🐳 Day 5: Mastering docker exec Command - With examples!

Welcome to this Docker tutorial! In this guide, we cover the docker exec command. Whether you're new to Docker or just need a refresher, this will help you understand how to run commands inside a running container. Let's dive in! 🎯

🎬 Video Demonstration

📌 1. What is docker exec?

The docker exec command allows you to run a command in a running container.

🧠 It's like opening a terminal inside your container!

❔ Get Help using help command

docker exec --help

Usage:  docker exec [OPTIONS] CONTAINER COMMAND [ARG...]

Execute a command in a running container

Aliases:
  docker container exec, docker exec

Options:
  -d, --detach               Detached mode: run command in the background
      --detach-keys string   Override the key sequence for detaching a container
  -e, --env list             Set environment variables
      --env-file list        Read in a file of environment variables
  -i, --interactive          Keep STDIN open even if not attached
      --privileged           Give extended privileges to the command
  -t, --tty                  Allocate a pseudo-TTY
  -u, --user string          Username or UID (format: "<name|uid>[:<group|gid>]")
  -w, --workdir string       Working directory inside the container

❓ 2. Why Use docker exec?

Here are some common reasons to use docker exec:

• 🐞 Debugging: Access a container’s shell to troubleshoot issues.

• 🔍 Inspection: Check files, processes, or environment variables inside a container.

• 🛠️ Management: Run one-off commands or scripts without modifying the container’s image.

• 🗨️ Interactivity: Start an interactive session (e.g., a bash shell) to work inside the container.

🛠️ 3. Syntax

docker exec [OPTIONS] CONTAINER COMMAND [ARG...]

• CONTAINER: The name or ID of the running container (find it using docker ps).

• COMMAND: The command you want to run inside the container.

• ARG: Optional arguments for the command.

• OPTIONS:

  • -i, --interactive: Keep STDIN open.

  • -t, --tty: Allocate a pseudo-TTY.

  • -d, --detach: Run the command in the background.

  • -u, --user: Run the command as a specific user.

  • -e, --env: Set environment variables.

  • --workdir: Set the working directory inside the container.

Summery

🧩 -i → Interactive

• Keeps STDIN (standard input) open even if not attached.

• Allows you to send input to the container, such as typing commands or piping input.

🖥️ -t → TTY (Pseudo-TTY)

• Allocates a pseudo-terminal.

• Makes the session act like a real terminal — this enables features like:

  • Colored output

  • Line editing

  • Clear formatting

Test Case

docker exec -i — you can pipe input in, but the terminal behaves oddly.
docker exec -t — may hang, since it expects interactive input.
docker exec -it ubuntu apt update -y

🚀 4. Basic Example

Check the current directory inside the container:

docker exec ubuntu pwd

➡️ Use Case: Quickly verify the working directory or check system details.

🖥️ 5. Open an Interactive Shell

To start a bash shell inside the container:

docker exec -it ubuntu bash
docker exec -it alpine bash

🔁 If bash isn't available, try sh:

docker exec -it alpine sh

🔒 6. Run as a Specific User

Run a command as the root user:

docker exec -u root ubuntu whoami

➡️ Use Case: Perform actions requiring elevated permissions.

📜 7. Run a Script Inside the Container

To run a script in the background:

docker exec -it ubuntu /root/setup.sh

➡️ Use Case: Execute long-running tasks without tying up your terminal.

🌱 8. Set Environment Variables

Set an environment variable for the command:

docker exec -e user=postgres ubuntu env

➡️ Use Case: Test how environment variables affect a command.

📂 9. Run a Command in a Specific Directory

Run a command in a specific working directory:

docker exec --workdir /root ubuntu pwd
docker exec --workdir /root ubuntu ls -alsh

➡️ Use Case: Execute commands in a specific directory without altering container defaults.

✅ 10. Best Practices

• ✅ Always use -it for interactive commands.

• ✅ Ensure your container is running before executing commands.

• ✅ Use docker ps to check container status.

• ❌ Avoid running long-lived processes via exec — it's for one-off tasks.

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🔗 Additional Resources

• 📘 Official Docker exec Documentation

• 🧰 Docker CLI Reference

• 🐋 Docker Hub

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Happy Docking! 🐳💻