What is the difference between CMD and ENTRYPOINT

Question

In Dockerfiles there are two commands that look similar to me: CMD and ENTRYPOINT. But I guess that there is a (subtle?) difference between them - otherwise it would not make any sense to have two commands for the very same thing.

The documentation states for CMD

The main purpose of a CMD is to provide defaults for an executing container.

and for ENTRYPOINT:

An ENTRYPOINT helps you to configure a container that you can run as an executable.

So, what's the difference between those two commands?

Answer 1

Comments on EntryPoint function in code

// ENTRYPOINT /usr/sbin/nginx.

// Set the entrypoint (which defaults to sh -c) to /usr/sbin/nginx.

// Will accept the CMD as the arguments to /usr/sbin/nginx.

Another reference from documents

You can use the exec form of ENTRYPOINT to set fairly stable default commands and arguments and then use CMD to set additional defaults that are more likely to be changed.

Example:

FROM ubuntu:14.04.3
ENTRYPOINT ["/bin/ping"]
CMD ["localhost", "-c", "2"]

Build: sudo docker build -t ent_cmd .

CMD arguments are easy to override.

NO argument (sudo docker -it ent_cmd)                :  ping localhost 
argument    (sudo docker run -it ent_cmd google.com) :  ping google.com

.

To override EntryPoint argument, you need to supply entrypoint
sudo docker run -it --entrypoint="/bin/bash" ent_cmdd

p.s: In presence of EntryPoint, CMD will hold arguments to fed to EntryPoint. In absense of EntryPoint, CMD will be the command which will be run.

Answer 2

In a nutshell:

• CMD sets default command and/or parameters, which can be overwritten from command line when docker container runs.
• ENTRYPOINT command and parameters will not be overwritten from command line. Instead, all command line arguments will be added after ENTRYPOINT parameters.

If you need more details or would like to see difference on example, there is a blog post that comprehensively compare CMD and ENTRYPOINT with lots of examples - http://goinbigdata.com/docker-run-vs-cmd-vs-entrypoint/

Answer 3

The accepted answer is fabulous in explaining the history. I find this table explain it very well from official doc on 'how CMD and ENTRYPOINT interact':

Answer 4

CMD:

• CMD ["executable","param1","param2"]: ["executable","param1","param2"] is the first process.
• CMD command param1 param2: /bin/sh -c CMD command param1 param2 is the first process. CMD command param1 param2 is forked from the first process.
• CMD ["param1","param2"]: This form is used to provide default arguments for ENTRYPOINT.

ENTRYPOINT (The following list does not consider the case where CMD and ENTRYPOINT are used together):

• ENTRYPOINT ["executable", "param1", "param2"]: ["executable", "param1", "param2"] is the first process.
• ENTRYPOINT command param1 param2: /bin/sh -c command param1 param2 is the first process. command param1 param2 is forked from the first process.

As creack said, CMD was developed first. Then ENTRYPOINT was developed for more customization. Since they are not designed together, there are some functionality overlaps between CMD and ENTRYPOINT, which often confuse people.

Answer 5

Yes, that is a good question. I don't understand it fully yet, but:

I understand that ENTRYPOINT is the binary that is being executed. You can overide entrypoint by --entrypoint="".

docker run -t -i --entrypoint="/bin/bash" ubuntu

CMD is the default argument to container. Without entrypoint, default argument is command that is executed. With entrypoint, cmd is passed to entrypoint as argument. You can emulate a command with entrypoint.

# no entrypoint
docker run ubuntu /bin/cat /etc/passwd

# with entry point, emulating cat command
docker run --entrypoint="/bin/cat" ubuntu /etc/passwd

So, main advantage is that with entrypoint you can pass arguments (cmd) to your container. To accomplish this, you need to use both:

# Dockerfile
FROM ubuntu
ENTRYPOINT ["/bin/cat"]

and

docker build -t=cat .

then you can use:

docker run cat /etc/passwd
#              ^^^^^^^^^^^
#                   CMD
#          ^^^      
#          image (tag)- using the default ENTRYPOINT

Answer 6

Difference between CMD and ENTRYPOINT by intuition:

• ENTRYPOINT: command to run when container starts.
• CMD: command to run when container starts or arguments to ENTRYPOINT if specified.

Yes, it's mixing up.

You can override any of them when running docker run.

Difference between CMD and ENTRYPOINT by example:

docker run -it --rm yourcontainer /bin/bash            <-- /bin/bash overrides CMD
                                                       <-- /bin/bash does not override ENTRYPOINT
docker run -it --rm --entrypoint ls yourcontainer      <-- overrides ENTRYPOINT with ls
docker run -it --rm --entrypoint ls yourcontainer  -la  <-- overrides ENTRYPOINT with ls and overrides CMD with -la

More on difference between CMD and ENTRYPOINT:

Argument to docker run such as /bin/bash overrides any CMD command we wrote in Dockerfile.

ENTRYPOINT cannot be overriden at run time with normal commands such as docker run [args]. The args at the end of docker run [args] are provided as arguments to ENTRYPOINT. In this way we can create a container which is like a normal binary such as ls.

So CMD can act as default parameters to ENTRYPOINT and then we can override the CMD args from [args].

ENTRYPOINT can be overriden with --entrypoint.