Opening a shell in container in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Opening an interactive shell inside a running container

You have a running Docker container named webapp. Which command opens an interactive bash shell inside it?

A docker exec -it webapp bash

B docker run -it webapp bash

C docker start -it webapp bash

D docker attach webapp bash

💡 Hint

Use docker exec to run commands inside a running container.

docker exec -it webapp bash opens an interactive bash shell inside the running container named webapp. docker run starts a new container, docker start only starts a stopped container without running commands, and docker attach connects to the main process's input/output but does not accept a command argument.

💻 Command Output

intermediate

Opening a shell in a container without bash

You want to open a shell inside a running container named db, but it does not have bash installed. Which command will open a shell using sh instead?

A docker exec -it db bash

B docker exec -it db sh

C docker run -it db sh

D docker attach db sh

💡 Hint

Try using a simpler shell like sh if bash is missing.

docker exec -it db sh opens an interactive shell using sh inside the running container named db. docker run starts a new container, and docker attach does not accept a command argument.

❓ Troubleshoot

advanced

Why does 'docker exec -it container_name bash' fail with 'exec: "bash": executable file not found'?

You run docker exec -it mycontainer bash but get the error exec: "bash": executable file not found in $PATH. What is the most likely cause?

A The container image does not have bash installed.

B The container is not running.

C You forgot to use sudo before the command.

D The Docker daemon is not running.

💡 Hint

Check what shells are available inside the container.

The error means the bash program is missing inside the container. Some minimal images only have sh or other shells. The container being stopped or Docker daemon down would cause different errors. sudo is not always required.

🔀 Workflow

advanced

Steps to open a shell in a stopped container

You have a stopped container named app. Which sequence of commands correctly opens an interactive shell inside it?

A 2,1

B 3,4

C 1,2

D 4,1

💡 Hint

You must start the container before executing commands inside it.

First, docker start app starts the stopped container. Then, docker exec -it app bash opens an interactive shell inside it. docker run creates a new container, and docker attach connects to the main process but does not start the container if stopped.

✅ Best Practice

expert

Best practice for opening a shell in a container for debugging

Which practice is best when you want to debug a running container without affecting its main process?

A Use <code>docker attach container_name</code> to connect to the main process.

B Stop the container and run <code>docker run -it container_name bash</code>.

C Restart the container with <code>docker restart container_name</code> and then open a shell.

D Use <code>docker exec -it container_name sh</code> to open a new shell session.

💡 Hint

Opening a separate shell session avoids interfering with the main process.

docker exec -it container_name sh opens a new shell inside the running container, allowing safe debugging without disturbing the main process. docker attach connects to the main process's input/output and can disrupt it. Restarting or stopping the container interrupts service.

Practice

1. What is the main purpose of the command docker exec -it <container> /bin/bash?

easy

A. To stop a running container

B. To start a new container from an image

C. To open an interactive shell inside a running container

D. To build a Docker image

Solution

1. Step 1: Understand the command components

   The command uses docker exec to run a command inside a running container. The -it flags make the session interactive and allocate a terminal.

2. Step 2: Identify the command's effect

   Using /bin/bash opens a shell inside the container, allowing direct interaction.

3. Final Answer:

   To open an interactive shell inside a running container -> Option C

4. Quick Check:

   docker exec -it container /bin/bash = open shell [OK]

Hint: Remember exec + -it + shell opens container shell [OK]

Common Mistakes:

• Confusing exec with docker run
• Forgetting the -it flags for interactivity
• Trying to open shell in a stopped container

2. Which of the following is the correct syntax to open a shell inside a running Docker container named webapp?

easy

A. docker exec -it webapp /bin/sh

B. docker start -it webapp /bin/sh

C. docker run -it webapp /bin/sh

D. docker exec webapp -it /bin/sh

Solution

1. Step 1: Recall the correct order of docker exec flags

   The -it flags must come immediately after docker exec and before the container name.

2. Step 2: Check the command structure

   The correct syntax is docker exec -it <container> <shell>. docker exec -it webapp /bin/sh matches this exactly.

3. Final Answer:

   docker exec -it webapp /bin/sh -> Option A

4. Quick Check:

   Flags -it come before container name [OK]

Hint: Flags -it always come right after exec command [OK]

Common Mistakes:

• Placing -it after container name
• Using docker run instead of exec
• Using docker start with shell command

3. Given a running container named db, what will be the output of the command docker exec -it db /bin/false?

medium

A. The command exits immediately with no shell opened

B. Error: container not found

C. An interactive shell opens inside the container

D. The container stops running

Solution

1. Step 1: Understand the command /bin/false

   The command /bin/false is a program that immediately exits with a failure status and does not open a shell.

2. Step 2: Predict the command behavior inside the container

   Running docker exec -it db /bin/false will run /bin/false inside the container, which exits immediately without opening a shell.

3. Final Answer:

   The command exits immediately with no shell opened -> Option A

4. Quick Check:

   /bin/false exits immediately, no shell [OK]

Hint: Commands like /bin/false exit immediately, no shell [OK]

Common Mistakes:

• Expecting a shell to open
• Confusing container not found error
• Thinking container stops after exec

4. You try to open a shell inside a running container with docker exec -it mycontainer /bin/bash but get the error: OCI runtime exec failed: exec failed: container_linux.go:xxx: exec: "/bin/bash": stat /bin/bash: no such file or directory. What is the best way to fix this?

medium

A. Restart the Docker daemon

B. Use /bin/sh instead of /bin/bash

C. Rebuild the container image

D. Stop and remove the container

Solution

1. Step 1: Analyze the error message

   The error says /bin/bash does not exist inside the container, so the shell is missing.

2. Step 2: Choose an alternative shell

   Many containers have /bin/sh instead of /bin/bash. Using /bin/sh usually works.

3. Final Answer:

   Use /bin/sh instead of /bin/bash -> Option B

4. Quick Check:

   Use available shell like /bin/sh if /bin/bash missing [OK]

Hint: Try /bin/sh if /bin/bash not found inside container [OK]

Common Mistakes:

• Restarting Docker daemon unnecessarily
• Rebuilding image without checking shell availability
• Removing container without trying alternative shells

5. You want to open a shell inside a running container named app but it does not have /bin/bash or /bin/sh. Which command can you try to open a shell if ash is installed inside the container?

hard

A. docker exec -it app /bin/zsh

B. docker exec -it app /bin/bash

C. docker exec -it app /bin/sh

D. docker exec -it app /bin/ash

Solution

1. Step 1: Identify available shells inside container

   If /bin/bash and /bin/sh are missing, but ash is installed, you can use /bin/ash to open a shell.

2. Step 2: Use docker exec with the correct shell path

   The command docker exec -it app /bin/ash opens an interactive shell using ash inside the container.

3. Final Answer:

   docker exec -it app /bin/ash -> Option D

4. Quick Check:

   Use shell installed inside container, e.g. /bin/ash [OK]

Hint: Try shells like /bin/ash if bash and sh missing [OK]

Common Mistakes:

• Assuming bash or sh always exist
• Using wrong shell path
• Not checking installed shells inside container