Docker Series - Part 14: Unveiling the Power of Linux Processes & Docker Internals

Behind every Docker container lies a fascinating story of Linux processes and the kernel magic that runs the show.

In this session, we’ll go deeper into how the Linux kernel manages processes, how Docker leverages that to isolate containers, and how every process has its own world inside the /proc directory.

1. Understanding Process IDs in Linux

Every time you run a command or start a program in Linux, a process is created. To inspect it:

pgrep firefox  # Get the PID of Firefox
ps -aux        # Show all running processes

The kernel assigns a unique Process ID (PID) to every process and links it to a directory in /proc.

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2. Meet the /proc Directory – A Window into Your RAM

cd /proc
ls

You’ll see folders like 2771, 3442, etc. — these are not random numbers; they are active process directories!

Each folder under /proc/[PID] contains all the information about that specific process, like:

bashCopyEdit/proc/3442/
├── cmdline
├── environ
├── exe
├── root
├── status
└── ...

As soon as a process stops, its PID folder disappears from /proc.

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3. BASH is a Process Too!

Even your terminal is powered by a process:

echo $SHELL
pgrep bash
ps -aux | grep bash

You can even run another bash inside bash (yes, a nested process!).

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4. Docker = Process in a Box

Let’s connect this with Docker:

docker run -dit --name os1 centos:7
docker ps

Now check processes:

ps -aux | grep bash

You’ll see /bin/bash running — that’s the container’s primary process.

If you kill the process, the container stops.

kill -9 <PID>
docker ps  # The container exits!

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5. Containers = Process Trees with a Private Root

In containers, / is remapped:

docker attach os1
cd /
ls
cat hi.txt

The data in / comes from the image, not the host system.

Try removing the image while a container is running — it’ll fail because the container is using it.

docker rmi centos:7
# Error: image in use

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6. CMD & ENTRYPOINT – Deciding the Process at Runtime

Inside the Docker image:

docker history centos:7

It uses CMD ["/bin/bash"] by default. That’s why you land in a bash shell unless overridden.

This is the reason containers can run custom scripts, web servers, or anything — all thanks to how processes work under the hood.

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Recap

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Why This Matters

Knowing how Docker leverages native Linux process isolation gives you deeper control over containers:

✅ Process inspection
✅ Runtime debugging
✅ Image layering and isolation
✅ Docker performance optimization

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Curious about how Docker isolates processes or how /proc maps the memory world?
Let’s chat in the comments — always happy to geek out on containers!