docker-faq.md

Docker FAQ

RUN, CMD, ENTRYPOINT

Instruction	RUN	CMD	ENTRYPOINT
Summary	Execute "build commands".	Specify default commands.	Specify default executable.
Description	RUN executes any commands to create a new layer on top of current image	CMD sets the command to be executed when running a container from image.	An ENTRYPOINT allows you to configure a container that will run as an executable.
Example	RUN apt-get update && apt-get install -y neofetch
Notes		- The main purpose of a CMD is to provide (default command) and its parameters for an executing container.1
		- CMD can also be used to only provide default parameters for the ENTRYPOINT instruction. 2	- Command line arguments to docker run <image> will be appended after all elements in an exec form ENTRYPOINT, and will override all elements specified using CMD.
			- You can override the ENTRYPOINT instruction using the docker run --entrypoint flag.
		Provide defaults (executable and/or parameters) for an executing container	The container's main executable.

Example:

• A simple Dockerfile use RUN to install neofetch and add a layer

  FROM ubuntu:latest
  
  RUN apt-get update && apt-get install -y neofetch

• MySQL's Official Dockerfile

  FROM debian-slim:latest
  # ...
  ENTRYPOINT ["docker-entrypoint.sh"]
  # ...
  CMD ["mysqld"]

• Nginx's Official Dockerfile

  FROM debian:bookworm-slim
  # ...
  ENTRYPOINT ["/docker-entrypoint.sh"]
  # ...
  CMD ["nginx", "-g", "daemon off;"]

Exec form vs shell form

	Exec form	Shell form
Syntax	CMD [ "executable" , "param1" , "param2" ]	RUN executable param1 param2
Example	CMD [ "nginx" , "-g", "daemon off;" ]	RUN apt-get update && apt-get install -y neofetch
Advantages	- Avoid shell string munging	More relaxed, and emphasizes ease of use, flexibility, and readability
	- Invoke commands using a specific command shell, or any other executable3	- Long commands can be split up into multiple lines4
Disadvantages	There is no automatically5 using of a command shell:	There is an automatically using of a command shell6 (the default shell7).
	- There is no shell processing, e.g. variable substitution8
	- The backslashes need to be escaped9
How is it parsed?	As a JSON array10	As a regular string4

For more information, see exec and shell form | Docker

Footnotes

1. CMD [ "executable" , "param1" , "param2" ] (exec form) ↩

2. CMD [ "param1" , "param2" ] (exec form, as default parameters to ENTRYPOINT) ↩

3. You can invoke the command with any shell you want, but you need to do it for every command in the exec form. ↩

4. For example, RUN source $HOME/.bashrc && echo $HOME can be split into multiple lines

   • Using escape character

     RUN apt-get update && \
       apt-get install -y neofetch
     

   • Using heredocs

     RUN <<EOF
       apt-get update
       apt-get install -y neofetch
     EOF
     

     For more information about heredocs, see

     • Here document | Wikipedia
     • Here-Documents | Docker
   ↩ ↩²

5. You can execute a shell directly with the exec form, for example: RUN [ "sh", "-c", "echo $HOME" ] ↩

6. You can override the default shell used for the shell form of command by using SHELL instruction

   For example:

   # Change the default shell to bash
   SHELL ["/bin/bash", "-c"]
   

   ↩

7. The default shell used for the shell form of command:

   • On Linux: ["/bin/sh", "-c"]
   • On Windows: ["cmd", "/S", "/C"]
   ↩

8. RUN [ "echo", "$HOME" ] won't handle variable substitution for $HOME, but ↩

9. On a Windows machine: RUN ["c:\\windows\\system32\\tasklist.exe"] ↩

10. You must use double-quotes (") around words, not single-quotes ('). e.g. ENTRYPOINT ["/bin/bash", "-c", "echo hello"] ↩