Intro

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Entrypoint vs CMD

Shell and Exec forms

All three instructions (RUN, CMD and ENTRYPOINT) can be specified in shell form or exec form. Let’s get familiar with these forms first, because the forms usually cause more confusion than instructions themselves.

Shell form

<instruction> <command>

Examples:

RUN apt-get install python3
CMD echo "Hello world"
ENTRYPOINT echo "Hello world"

When instruction is executed in shell form it calls /bin/sh -c \ under the hood and normal shell processing happens. For example, the following snippet in Dockerfile

ENV name John Dow
ENTRYPOINT echo "Hello, $name"

when container runs as docker run -it \ will produce output

Hello, John Dow

Note that variable name is replaced with its value.

Exec form

This is the preferred form for CMD and ENTRYPOINT instructions.

\ ["executable", "param1", "param2", ...]

Examples:

RUN ["apt-get", "install", "python3"]
CMD ["/bin/echo", "Hello world"]
ENTRYPOINT ["/bin/echo", "Hello world"]

When instruction is executed in exec form it calls executable directly, and shell processing does not happen. For example, the following snippet in Dockerfile

ENV name John Dow
ENTRYPOINT ["/bin/echo", "Hello, $name"]

when container runs as docker run -it \ will produce output Hello, $name

Note that variable name is not substituted.

CMD

CMD instruction allows you to set a default command, which will be executed only when you run container without specifying a command. If Docker container runs with a command, the default command will be ignored. If Dockerfile has more than one CMD instruction, all but last CMD instructions are ignored.

CMD has three forms:

• CMD ["executable","param1","param2"] (exec form, preferred)
• CMD ["param1","param2"] (sets additional default parameters for ENTRYPOINT in exec form)
• CMD command param1 param2 (shell form)

The first and third forms were explained in Shell and Exec forms section. The second one is used together with ENTRYPOINT instruction in exec form. It sets default parameters that will be added after ENTRYPOINT parameters if container runs without command line arguments. See ENTRYPOINT for example.

ENTRYPOINT

ENTRYPOINT instruction allows you to configure a container that will run as an executable. It looks similar to CMD, because it also allows you to specify a command with parameters. The difference is ENTRYPOINT command and parameters are not ignored when Docker container runs with command line parameters. (There is a way to ignore ENTTRYPOINT, but it is unlikely that you will do it.)

ENTRYPOINT has two forms:

• ENTRYPOINT ["executable", "param1", "param2"] (exec form, preferred)
• ENTRYPOINT command param1 param2 (shell form)

Be very careful when choosing ENTRYPOINT form, because forms behaviour differs significantly.

Exec form

Exec form of ENTRYPOINT allows you to set commands and parameters and then use either form of CMD to set additional parameters that are more likely to be changed. ENTRYPOINT arguments are always used, while CMD ones can be overwritten by command line arguments provided when Docker container runs.

ENTRYPOINT ["/bin/echo", "Hello"]
CMD ["world"]

when container runs as docker run -it <image> will produce output

Hello world

but when container runs as docker run -it <image> John will result in

Hello John

Shell form

Shell form of ENTRYPOINT ignores any CMD or docker run command line arguments.