Docker Cleaning Up Cli Lab

Lab 1: The "Junk Generator" (Visualize the problem)

• Check Space: docker system df (Note the numbers).

• Create Junk:

• Run a container: docker run -d --name junk-app alpine sleep 1000

• Stop it: docker stop junk-app

• Pull an image: docker pull busybox

• Check Space Again: docker system df (Reclaimable space increased).

• Clean Up: docker system prune -f

• Verify: docker ps -a (The stopped container is gone). docker images (Busybox is still there! Why? Because it is not "dangling", just unused).

• Deep Clean: docker system prune -a -f

• Verify: docker images (Busybox is gone).

Lab 2: The "Orphan Volume" Hunt

• Create Volume: docker volume create orphan-vol

• Check List: docker volume ls

• Prune: docker volume prune -f

• Check List: docker volume ls (It’s gone).

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Lab 1: The "Time Machine" Clean (Using Filters)

Goal: Automate cleanup safely by only deleting old junk.

• Create "Old" and "New" Junk:

• We can't actually fake time easily, so we will use the logic.

• Pull an image: docker pull hello-world (This is "New").

• The Command:

• docker system prune -a --filter "until=24h"

• Explanation: This command looks for junk created more than 24 hours ago.

• Result: Since you just pulled hello-world, it is preserved.

• The Comparison:

• Now run: docker system prune -a (Without filter).

• Result: hello-world gets deleted because it is not currently running.

• Lesson: Use until=24h in your cron jobs (automated scripts) to avoid deleting images you just built but haven't started yet.

Lab 2: The "Dependency Hell" (Why can't I delete this?)

Goal: Understand image layers and why Docker sometimes refuses to delete.

• Create Base Image:

• Create file Dockerfile.base: FROM alpine

• Build: docker build -t my-base:v1 -f Dockerfile.base .

• Create Child Image:

• Create file Dockerfile.child: FROM my-base:v1

• Build: docker build -t my-child:v1 -f Dockerfile.child .

• Attempt Deletion:

• docker rmi my-base:v1

• Result:

• Error: image is being used by ...

• Why? Docker protects you. If you delete the base, the child breaks.

• Fix:

• Delete the child first: docker rmi my-child:v1

• Now delete the base: docker rmi my-base:v1

• Success!

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Lab 3: The "Silent Disk Killer" (Log Explosion)

Goal: Simulate a container filling the disk with logs and fix it without restarting.

• Create the Problem:

• Run a container that spams logs endlessly.

• docker run -d --name spammer alpine sh -c "while true; do echo 'CRITICAL ERROR: Disk is filling up fast...'; sleep 0.01; done"

• Observe the Growth:

• Let it run for 30 seconds.

• Find the log file: LOG_PATH=$(docker inspect --format='{{.LogPath}}' spammer)

• Check size: ls -lh $LOG_PATH (It will grow by MBs every second).

• Attempt Prune (Fail):

• Run docker system prune.

• Result: Does nothing. The container is running, so logs are "active."

• The Architect Fix (Truncate):

• You cannot delete the file while open, but you can empty it.

• truncate -s 0 $LOG_PATH

• Check size again: ls -lh $LOG_PATH (It is now 0 bytes!).

• The Permanent Fix (Log Rotation):

• Stop the spammer: docker rm -f spammer

• Run with limits: docker run -d --log-opt max-size=1m --log-opt max-file=3 alpine ...

• Result: Docker will now automatically delete old logs when they reach 1MB.