Executing commands with docker exec - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the docker exec command do?

It runs a new command inside a running Docker container without starting a new container.

beginner

How do you run an interactive shell inside a running container named webapp?

Use docker exec -it webapp /bin/bash to open an interactive bash shell inside the container.

intermediate

What does the -it option mean in docker exec?

-i keeps STDIN open, and -t allocates a pseudo-TTY, allowing interactive commands.

beginner

Can you use docker exec to run commands in a stopped container?

No, the container must be running to use docker exec.

beginner

How do you run a one-time command like ls /app inside a container named backend?

Run docker exec backend ls /app to list the contents of the /app directory inside the container.

What is required before you can use docker exec to run a command inside a container?

A The container must be running

B The container must be stopped

C Docker daemon must be stopped

D The container must be paused

You can only run commands inside containers that are currently running.

Which option allows you to run an interactive shell inside a container?

A -it

B -d

C -rm

D -p

The -it option keeps input open and allocates a terminal for interaction.

What happens if you run docker exec without specifying a command?

A It starts a new container

B It lists running containers

C It stops the container

D It returns an error

You must specify a command to run inside the container with docker exec.

How do you run a command inside a container named db to check the current directory?

A docker start db pwd

B docker run db pwd

C docker exec db pwd

D docker stop db pwd

docker exec db pwd runs the pwd command inside the running container named db.

Which of these commands opens an interactive shell inside a container named app?

A docker run -it app /bin/sh

B docker exec -it app /bin/sh

C docker exec app ls

D docker start -it app

Using docker exec -it app /bin/sh opens an interactive shell inside the running container.

Explain how to use docker exec to run an interactive shell inside a running container.

Think about how to keep input open and allocate a terminal.

Describe the limitations of docker exec regarding container state and command execution.

Consider when and how <code>docker exec</code> works.

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