How do we tell what command docker container will run after starting

To determine what command a container will run after it starts, you can examine the ENTRYPOINT and CMD instructions specified in the Dockerfile used to build the container image. These instructions define the default executable or command that will run when a container starts.

Understanding ENTRYPOINT and CMD

• ENTRYPOINT: Sets the command and arguments that will be executed when the container starts. It is intended to define the main purpose of the container.
• CMD: Provides default arguments for the ENTRYPOINT instruction. If ENTRYPOINT is not specified, CMD can also act as the command to run.

How to Check ENTRYPOINT and CMD

1. Dockerfile: If you have access to the Dockerfile, you can look for the ENTRYPOINT and CMD instructions. Here is an example Dockerfile:

   FROM ubuntu:latest
   ENTRYPOINT ["python3"]
   CMD ["app.py"]
   

   In this case, the container will run python3 app.py when it starts.

2. Docker Inspect: If you do not have access to the Dockerfile but have the image or a running container, you can use the docker inspect command to retrieve this information.

   • For an image:

     docker inspect <image_name_or_id>
     

   • For a running container:

     docker inspect <container_name_or_id>
     

   Look for the Config section in the output, which will contain Entrypoint and Cmd fields. Here's an example output snippet:

   "Config": {
       "Cmd": [
           "app.py"
       ],
       "Entrypoint": [
           "python3"
       ],
       ...
   }
   

   This output indicates that the container will run python3 app.py.

Practical Examples

1. Using Docker Inspect:

   Suppose you have a running container named my_container. To inspect it, you would run:

   docker inspect my_container
   

   This command outputs detailed information about the container, including the command it will execute.

2. Example Output Interpretation:

   [
       {
           "Id": "c3d0bfc96c8f",
           "Config": {
               "Cmd": [
                   "app.py"
               ],
               "Entrypoint": [
                   "python3"
               ],
               ...
           },
           ...
       }
   ]
   

   From this, you can see that python3 app.py is the command executed when the container starts.