JIYIK CN >

We docker runuse the entrypoint ( --entrypoint ) flag in the command to specify the command to be executed when we start the Docker container. This flag allows us to customize the behavior of the container and determine the command that Docker will run beforehand.

This article provides an overview of the entrypoint directive and how Docker docker runuses it when running commands.

Understanding the entry point directive

By default, the entrypoint 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 entrypoint for the image as the echo command.

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

We can use the docker buildand docker runcommands to build images and run containers based on them.

Sample code:

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

In this example, docker runthe 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 entrypoint for the container, we can use the --entrypoint option when running the docker run command.

Sample code:

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

Passing parameters using entry point flags

In this command, the **--entrypoint** option specifies the entrypoint for the container as /bin/shthe command. This command overrides the default entrypoint specified in the Dockerfile.

When Docker starts a container instead of the CMD defined in the Dockerfile, it causes /bin/sh to execute the command.

In addition to specifying the container's entrypoint, we can also pass arguments to the entrypoint command using entrypoint flags.

Sample code:

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

docker run --entrypointThe command uses the following syntax.

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

Summarize

We execute the command with the --entrypoint flag to help specify the default command shell on the launched container. Docker uses this to customize the behavior of the container and run a specific command or script when it is first used.

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