Mastering Docker Exec: Your Guide to Accessing and Managing Containers

Docker containers offer amazing portability and consistency for your applications. But what happens when you need to peek inside a running container to check its status or fix a problem? That's where docker exec comes to the rescue. This guide provides practical examples of how to use docker exec to manage and interact with your Docker containers effectively. Learn how to run commands, get an interactive shell, and troubleshoot issues directly within your containers.

What is docker exec and Why Should You Care?

docker exec is a powerful Docker command that allows you to execute commands inside a running container. It's like having a remote control to interact with your container's internal environment. This is useful when you need to:

• Debug applications: Investigate issues by running debugging tools or inspecting log files.
• Manage processes: Start, stop, or restart services within the container.
• Inspect the filesystem: Examine files, directories, and configurations.

Prerequisites: Getting Ready to Use docker exec in Docker Container

Before diving in, make sure you have the following:

• Docker installed: Ensure Docker Engine is installed and running on your system.
• User Permissions: If you encounter permissions issues (sudo required), configure Docker to run without sudo or prepend sudo to the commands.

Launching a Test Container for Practice

Let's start by creating a simple container to experiment with. This command uses the Alpine Linux image, a lightweight distribution perfect for testing:

docker run -d --name my-test-container alpine watch "date >> /var/log/date.log"

This command does the following:

• -d: Runs the container in detached mode (in the background).
• --name my-test-container: Assigns the name "my-test-container" to the container.
• alpine: Specifies the Alpine Linux image.
• watch "date >> /var/log/date.log": Executes a command that appends the current date and time to a log file every two seconds. Useful for generating data.

Finding Your Docker Container's Name or ID

To use docker exec, you need to identify the container you want to access. Use this command to list running containers:

docker ps

The output will show you the CONTAINER ID and NAMES of your running containers. You can use either of these to target a container with docker exec.

Want to give that container a new name? Use the command below.

docker rename my-test-container new-container-name

Opening an Interactive Shell Inside a Docker Container

Need direct access to the container's shell? This is where the -it flags shine:

docker exec -it my-test-container sh

• -i: Keeps input open, allowing you to send commands to the container. It is the Interactive flag.
• -t: Allocates a pseudo-terminal, providing a more interactive shell experience. It is the TTY Flag.
• sh: Specifies the shell to use (e.g., bash if available).

Type exit and press ENTER to terminate the connection and return to your local terminal.

Executing a Single Command in a Docker Container Non-Interactively via docker exec

For simple tasks, you might not need an interactive shell. Run commands directly like so:

docker exec my-test-container tail /var/log/date.log

This command executes tail /var/log/date.log inside the container and prints the last ten lines of the log file to your terminal.

Specifying a Working Directory for Your Commands

Want to run a command in a specific directory inside the container? Use the --workdir flag:

docker exec --workdir /tmp my-test-container pwd

This will execute the pwd (print working directory) command in the /tmp directory within the container.

Running Commands as a Different User Within the Container

Need to execute commands as a specific user? The --user flag is your friend:

docker exec --user guest my-test-container whoami

This command executes the whoami command as the "guest" user inside the container.

Passing Environment Variables to Your Commands

Sometimes, commands need environment variables. Use the -e flag to pass them:

docker exec -e TEST_VAR=hello my-test-container env

This sets the environment variable TEST_VAR to "hello" and then runs the env command to display all environment variables. To pass multiple variables, you can repeat the -e flag. Additionally, you can pass in a file full of environmental variables using the --env-file flag.

Here is an example of using it.

nano .env

Example usage:

TEST=sammy
ENVIRONMENT=prod

Executing the command:

docker exec --env-file .env container_name env