📘 Linux Summary Guide

 📘 Linux Summary Guide

I. What is Linux?

Linux is an open-source, Unix-like operating system kernel used widely in servers, desktops, embedded systems, and cloud environments. Popular Linux distributions include Ubuntu, CentOS, RHEL, Debian, Fedora, and Alpine.

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II. Linux Architecture Overview

1. Kernel

   • The core of the OS that manages hardware, processes, memory, I/O, file systems, and system calls.

2. System Libraries

   • Provide functions for programs to interact with the kernel (e.g., GNU C Library glibc).

3. System Utilities

   • Basic tools for managing the system (ls, cat, mkdir, etc.).

4. Shell

   • Command interpreter (e.g., Bash, Zsh) used to interact with the system.

5. User Applications

   • Programs installed by users (e.g., Vim, Docker, Firefox).

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III. File System Hierarchy

Directory	Description
/	Root directory
/bin	Essential binary commands
/sbin	System binaries
/etc	Configuration files
/home	User home directories
/var	Variable data (logs, spool files)
/usr	User programs and data
/tmp	Temporary files
/dev	Device files
/proc	Kernel and process info (virtual FS)
/boot	Boot loader and kernel files

IV. Basic Linux Commands

Navigation

bash

pwd          # Show current directory
cd /path     # Change directory
ls -l        # List directory contents

File Management

bash

touch file.txt               # Create file
mkdir dir/                   # Create directory
cp file1 file2               # Copy file
mv file1 /path/              # Move/rename
rm file.txt                  # Delete file

Viewing & Searching

bash

cat file.txt                 # View file
less file.txt                # Scroll through file
grep "text" file.txt         # Search text in file
find / -name "*.log"         # Search by name

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V. User and Permission Management

Users & Groups

bash

adduser user1                # Add user
passwd user1                # Set password
usermod -aG sudo user1       # Add to sudo group
id user1                     # View user info

Permissions

bash

chmod 755 file.sh            # Set file permissions
chown user:group file.txt    # Change owner
ls -l                        # View permissions

Symbol	Meaning
r	Read
w	Write
x	Execute

VI. Process Management

bash

ps aux                      # List processes
top                         # Real-time process view
kill <pid>                  # Kill process
htop                        # Interactive process viewer
nice/renice                 # Adjust process priority

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VII. Package Management

Debian/Ubuntu

bash

apt update && apt upgrade
apt install nginx
apt remove nginx

RHEL/CentOS

bash

yum install nginx
yum remove nginx
dnf update

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VIII. System Services (Systemd)

bash

systemctl status nginx
systemctl start nginx
systemctl stop nginx
systemctl restart nginx
systemctl enable nginx     # Start on boot

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IX. Networking

bash

ip a                       # View IP addresses
ip r                       # View routing table
ping google.com            # Check connectivity
netstat -tuln              # Show open ports
ss -tuln                   # Faster alternative
hostnamectl                # View/modify hostname

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X. Disk and File Systems

bash

df -h                      # Disk usage
du -sh *                   # Directory size
lsblk                      # Block devices
mount /dev/sdb1 /mnt       # Mount drive
umount /mnt                # Unmount
fdisk -l                   # Partition info

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XI. Logs and Monitoring

journalctl bash-xe             # View logs (systemd)
tail -f /var/log/syslog    # Live log view
uptime                     # System uptime
free -h                    # Memory usage
top/htop                   # CPU, memory, processes

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XII. Shell Scripting Basics

bash

#!/bin/bash
echo "Hello, $USER"
date

Run:

bash

chmod +x script.sh
./script.sh

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XIII. Crontab (Scheduled Tasks)

bash

crontab -e                 # Edit user's cron
crontab -l                 # List crons

Cron syntax:

Bash

* * * * * /path/to/script.sh
│ │ │ │ │
│ │ │ │ └─ Day of week (0-6)
│ │ │ └─── Month (1-12)
│ │ └───── Day of month (1-31)
│ └─────── Hour (0-23)
└───────── Minute (0-59)

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XIV. Linux Best Practices

1. Use SSH keys instead of passwords for secure access.

2. Regularly update packages and monitor logs.

3. Limit root access; use sudo for admin tasks.

4. Automate tasks using cron or systemd timers.

5. Maintain backups and versioned configurations.

6. Use SELinux/AppArmor for enhanced security (if applicable).

7. Monitor disk, CPU, memory to prevent outages.