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Docker: Executing Commands in a Running Container

Sometimes you may need to execute commands inside a running Docker container, without disturbing the running process. This can be useful for debugging, troubleshooting, or performing administrative tasks.

Step 1: Start an nginx Container in detached mode​

To start an nginx container, you can use the following command:

docker run -d --name my-nginx -p 8090:80 nginx

This command will start a new nginx container in detached mode, mapping port 8090 on the host to port 80 in the container, and give the custom name my-nginx to the container.

Step 2: Tapping into the container​

To execute commands inside the running my-nginx container, without disturbing the running process, you can use the docker exec command.

To list files inside the container, you could run:

docker exec -it my-nginx ls
  • You can use either the container name or the container ID to reference the container.

But this may not be that useful, as you might want to run more complex commands inside the container. So instead we want to open a bash shell:

docker exec -it my-nginx bash

This will give you an interactive shell inside the container, where you can run any command as if you were logged into the container's operating system. For example, you could update packages, modify configuration files, or inspect logs. When you're done, you can type exit to leave the shell and return to your host system.

Step 3: Updating packages inside the container​

Once you have a shell inside the container, you can update the package lists and upgrade the installed packages just like you would on a regular Linux system. For example, to update the package lists and upgrade all packages in an Ubuntu-based container, you could run:

apt update
apt upgrade

Keep in mind that changes made inside the container will not persist if the container is removed.

Step 4: Installing packagess inside the container​

When inside the container, try installing a text editor like vim or nano:

apt install vim

or

apt install nano

This will allow you to edit files within the container using your preferred text editor.

Step 5: Changing webcontent inside the nginx webserver​

To change the web content served by the nginx server, you need to modify the files in the /usr/share/nginx/html directory inside the container. You can use your preferred text editor (nano is the easiest) to edit these files.

First you need to access the shell inside the container:

docker exec -it my-nginx bash

For example, to change the default index.html file, you could run:

nano /usr/share/nginx/html/index.html

This will open the index.html file in the nano text editor.

Try changing the content of the <h1>-tag to something like:

<h1>Hello from my-nginx container!</h1>

To save the changes, press CTRL+X, then Y, and finally ENTER.

Step 6: Verifying the changes​

After making changes to the index.html file, you can verify that the changes have taken effect by accessing the web page in your browser. Navigate to http://localhost:8090 and you should see the updated content.