What is the difference between CMD and ENTRYPOINT in a Dockerfile?
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?
Docker has a default entrypoint which is /bin/sh -c but does not have a default command.
When you run docker like this: docker run -i -t ubuntu bash the entrypoint is the default /bin/sh -c , the image is ubuntu and the command is bash .
The command is run via the entrypoint. ie, the actual thing that gets executed is /bin/sh -c bash . This allowed Docker to implement RUN quickly by relying on the shell's parser.
Later on, people asked to be able to customize this, so ENTRYPOINT and --entrypoint were introduced.
Everything after ubuntu in the example above is the command and is passed to the entrypoint. When using the CMD instruction, it is exactly as if you were doing docker run -i -t ubuntu <cmd> . <cmd> will be the parameter of the entrypoint.
You will also get the same result if you instead type this command docker run -i -t ubuntu . You will still start a bash shell in the container because of the ubuntu Dockerfile specified a default CMD: CMD ["bash"]
As everything is passed to the entrypoint, you can have a very nice behavior from your images. @Jiri example is good, it shows how to use an image as a "binary". When using ["/bin/cat"] as entrypoint and then doing docker run img /etc/passwd , you get it, /etc/passwd is the command and is passed to the entrypoint so the end result execution is simply /bin/cat /etc/passwd .
Another example would be to have any cli as entrypoint. For instance, if you have a redis image, instead of running docker run redisimg redis -H something -u toto get key , you can simply have ENTRYPOINT ["redis", "-H", "something", "-u", "toto"] and then run like this for the same result: docker run redisimg get key .
The ENTRYPOINT specifies a command that will always be executed when the container starts.
The CMD specifies arguments that will be fed to the ENTRYPOINT .
If you want to make an image dedicated to a specific command you will use ENTRYPOINT ["/path/dedicated_command"]
Otherwise, if you want to make an image for general purpose, you can leave ENTRYPOINT unspecified and use CMD ["/path/dedicated_command"] as you will be able to override the setting by supplying arguments to docker run .
For example, if your Dockerfile is:
FROM debian:wheezy
ENTRYPOINT ["/bin/ping"]
CMD ["localhost"]
Running the image without any argument will ping the localhost:
$ docker run -it test
PING localhost (127.0.0.1): 48 data bytes
56 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.096 ms
56 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.088 ms
56 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.088 ms
^C--- localhost ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.088/0.091/0.096/0.000 ms
Now, running the image with an argument will ping the argument:
$ docker run -it test google.com
PING google.com (173.194.45.70): 48 data bytes
56 bytes from 173.194.45.70: icmp_seq=0 ttl=55 time=32.583 ms
56 bytes from 173.194.45.70: icmp_seq=2 ttl=55 time=30.327 ms
56 bytes from 173.194.45.70: icmp_seq=4 ttl=55 time=46.379 ms
^C--- google.com ping statistics ---
5 packets transmitted, 3 packets received, 40% packet loss
round-trip min/avg/max/stddev = 30.327/36.430/46.379/7.095 ms
For comparison, if your Dockerfile is:
FROM debian:wheezy
CMD ["/bin/ping", "localhost"]
Running the image without any argument will ping the localhost:
$ docker run -it test
PING localhost (127.0.0.1): 48 data bytes
56 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.076 ms
56 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.087 ms
56 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.090 ms
^C--- localhost ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.076/0.084/0.090/0.000 ms
But running the image with an argument will run the argument:
docker run -it test bash
root@e8bb7249b843:/#
See this article from Brian DeHamer for even more details: https://www.ctl.io/developers/blog/post/dockerfile-entrypoint-vs-cmd/
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
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