Executing commands with docker exec - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of docker exec with environment variable

What is the output of this command if the container named webapp has an environment variable APP_MODE=production set?

docker exec webapp printenv APP_MODE

Docker

docker exec webapp printenv APP_MODE

A APP_MODE

B Error: No such container: webapp

C printenv APP_MODE

D production

💡 Hint

The command prints the value of the environment variable inside the container.

The docker exec command runs printenv APP_MODE inside the webapp container, which outputs the value of the environment variable APP_MODE set to 'production'.

💻 Command Output

intermediate

Effect of docker exec with interactive shell

What happens when you run this command?

docker exec -it mycontainer /bin/bash

Docker

docker exec -it mycontainer /bin/bash

A Starts an interactive bash shell inside the running container named mycontainer

B Stops the container named mycontainer

C Shows the logs of the container named mycontainer

D Creates a new container named mycontainer

💡 Hint

The flags -i and -t are used for interactive terminal sessions.

The command opens an interactive bash shell inside the running container named mycontainer, allowing you to run commands inside it.

❓ Troubleshoot

advanced

Troubleshooting docker exec permission error

You run docker exec mycontainer ls /root and get a permission denied error. What is the most likely cause?

A The /root directory does not exist inside the container

B The container is not running

C The user inside the container does not have permission to access /root directory

D The docker daemon is not started

💡 Hint

Consider user permissions inside the container.

The /root directory is usually accessible only by the root user. If docker exec runs as a non-root user, permission is denied.

🔀 Workflow

advanced

Correct sequence to update a file inside a running container

You want to update a configuration file inside a running container named app. Which sequence of commands is correct?

A 2,1,3,4

B 1,2,3,4

C 1,3,2,4

D 4,1,2,3

💡 Hint

Copy the file first, then move it, verify, and restart the service.

First, copy the file into the container's temporary location, then move it to the correct path, verify the content, and finally restart the service to apply changes.

✅ Best Practice

expert

Best practice for running a command as root inside a container

Which docker exec command correctly runs apt-get update as root inside a container named db?

A docker exec -u root db apt-get update

B docker exec db -u root apt-get update

C docker exec db apt-get update -u root

D docker exec --user db root apt-get update

💡 Hint

The -u or --user flag must come before the container name.

The correct syntax is docker exec -u root db apt-get update. The -u flag specifies the user and must come before the container name.

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