What is the Difference Between RUN, CMD, and ENTRYPOINT in a Dockerfile

To build Docker images, a Dockerfile is used that contains instructions. These instructions are CMD, RUN, and ENTRYPOINT respectively that are used to define multiple aspects of a Docker image. Developers are often confused between them because of their work.

This blog will list and explain multiple differences among RUN, CMD, and ENTRYPOINT commands.

What is the Difference Between RUN, CMD, and ENTRYPOINT in a Dockerfile?

The RUN is used to execute the command instructions and create new image layers. The CMD is used for setting the commands and their parameters to be executed by default after the container is started. The ENTRYPOINT configures the command to execute when the container starts, similar to CMD from a functionality perspective.

Let’s check out the working of RUN, CMD, and ENTRYPOINT with the help of the below-provided instructions.

Initially, create a Dockerfile and paste the following code into it that contains the alpine base image and the RUN time instruction for Docker containers:

FROM alpine

RUN ["echo", "My new image created"]

After that, run the docker build command to build the Docker image using the Dockerfile:

docker build -t alphine_img .

As you can see, the instructions that are provided inside the Dockerfile have been executed while building the image:

Next, check the list of existing images by running the following command:

docker images

As you can see, the particular image has been created successfully:

Now, add the CMD instructions in the existing Dockerfile that will be executed when the container starts:

FROM alpine

RUN ["echo", "My new image created"]

CMD ["Hi Welcome to my LinuxHint World!!"]

After doing so, build the new Docker image by executing the provided command:

docker build -t alphine_img1 .

Now, use the docker run command to create a Docker container:

docker run --name gol_con1 alphine_img1

As you can see, the container has been created and the CMD instructions executed successfully:

To override the content of the CMD instructions, now build a container along with the new instructions by executing the following command:

docker run --name gol_container alphine_img1 "it's my LinuxHint

World"

The output of the above-executed command shows that the CMD instructions have been overriding:

Now, add the ENTRYPOINT instructions in the previously created Dockerfile that will tell the entry point to the container:

FROM nginx:latest

COPY t_prg.html /usr/share/nginx/html/index.html

ENTRYPOINT ["nginx", "-g", "daemon off;"]

Then, build a new image by executing the below-mentioned command:

docker build -t oreo_img .

Next, run the docker images command to list all the existing images:

docker images

As you can see, the newly created image exists in the list:

Now, run the following command to generate a container with an image:

docker run -d --name=sticky_con oreo_img:latest

Lastly, list the content of the container by running the ls command:

That’s all! We have described and illustrated with examples some of the differences between RUN, CMD, and ENTRYPOINT.

Conclusion

In Docker, the RUN is utilized for executing the command instructions and creating new image layers. While the CMD is used for setting the commands and their parameters, the ENTRYPOINT configures the command to execute when the container starts. This blog illustrated some of the important differences among the RUN, CMD, and ENTRYPOINT.

What is the Difference Between RUN, CMD, and ENTRYPOINT in a Dockerfile

What is the Difference Between RUN, CMD, and ENTRYPOINT in a Dockerfile?

Conclusion

About the author

Maria Naz