuntu sleep 10