What does the CMD instruction in a Dockerfile define

The CMD instruction in a Dockerfile provides defaults for an executing container. These can include an executable, or they can omit the executable, in which case you must specify an ENTRYPOINT instruction.

There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the last CMD will take effect.

The main purpose of a CMD is to provide defaults for an executing container. These defaults can include an executable, or they can omit the executable, in which case you must specify an ENTRYPOINT instruction.

CMD has three forms:

• CMD ["executable","param1","param2"] (exec form, this is the preferred form)
• CMD ["param1","param2"] (as default parameters to ENTRYPOINT)
• CMD command param1 param2 (shell form)

When used in the shell or exec formats, the CMD instruction sets the command to be executed when running the image.

If you use the shell form of the CMD, then the <command> will execute in /bin/sh -c:

For example:

CMD echo "This is a test." | wc -

If you use the exec form, it is recommended to be CMD ["executable","param1","param2"].

For example:

CMD ["echo", "This is a test."]