Executing commands with docker exec - Mini Project: Build & Apply

Executing commands with docker exec

📖 Scenario: You are managing a Docker container running a simple web server. You want to check the server's status and list files inside the container without stopping it.

🎯 Goal: Learn how to use docker exec to run commands inside a running container.

📋 What You'll Learn

Use docker exec to run commands inside a container

Identify the container by its name

Run simple commands like ls and cat inside the container

💡 Why This Matters

🌍 Real World

In real life, developers and system admins often need to check or fix things inside running containers without stopping them. Using <code>docker exec</code> lets you do that safely.

💼 Career

Knowing how to use <code>docker exec</code> is essential for troubleshooting, monitoring, and managing Docker containers in development and production environments.

1

Create and run a container named webserver

Run a Docker container named webserver using the nginx image in detached mode.

Docker

# Your code here

Hint

Use docker run -d --name webserver nginx to start the container in the background.

2

Set a variable with the container name

Create a shell variable called container and set it to the string webserver.

Docker

docker run -d --name webserver nginx
# Your code here

Hint

Use container=webserver to store the container name in a variable.

3

List files inside the container's /usr/share/nginx/html directory

Use docker exec with the variable container to run ls /usr/share/nginx/html inside the container.

Docker

docker run -d --name webserver nginx
container=webserver
# Your code here

Hint

Use docker exec $container ls /usr/share/nginx/html to list files inside the container.

4

Display the content of the index.html file inside the container

Use docker exec with the variable container to run cat /usr/share/nginx/html/index.html inside the container and print the output.

Docker

docker run -d --name webserver nginx
container=webserver
docker exec $container ls /usr/share/nginx/html
# Your code here

Hint

Use docker exec $container cat /usr/share/nginx/html/index.html to show the file content.

Practice

1. What does the docker exec command do?

easy

A. Runs a command inside a running container

B. Starts a new container from an image

C. Stops a running container

D. Builds a Docker image from a Dockerfile

Solution

1. Step 1: Understand the purpose of docker exec

   The docker exec command is used to run commands inside an already running container without stopping it.

2. Step 2: Compare with other Docker commands

   Starting a container is done with docker run, stopping with docker stop, and building images with docker build. These are different from docker exec.

3. Final Answer:

   Runs a command inside a running container -> Option A

4. Quick Check:

   docker exec = run command inside container [OK]

Hint: Remember: exec means execute inside running container [OK]

Common Mistakes:

• Confusing exec with docker run
• Thinking exec stops containers
• Mixing exec with image build commands

2. Which of the following is the correct syntax to start an interactive bash shell inside a running container named webapp?

easy

A. docker exec webapp bash -it

B. docker exec bash -it webapp

C. docker exec -ti bash webapp

D. docker exec -it webapp bash

Solution

1. Step 1: Recall the correct option order for docker exec

   The correct syntax places options like -it immediately after docker exec, then the container name, then the command.

2. Step 2: Validate each option

   docker exec -it webapp bash correctly uses docker exec -it webapp bash. Options A, B, and C have incorrect order of arguments or options.

3. Final Answer:

   docker exec -it webapp bash -> Option D

4. Quick Check:

   Options come right after exec, then container, then command [OK]

Hint: Options (-it) come right after exec, before container name [OK]

Common Mistakes:

• Placing options after container name
• Swapping container name and command
• Using wrong option order like -ti instead of -it

3. Given a running container named db, what will be the output of this command?

docker exec db ls /var/lib/mysql

medium

A. Shows an error because ls is not a valid command

B. Lists files on the host machine's /var/lib/mysql directory

C. Lists files and folders inside /var/lib/mysql in the container

D. Starts a new container named db and runs ls

Solution

1. Step 1: Understand what docker exec does with ls /var/lib/mysql

   The command runs ls /var/lib/mysql inside the running container named db, listing files in that path inside the container.

2. Step 2: Eliminate incorrect options

   ls is a valid command inside Linux containers, so no error. It does not list host files or start a new container.

3. Final Answer:

   Lists files and folders inside /var/lib/mysql in the container -> Option C

4. Quick Check:

   docker exec runs command inside container filesystem [OK]

Hint: Exec runs commands inside container, not on host [OK]

Common Mistakes:

• Thinking exec runs commands on host
• Assuming exec starts new containers
• Believing ls is invalid inside containers

4. You run docker exec webapp bash but get the error: the input device is not a TTY. What is the likely fix?

medium

A. Stop the container and restart it

B. Add the -it flags to enable interactive terminal

C. Use docker run instead of docker exec

D. Remove the bash command from the exec

Solution

1. Step 1: Understand the error about TTY

   The error means the command expects an interactive terminal but none was allocated.

2. Step 2: Fix by adding -it flags

   Adding -it allocates a pseudo-TTY and keeps stdin open, enabling interactive shells inside containers.

3. Final Answer:

   Add the -it flags to enable interactive terminal -> Option B

4. Quick Check:

   Interactive shells need -it flags [OK]

Hint: Use -it for interactive commands to avoid TTY errors [OK]

Common Mistakes:

• Ignoring the need for -it flags
• Restarting container unnecessarily
• Confusing exec with run command

5. You want to check the environment variables inside a running container named api. Which command correctly does this and shows output interactively?

hard

A. docker exec -it api env

B. docker exec env api

C. docker exec -it env api

D. docker exec api -it env

Solution

1. Step 1: Identify the need for interactive output

   To see environment variables interactively, you need to allocate a TTY and keep stdin open using -it.

2. Step 2: Check command syntax

   docker exec -it api env correctly places -it after docker exec, then container name api, then command env. Other options have wrong order or missing flags.

3. Final Answer:

   docker exec -it api env -> Option A

4. Quick Check:

   Use -it before container name for interactive commands [OK]

Hint: Place -it right after exec for interactive commands [OK]

Common Mistakes:

• Placing -it after container or command
• Omitting -it for interactive output
• Swapping command and container name