Opening a shell in container in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What command lets you open an interactive shell inside a running Docker container?

Use docker exec -it <container_name_or_id> /bin/bash to open an interactive bash shell inside a running container.

beginner

What does the -it option mean in docker exec -it?

-i means interactive mode to keep STDIN open, and -t allocates a pseudo-TTY so you get a terminal interface.

beginner

How do you open a shell in a container that uses sh instead of bash?

Use docker exec -it <container> /bin/sh because some containers have only sh shell.

intermediate

What is the difference between docker exec and docker attach?

docker exec runs a new command (like a shell) inside the container, while docker attach connects to the main process's input/output of the container.

beginner

Why might you want to open a shell inside a running container?

To inspect the container's file system, debug running processes, or manually run commands inside the container environment.

Which command opens an interactive bash shell inside a running Docker container named 'webapp'?

A docker run -it webapp /bin/bash

B docker exec -it webapp /bin/bash

C docker start -it webapp /bin/bash

D docker attach webapp /bin/bash

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

What does the -t option do in the command docker exec -it?

A Runs the container in detached mode

B Stops the container after command finishes

C Specifies the target container

D Allocates a pseudo-TTY for terminal interface

-t allocates a pseudo-terminal so you get a terminal interface.

If a container does not have bash installed, which shell is commonly available?

A /bin/sh

B /bin/zsh

C /bin/fish

D /bin/csh

Most containers have /bin/sh as a minimal shell.

Which command connects to the main process's input/output of a running container?

A docker attach

B docker exec

C docker run

D docker shell

docker attach connects to the main process's input/output streams.

Why is opening a shell inside a container useful?

A To permanently change container settings

B To delete the container

C To inspect and debug the container environment

D To update Docker software

Opening a shell helps inspect files and debug inside the container.

Explain how to open an interactive shell inside a running Docker container and why you might do this.

Think about the command and the purpose of opening a shell.

Describe the difference between docker exec and docker attach commands.

Consider how each command interacts with the container.

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