How to use docker exec?

To use the docker exec command, follow these steps:

Identify the Running Container: First, you need to know the name or ID of the running container you want to interact with. You can list all running containers using:
```
docker container ls
```
Run a Command Inside the Container: Use the docker exec command to execute a command inside the container. The basic syntax is:
```
docker exec [OPTIONS] CONTAINER COMMAND [ARG...]
```
Here are some common examples:
- Start an Interactive Shell:
```
docker exec -it <container_name_or_id> /bin/bash
```
  This command opens an interactive bash shell inside the specified container.
- Run a Command:
```
docker exec <container_name_or_id> ls /app
```
  This command lists the contents of the /app directory inside the specified container.
- Run a Command with Environment Variables:
```
docker exec -e VAR_NAME=value <container_name_or_id> printenv VAR_NAME
```
  This command runs printenv to display the value of the specified environment variable inside the container.
Using Options: You can use various options with docker exec, such as:
- -i: Keep STDIN open even if not attached.
- -t: Allocate a pseudo-TTY.

Here’s a complete example of starting a bash shell in a running container named my_container:

docker exec -it my_container /bin/bash

This will give you a command prompt inside the container, allowing you to run commands as if you were logged into that container directly.