What is the difference between CMD and ENTRYPOINT in a Dockerfil

ghz 8months ago ⋅ 97 views

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?

Answers

Both CMD and ENTRYPOINT are used in Dockerfiles to define the command that will be executed when a container starts. However, there are key differences between them:

CMD:

  1. Default Command: CMD is used to specify the default command and/or arguments that will be executed when the container starts if no other command is provided at runtime.
  2. Overridable: The command specified in CMD can be overridden by providing a command when running the container with docker run.
  3. Multiple CMDs: If multiple CMD instructions are provided in a Dockerfile, only the last one will take effect.

Example:

CMD ["nginx", "-g", "daemon off;"]

ENTRYPOINT:

  1. Executable Command: ENTRYPOINT is used to specify the executable command that will run when the container starts. It's similar to specifying a binary or script to run.
  2. Arguments: Any arguments provided to docker run will be passed as arguments to the command specified in ENTRYPOINT.
  3. Combining with CMD: You can use ENTRYPOINT with CMD to provide a default command and still allow it to be overridden. If both are specified, CMD provides default arguments to the command specified in ENTRYPOINT.
  4. Form of Use: It can be used in both exec form (["executable", "param1", "param2"]) and shell form (executable param1 param2).

Example:

ENTRYPOINT ["nginx", "-g", "daemon off;"]

Differences:

  • CMD is used to provide default arguments to the main command, while ENTRYPOINT specifies the main command itself.
  • CMD can be overridden by providing a command when running the container, while ENTRYPOINT cannot be overridden but can have its arguments appended.
  • ENTRYPOINT is often used for running the main application or script in the container, while CMD is used for specifying default options or arguments for the main command.

Combined Use:

You can combine ENTRYPOINT and CMD in a Dockerfile to create more flexible and reusable images. For example:

FROM ubuntu

# Set the default command
ENTRYPOINT ["nginx"]

# Provide default arguments to the command
CMD ["-g", "daemon off;"]

This Dockerfile would run nginx by default with the provided arguments, but you could override the arguments by specifying a different command when running the container.

#docker