Docker RUN vs CMD vs ENTRYPOINT

Differences between three Docker build instructions

In Docker build, the three instructions RUN , CMD and ENTRYPOINT look similar and all are instructions used in a Dockerfile to define various aspects of a Docker image. Sometimes it can easily cause confusions. Let’s discuss their differences in this article.

RUN vs CMD vs ENTRYPOINT

• RUN executes commands and creates new image layers.
• CMD sets the command and its parameters to be executed by default after the container is started. However CMD can be replaced by docker run command line parameters.
• ENTRYPOINT configures the command to run when the container starts, similar to CMD from a functionality perspective.

Shell Format vs Exec Format

We can specify the command to be run by RUN, CMD and ENTRYPOINT in two ways: Shell format and Exec format, which have subtle differences in usage.

Shell Format

Shell format has the following form:

<instruction> <command>

For example:

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