Dockerfile Instructions (CMD, ENTRYPOINT, COPY, ADD)

Understanding the nuances of core Dockerfile commands.

Edit Concept ✏️

Dockerfile Instructions

Writing efficient Dockerfiles requires understanding the specific behavior of each instruction.

1. CMD vs. ENTRYPOINT

Both instructions define what command runs when a container starts, but they behave differently:

  • ENTRYPOINT: Sets the main executable for the image. It is not easily overridden by command-line arguments. Use this for containers that should act like executables (e.g., a CLI tool).
  • CMD: Sets default arguments for the ENTRYPOINT or a default command. It is easily overridden by providing arguments to docker run.

The Pattern: Usually, you use ENTRYPOINT for the binary and CMD for the default flags.

2. COPY vs. ADD

  • COPY: The preferred instruction. It simply copies files or directories from the host into the container.
  • ADD: Does everything COPY does, but also has extra features: it can fetch files from remote URLs and automatically extract tarballs. Best Practice: Use COPY unless you explicitly need ADD’s extra features to keep the build process transparent.

3. Shell vs. Exec Form

Most instructions can be written in two ways:

  • Shell form: CMD executable param1 (runs as /bin/sh -c)
  • Exec form: CMD ["executable", "param1"] (preferred; runs the executable directly, allowing it to receive Unix signals like SIGTERM).