Docker Entrypoint vs Cmd

Docker ENTRYPOINT vs CMD

Dockerfile should specify at least one of CMD or ENTRYPOINT commands. Both instructions might have exec (CMD ["executable","param1","param2"]) and (CMD command param1 param2) shell forms. The exec form is the preferred form.

ENTRYPOINT

ENTRYPOINT contains a default command, that is executed when the container is run. If the container shouldn’t have a default command, then ENTRYPOINT can be omitted.

You can override the ENTRYPOINT instruction using the docker run --entrypoint flag.

CMD

CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.

CMD will be overridden when running the container with alternative arguments.

$ docker run image [command 1] [arg1] [arg2]

Samples

User isn’t supposed to change container executable and its default arguments

The container always executes the same command - ping www.google.com

$ cat Dockerfile 
FROM alpine  
ENTRYPOINT ["ping", "www.google.com"]
$ docker run image
# Container executes - ping www.google.com

User isn’t supposed to change container executable but can change its default arguments

The container always executes the same command - ping, but argument can be changed by a user. If a user doesn’t specify an argument, then the default argument is used - www.google.com.

$ cat Dockerfile
FROM alpine
ENTRYPOINT ["ping"]
CMD ["www.google.com"]
$ docker run image
# Container executes - ping www.google.com
$ docker run image www.yahoo.com
# Container executes - ping www.yahoo.com

User is supposed to change container executable and its default arguments

The container should execute commands specified by a user. If user doesn’t pass arguments then default command is executed - ping www.google.com.

$ cat Dockerfile
FROM alpine  
CMD ["ping", "www.google.com"]
$ docker run image
# Container executes - ping www.google.com
$ docker run image curl www.yahoo.com
# Container executes - curl www.yahoo.com