How to Understand the Entrypoint Flag in Docker

1. Understanding the Entrypoint Instruction
2. Use the Entrypoint Flag to Pass Arguments
3. Conclusion

We use the entry point (--entrypoint) flag in the docker run command to specify the command to execute when we start a Docker container. This flag allows us to customize the container’s behavior and determine the command that Docker will run beforehand.

This article will discuss an overview of the entry point instruction and Docker uses it when running the docker run command.

Understanding the Entrypoint Instruction

By default, the entry point for a Docker container is the CMD instruction in the Dockerfile used to build the image. The CMD instruction specifies the default command that Docker executes when it starts the container.

For example, the following Dockerfile defines the default entry point for the image as the echo command.

FROM ubuntu:18.04
CMD ["echo", "Hello, World!"]

We can use the docker build and docker run commands to build the image and run a container based on the image.

Example Code:

docker build -t my-image .
docker run my-image

In this example, the docker run command will start a container based on the my-image image and execute the echo command as the container’s entry point. The above command will print the "Hello, World!" message to the console.

However, if we want to specify a different entry point for the container, we can use the --entrypoint option when running the docker run command.

Example Code:

docker run --entrypoint /bin/sh my-image:latest

Use the Entrypoint Flag to Pass Arguments

In this command, the --entrypoint option specifies the container’s entry point as the /bin/sh command. This command overrides the default entry point specified in the Dockerfile.

It causes the /bin/sh to execute the command when Docker starts the container instead of the defined CMD inside the Dockerfile.

In addition to specifying the entry point of a container, we can also use the entry point flag to pass arguments to the entry point command.

Example Code:

docker run --entrypoint /bin/sh my-image:latest -c

The docker run --entrypoint command uses the following syntax.

docker run --entrypoint <entrypoint.sh> <image:tag> <arg1> <arg2> <arg3>

Conclusion

We execute the command with an --entrypoint flag to help specify the default command shell on a started container. Docker uses it to customize the container’s behavior and run specific commands or scripts on its first use.

This flag can be handy when running containers in a production environment, where you may want to ensure that the container always starts and executes a specific command.