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
The command is run via the entrypoint. i.e., 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 were introduced.
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:
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
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.