Linux Fundamentals

Deep-Dive

FUNDAMENTALS REFERENCE

Click any layer to explore concepts, commands, and production-tested guidance.

📁

The FHS Standard

The Filesystem Hierarchy Standard defines what belongs where — so any two Linux distros are navigable the same way.

Must Know

Must Know▾

Path	Contains
`/etc`	System-wide configuration files
`/var`	Variable data — logs, spool files, caches
`/usr`	User-installed software & libraries (most of the OS actually lives here)
`/opt`	Third-party/manually installed applications
`/proc, /sys`	Virtual filesystems — kernel & process state, not real files on disk

🔗

Mounting & /etc/fstab

A filesystem isn't accessible until it's mounted somewhere in the tree — fstab is what makes that happen automatically at boot.

Important

Important▾

/etc/fstab entry

fstab

# device mountpoint fstype options dump pass

UUID=a1b2c3 /data ext4 defaults 0 2

Commands

mount

❯mount -a # mount everything in fstab

❯df -hT # what's mounted where, with type

❯findmnt /data

🔀

Symbolic vs Hard Links

Two ways to make one file answer to two names — with very different behavior when the original is deleted.

Concept

Recommended▾

Symbolic (soft) link

🔵A pointer to a path — ln -s target link

🔵Breaks ("dangling") if the target is deleted or moved

🔵Can cross filesystems, can link to directories

Hard link

ln target link — both names point to the same inode

File data isn't freed until every hard link is removed

Cannot cross filesystems, cannot link to directories

🔐

rwx Permission Bits & chmod/chown

Three permission triplets — owner, group, other — each with read/write/execute. Everything else is built on this.

Must Know

Must Know▾

permissions

❯ls -l app.sh

-rwxr-xr-- 1 vishal devs 842 app.sh

# owner: rwx group: r-x other: r--

❯chmod 750 app.sh # same thing, numeric form

❯chown vishal:devs app.sh

Never chmod 777

🔴777 means anyone, anywhere, can read/write/execute — it "fixes" a permission error by removing the security model entirely

🟠The real fix is almost always the correct owner or group, not wider permissions

🛡️

sudo & the sudoers File

Controlled privilege escalation — who can run what as root, logged and auditable.

Important

Important▾

/etc/sudoers (edit via visudo only)

sudoers

%devs ALL=(ALL) NOPASSWD: /usr/bin/systemctl restart billing-api

# group "devs" can restart exactly this service, no password

Always edit with visudo — it syntax-checks before saving, a broken sudoers file can lock everyone out

Scope rules to specific commands, not blanket ALL=(ALL) ALL, wherever possible

⚡

SetUID, SetGID & Sticky Bit

The three special permission bits that change how a file or directory behaves beyond plain rwx.

Advanced

Recommended▾

The Three Bits

🔵SetUID — executable runs as the file's owner, not the caller (e.g. passwd)

🔵SetGID — new files in a directory inherit the directory's group

🔵Sticky bit — only the file owner can delete, even with group write (e.g. /tmp)

Audit Command

find

❯find / -perm -4000 -type f 2>/dev/null

# list every SetUID binary — audit regularly

📦

apt vs yum/dnf

The two package-manager families — Debian/Ubuntu vs RHEL/CentOS/Fedora — same job, different commands and package formats.

Must Know

Must Know▾

Task	Debian / Ubuntu (apt, .deb)	RHEL / Fedora (dnf, .rpm)
Install	`apt install nginx`	`dnf install nginx`
Update index	`apt update`	`dnf check-update`
Upgrade all	`apt upgrade`	`dnf upgrade`
Search	`apt search nginx`	`dnf search nginx`
Remove	`apt remove nginx`	`dnf remove nginx`

🔑

Repository Management & GPG Keys

A repo tells the package manager where to look; a GPG key proves what it finds there hasn't been tampered with.

Important

Important▾

Adding a third-party repo safely

apt

❯curl -fsSL https://repo.example.com/key.gpg | gpg --dearmor -o /etc/apt/keyrings/example.gpg

❯echo "deb [signed-by=/etc/apt/keyrings/example.gpg] https://repo.example.com stable main" | tee /etc/apt/sources.list.d/example.list

❯apt update

🟠Never apt-get install --allow-unauthenticated in production — that's disabling the exact check that stops supply-chain attacks

🔨

Building from Source vs Packages

Sometimes the package repo doesn't have the version you need — knowing when compiling from source is actually the right call.

Decision

Recommended▾

Use Packages When

🟢A stable version exists in the repo — packages get security patches automatically

Build From Source When

You need a specific patch/version not yet packaged

Custom compile flags are required (e.g. hardware-specific optimizations)

Track it in config management — a hand-built binary nobody remembers building is a liability

📋

ps, top, and the Process Tree

Every process has a parent — understanding the tree is how you find what actually spawned that runaway job.

Must Know

Must Know▾

process inspection

❯ps aux --sort=-%cpu | head

❯ps -ef --forest # process tree view

❯pstree -p 1

❯top # or htop for the friendlier version

📡

Foreground/Background Jobs & Signals

Ctrl+C sends SIGINT. kill -9 sends SIGKILL. They are not the same thing, and using the wrong one has consequences.

Important

Important▾

Common Signals

🔵SIGTERM (15) — polite "please shut down," process can clean up

🔴SIGKILL (9) — immediate termination, no cleanup, last resort

🔵SIGHUP (1) — often used to tell a daemon "reload your config"

Job Control

ctrl+z suspends, bg resumes in background, fg brings back to foreground

nohup cmd & keeps a job alive after you log out

🔧

Starting Services (Intro to systemctl)

A first taste of systemd service control — the full deep-dive is in the Advanced issue, but you need this much on day one.

Basics

Recommended▾

systemctl

❯systemctl status nginx

❯systemctl restart nginx

❯systemctl enable nginx # start on boot

📜

Bash Scripting Basics

Variables, loops, and conditionals — the four constructs that turn a list of commands into an actual script.

Must Know

Must Know▾

backup.sh

bash

#!/usr/bin/env bash

set -euo pipefail

for f in /data/*.log; do

if [[ -s "$f" ]]; then

gzip "$f"

done

🟠set -euo pipefail at the top of every script — exit on error, undefined variable, or failed pipe. This alone prevents most silent script failures.

🔍

grep/sed/awk Essentials

The classic Unix text-processing trio — find it, transform it, extract it. Still the fastest way to slice a log file.

Important

Important▾

text processing

# grep — find lines matching a pattern

❯grep -i "error" /var/log/app.log

# sed — find & replace in place

❯sed -i 's/staging/production/g' config.yaml

# awk — extract & compute on columns

❯awk '{sum+=$5} END {print sum}' access.log

🔗

Pipes, Redirection & Environment Variables

How to chain commands together and control where their output goes — the connective tissue of everything above.

Fundamentals

Recommended▾

Redirection

🔵> overwrites, >> appends, 2>&1 merges stderr into stdout

🔵cmd1 | cmd2 — stdout of cmd1 becomes stdin of cmd2

Environment Variables

export VAR=value — makes it visible to child processes

.bashrc / .bash_profile — where persistent exports belong

Distro	Family	Package Manager
Ubuntu / Debian	Debian	apt (.deb packages)
RHEL / CentOS / Rocky / Alma	Red Hat	dnf / yum (.rpm packages)
Fedora	Red Hat	dnf (.rpm packages)
Amazon Linux 2023	Red Hat-based	dnf
Alpine	Independent	apk

FIVE LAYERS OF A LINUX BOX

FUNDAMENTALS REFERENCE

WHICH PACKAGE MANAGER AM I ON?

COMMAND CHEATSHEET