Master Linux Command Line: Essential Commands and Tips for System Administration

1. Linux Overview

Linux is a free, open‑source Unix‑like operating system that supports POSIX and runs on multiple hardware platforms. It provides a multi‑user, multitasking environment with a rich set of command‑line tools.

1.1 What is Linux?

Linux is a free operating system based on Unix, supporting 32‑bit and 64‑bit hardware. It runs most Unix tools, applications, and network protocols.

1.2 Differences between Unix and Linux

Open source : Linux is free and its source code can be modified and redistributed.

Cross‑platform : Linux runs on many hardware platforms, while Unix is more limited.

GUI : Linux provides both command‑line and graphical interfaces; Unix traditionally offers only a command line.

Hardware requirements : Linux works on lower‑end hardware; Unix often requires more powerful machines.

User base : Linux is used by a wide range of users, from individuals to enterprises.

1.3 Linux Kernel

The kernel manages hardware and software resources, handling memory, process, and device management.

1.4 Linux Architecture

Linux consists of two main spaces:

User space : Contains user applications, libraries, and utilities.

Kernel space : Includes system call interface, the kernel itself, and architecture‑dependent code.

2. Disk, Directory, File

In Linux, everything is treated as a file, including devices. Key directories include /bin, /etc, /home, /usr, /proc, /root, /sbin, /dev, /mnt, /boot, /lib, /tmp, /var, and /lost+found.

2.1 Inode

An inode stores metadata about a file (owner, size, timestamps, block pointers). It enables the mapping between logical file structure and physical storage.

2.2 Hard and Soft Links

Hard link : Direct reference to an inode; cannot span filesystems and cannot link directories.

Soft (symbolic) link : Path‑based reference; can cross filesystems and point to directories.

3. Security

Typical hardening steps after a fresh Linux installation include:

Add non‑root users and disable root SSH login.

Use SSH key authentication and disable password login.

Enable a firewall (iptables) and disable SELinux if not needed.

Install fail2ban to block brute‑force attacks.

Restrict SSH access to specific IP ranges or VPN.

Limit command history length.

Restrict outbound traffic to necessary services.

Apply application‑level security measures.

3.1 Common Attacks

CC (Challenge‑Response) attack : Simulates many legitimate requests to exhaust resources.

DDOS attack : Distributed denial‑of‑service using multiple compromised hosts.

SQL injection : Unsanitized input allows attackers to execute arbitrary queries.

4. Shell

A shell script is a text file containing a sequence of commands. The default login shell is /bin/bash.

4.1 Basic Syntax

Comments start with #.

Variables: system‑defined (uppercase) and user‑defined.

Exit status: $? equals 0 on success.

Special variables: $0, $1 … $9, $@, $#.

4.2 Control Structures

# if…then…else…fi
if [ condition ]; then
    # commands
else
    # commands
fi

# case statement
case $var in
    pattern1) commands ;;&
    pattern2) commands ;;
esac

# for loop
for i in list; do
    # commands
done

# while loop
while [ condition ]; do
    # commands
done

4.3 Common Commands

chmod

, chown, cp, mv, rm, ln, find, grep, sed, awk.

Redirection: >, >>, 2>&1, &>.

5. Programming Exercises

Examples include checking if a file is a character device and copying it, creating a group and adding users, deleting a range of user accounts, and writing sed scripts to modify files.

6. Practical Tasks

Choosing a Linux distribution (e.g., Ubuntu for desktop, CentOS/RHEL for servers), planning system installation, partitioning, creating users, and configuring services.

6.1 Performance Tuning

Disable unnecessary daemons.

Turn off the GUI.

Adjust kernel parameters.

Tune CPU, memory, filesystem, and network subsystems.

7. File Management Commands

cat

: Concatenate and display files. chmod: Change file permissions. chown: Change file owner/group. cp: Copy files/directories. ln: Create hard or symbolic links. locate, whereis, which: Find files or commands. more, less, head, tail, sed, awk, grep, wc.

8. Disk Management Commands

cd

: Change directory. df: Report filesystem disk space usage. du: Estimate file and directory space usage. ls: List directory contents with options for sorting, showing hidden files, and detailed listings. mkdir, pwd, rmdir.

9. Network Communication Commands

ifconfig

: Show or configure network interfaces. iptables: Manage firewall rules (e.g., block IP, open port 80). netstat: Display network connections, routing tables, and interface statistics. ping: Test reachability of a host. telnet: Remote login (useful for testing services).

10. System Management Commands

date

: Display or set system date and time. free: Show memory usage. kill, ps: Process management. rpm, yum: Package management for Red Hat based systems. top: Real‑time process monitoring.

11. Backup and Compression Commands

bzip2

, gzip: Compress and decompress files. tar: Archive files; combine with -z (gzip) or -j (bzip2) for compression. unzip: Extract ZIP archives.

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