CMD vs ENTRYPOINT in Docker: A Comparative Analysis for Container Orchestration

In Docker, both CMD and ENTRYPOINT instructions used to specify what command should be run when a container starts. However, they have different purposes and behaviors.

CMD

• The CMD instruction sets the default command to be executed when the container starts.
• CMD can be overridden at runtime by passing arguments to docker run after specifying the image name.
• If the Dockerfile contains multiple CMD instructions, only the last one will take effect.

ENTRYPOINT

• The ENTRYPOINT instruction sets the primary command that should be executed when the container starts.
• It’s used to provide a fixed command that cannot be overridden at runtime.
• Arguments passed to docker run will be appended to the entrypoint command.

Example 1

Let’s create a real-world example using a Python application that accepts command-line arguments.

• We’ll create a simple Python script that greets a user with a custom message.

# greet.py
import sys
def greet(name):
    print(f"Hello, {name}!\nWelcome to DevOps learning.\nAuthor - Sandeep Siyadri")
if __name__ == "__main__":
    args = sys.argv[1:]
    if args:
        greet(args[0])
    else:
        greet("There")

This script accepts a name as a command-line argument and greets the user with “Hello, {name}!”. If no argument is provided, it defaults to greeting “There”.

• Dockerfile to containerize this application.

# Base Image
FROM python:3.9-slim
LABEL maintainer="Sandeep Siyadri"
# Setup the project working directory 
WORKDIR /app
# Copy files from outside to the docker image
COPY greet.py .
# Using ENTRYPOINT to set the primary command. Executed at the runtime and cannot be overridden. 
ENTRYPOINT ["python", "greet.py"]
# Using CMD to provide default arguments. This can be overridden during the run time. This is passed as argument for the Entrypoint. 
CMD ["World"]

• Notice the default greeting in the greet.py script is set to “There” whereas the default CMD in the Dockerfile is defined as “World”. So when we run the container without any argument, ${name} will print “World” otherwise print the argument whatever passed during the runtime.

Let’s build the image.

ubuntu@ip-172-31-29-224:~/python-example$ docker build -t greeting-app .
Sending build context to Docker daemon  3.072kB
Step 1/5 : FROM python:3.9-slim
3.9-slim: Pulling from library/python
8a1e25ce7c4f: Pull complete 
1103112ebfc4: Pull complete 
6e52db3290c0: Pull complete 
937bce5dbc70: Pull complete 
05e63546fee1: Pull complete 
Digest: sha256:df78d66895cd3b12ec57c451f9776192a535688e27ab8a72c03896b17dbb4b98
Status: Downloaded newer image for python:3.9-slim
 ---> 500c1b793e9d
Step 2/5 : WORKDIR /app
 ---> Running in 8bed47b0c853
Removing intermediate container 8bed47b0c853
 ---> f20db9f7794e
Step 3/5 : COPY greet.py .
 ---> 45b887dd3385
Step 4/5 : ENTRYPOINT ["python", "greet.py"]
 ---> Running in 4652b100a38a
Removing intermediate container 4652b100a38a
 ---> 86be353eaa0e
Step 5/5 : CMD ["World"]
 ---> Running in c1c50da087cf
Removing intermediate container c1c50da087cf
 ---> dbf8d7d594bf
Successfully built dbf8d7d594bf
Successfully tagged greeting-app:latest

ubuntu@ip-172-31-29-224:~/python-example$ docker images
REPOSITORY          TAG        IMAGE ID       CREATED          SIZE
greeting-app        latest     dbf8d7d594bf   5 seconds ago    126MB

• Run the container without specifying user argument.

ubuntu@ip-172-31-29-224:~/python-example$ docker run greeting-app
Hello, World!
Welcome to the Docker Learning.
Presenter: Sandeep Siyadri

• Run the container specifying user argument and verify the message in the output.

ubuntu@ip-172-31-29-224:~/python-example$ docker run greeting-app Shiva
Hello, Shiva!
Welcome to the Docker Learning.
Presenter: Sandeep Siyadri

Note

• In a Dockerfile, only the last CMD instruction will take effect. So there is no practical use case to define multiple CMD instructions within the same Dockerfile.
• This behavior ensures clarity and avoids confusion about the container’s default behavior.

To explore more about these Dockerfile instructions and much more related to Docker and DevOps, refer https://github.com/sandeepsiyadri/learn-devops/blob/main/docker/cmd-vs-entrypoint/understand-cmd-vs-entrypont.md.

Thanks for reading this. Keep Learning!

Have any questions? Let us know in the comments.