02. Manipulating Containers with the Docker Client

Listing Containers

docker ps
docker ps --all

Container Lifecyle

docker run = docker create + docker start

Restarting Stopped Containers

docker start ID

Removing Stopped Containers

docker system prune

Retrieving Log Outputs

docker logs ID

Stopping Containers

docker stop ID
# SIGTERM, 10 Seconds

docker kill ID
# SIGKILL

Quiz

• Q1: You need to debug a container that ran yesterday and exited. Which command would help you see what happened without restarting it?
  • A1: docker logs CONTAINER_ID
• Q2: A container with status “Exited (0)” in docker ps -a indicates what?
  • A2: The container’s main process completed successfully.
• Q3: Your team member created a container that processes data files. The container ID is abc123. How would you run the same processing job again?
  • A3: docker start abc123
• Q4: A container was created with docker create ubuntu echo "test". After starting it once, can you change it to run echo "production" instead?
  • A4: No, you must create a new container with the new command. The command echo "test" becomes part of the container’s immutable configuration.

Executing Commands in Running Containers

docker exec -it <container id> <command>

• docker: Reference the Docker Client
• exec: Run another command
• -it: Allow us to provide input to the container

The Purpose of the IT Flag

• Process in Linux Environment
  • Stuff you type -> STDIN
  • STDOUT -> Stuff that shows up on the screen
  • STDERR -> Stuff that shows up on the screen
• -it
  • -i: STDIN in process
  • -t: text nicely

Getting a Command Prompt in a Container

• Starting with a Shell

docker exec -it <container id> sh
docker run -it <container id> sh

Quize

• A1: A container is running a Node.js application. You need to check with npm packages are installed. Which command would accomplish this?
  • Q1: docker exec CONTAINER_ID npm list
• A2: What is the relationship between stdin, stdout, and the -it flags?
  • Q2: -i connects to stdin, -t formats the terminal output.
• A3: A container process writes error messages. If you run the container without specifying any attach flags (such as -a stderr), where will the stderr output typically go?
  • Q3: Without proper flags, stderr output would be lost.
• A4: You need to debug a web application running in a container. Which approach would give you the most flexibility for troubleshooting?
  • Q4: Using docker exec -it CONTAINER_ID sh to get shell access. You can inspect logs, config files, environment variables, network settings, and running services from inside the environment.