Linux Command Fieldbook

The operating system underneath everything. Learn to move without a mouse.

This repo is not a list of commands. It's a map of how to operate a Linux machine — from creating a folder to debugging why production is down at 2am.

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What is Linux?

Before Windows had a Start menu (1995) or Mac had a Dock, computers were just a black screen and a shell.

• 1969: UNIX invented the idea: "everything is a file, everything is a command"
• 1991: Linus Torvalds rebuilt it as Linux — a free kernel that talks directly to hardware
• Today: Linux runs your phone (Android), your cloud (AWS), your router, your car. Windows runs desktops. Linux runs the world.

How it works: You → Terminal → Shell (bash/zsh) → Kernel → CPU/Disk/Network

Windows hides this behind Explorer. Linux gives you the steering wheel.

Shell vs Bash

	What it is
Terminal	The window (iTerm, GNOME Terminal)
Shell	The interpreter — any language that runs commands
Bash	The most common shell on servers. bash = Bourne Again SHell

All bash commands are shell commands, but not all shells are bash.

Why use commands when Windows exists?

Because production servers don't have a mouse.

1. Speed: mkdir -p app/{logs,src,config} = 3 folders in 0.1s
2. Scale: You can't right-click on 50 servers. You ssh and run a script.
3. Repeatable: Commands go in Git. Clicks don't.
4. Visibility: ls -l shows permissions, owner, size. Properties window hides it.

Linux Architecture — Explained Like a Human Body

Linux works in 5 layers, from bottom to top. Think of your computer like a person:

HARDWARE ← The body (CPU, RAM, Disk, Keyboard) ↓ KERNEL ← The brain (Linux) ↓ SYSTEM LIBRARIES ← Common tools (toolbox) ↓ SHELL ← Your voice (bash) ↓ USER APPS ← What you use (Chrome, VS Code, ls, nginx)

1. HARDWARE — The Body

The physical stuff. CPU, RAM, hard disk. Does nothing alone — like a body without a brain. Example: Your hard disk is a cupboard. It can't open itself.

2. KERNEL — The Brain

The Linux program that controls everything. Decides who uses CPU, gives memory, reads/writes disk, sends network. Example: When you save a file, the kernel is the one who actually writes 1s and 0s to disk.

3. SYSTEM LIBRARIES — The Toolbox

Ready-made tools (main one is glibc). So apps don't have to learn brain-language. Example: Every app uses open() to open files — like using the same screwdriver instead of making your own.

4. SHELL — Your Voice

bash — the black screen where you type. Translates your words into instructions. Example: You type mkdir photos. Shell hears it and passes it up.

5. USER APPLICATIONS — Your Hands

Programs you run — ls, chrome, python. They do the actual work. Example: ls just asks "show me files".

Memory trick: Hardware → Brain → Nerves → Voice → Hands

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THE 4 PILLARS — How this repo is organized

1. Navigation-and-Manipulation

Everything about moving around the system without a mouse.

Goal: Never get lost in the filesystem.

You'll learn:

• Core navigation: pwd, cd, ls, ll, tree, pushd/popd, dirs, realpath, readlink — plus absolute vs relative paths (/var/log vs ../logs vs ~/projects)
• Files & folders: mkdir, rmdir, touch, rm -rf, cp, mv, ln (hard/soft links), unlink, stat, file, basename, dirname — plus brace expansion touch file_{1..100}.log and loops
• Search & find: find, locate/updatedb, which, whereis, type, grep, egrep, rg, ack, xargs
• Shell fundamentals: > >> < 2> &> | tee, ; && || &, * ? [] {} ~ $, history, alias, export, env, variables $HOME $PATH, why rm -rf / is nuclear

Real scenario: You SSH into a fresh EC2. In 60 seconds you build the full app structure without touching a GUI.

CLI vs GUI: Right-click → New Folder vs mkdir -p project/{src,tests}

2. System-Observability

Everything related to top, df, free, tail -f, dmesg

Goal: Understanding why the application is crashing or slow.

You'll learn:

• Process view: ps, pgrep, pkill, pidof, top, htop, atop, uptime, w, who
• Resources: free, vmstat, mpstat, df -h, du -sh, ncdu, lsblk, iostat, sar
• Live logs & services: tail -f, head, less, journalctl, dmesg, systemctl status, watch
• Deep debug: lsof, lsof -i :8080, fuser, strace, ltrace, timeout

Real scenario: App is slow. You run top → see 1 process at 400% CPU, df -h → disk at 100%, tail -f → logs filling disk. Fix in 3 commands.

3. Networking-and-Security

Securing the blast radius and ensuring services can talk to each other.

Goal: Connect to machines, move files, lock them down.

You'll learn:

• Networking basics: ip a, hostname, ping, traceroute, mtr, ss -tulnp, netstat, curl, wget, dig, nslookup, host, nc
• Remote access: ssh, ssh-keygen, scp, sftp, rsync
• Users & permissions: id, whoami, groups, su, sudo, passwd, chmod, chown, chgrp, umask, getfacl, setfacl, lsattr, chattr
• Firewall & packages: ufw, iptables, nft, apt, yum/dnf, systemctl

Real scenario: You can't reach the API. ss -tulnp shows nothing on port 8080, curl localhost:8080 fails, chmod +x fixes the startup script.

4. Automation

Never doing the same task twice.

Goal: Turn commands into systems.

You'll learn:

• Bash basics: echo, printf, read, test, [ ], [[ ]], if, for, while, case, functions
• Scripting power: set -e, $?, $0 $1, source, ., exit codes, error handling, logging
• Scheduling: crontab -e, at, systemd timers, systemctl enable
• Environment: export, env, printenv, $PATH, .bashrc, .profile, alias, unalias

Real scenario: Instead of manually compressing logs daily, you write a 10-line script + cron that rotates, gzips, and uploads to S3 every night.

How to read a command

Open any folder — every lesson follows the same 4-part structure inside.

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⭐ If this helped you understand Linux better than a 2-hour video, star the repo. It helps others find it.

Built by @rahul7528 — learning in public.