🐧 Linux Command Line Guide – Complete with Explanation & Examples

This document summarizes essential Linux command-line concepts and tools, presented with clear explanations and practical examples. It is formatted for a README.md file on GitHub.

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📁 Linux Directory Structure (FHS - Filesystem Hierarchy Standard)

Directory	Description
/	Root directory – the top-level of the filesystem. All other directories branch from here.
/boot	Contains boot files (e.g., Linux kernel, GRUB).
/bin	Essential command binaries (e.g., ls, cp).
/sbin	System binaries for superuser tasks (e.g., shutdown, ifconfig).
/lib, /lib64	Shared libraries for binaries in /bin and /sbin.
/home	User home directories (e.g., /home/ahmed).
/root	Root user’s home directory.
/usr	User-installed apps, docs, and libraries.
/etc	System-wide configuration files.
/tmp	Temporary files (deleted on reboot).
/opt	Optional and 3rd-party software packages.
/proc, /sys, /run	Virtual filesystems for kernel, runtime, and system data.
/dev	Device files (e.g., /dev/sda).
/mnt, /media, /cdrom	Mount points for devices or removable media.
/srv	Service data for servers (e.g., FTP, HTTP).
/var	Variable files (logs, emails, etc.).

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💼 File Management Interfaces

• CLI (Command Line Interface) – Powerful, scriptable terminal-based interface.
• GUI (Graphical User Interface) – Visual file managers like Nautilus, Dolphin.

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🧾 Basic File Operations

• Browse: ls, cd, pwd
• Create & Rename: touch, mv
• Move: mv file path
• Delete: rm, rm -r
• Copy: cp, cp -r

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🧪 Terminal & System Commands

User & System Info

who         # Logged-in users
uname -a    # System & kernel info
cal         # Calendar
date        # Current date & time

Network & Monitoring

ifconfig    # Network interfaces
 top         # Live process viewer
time cmd    # Time taken by a command
times       # Shell and child process times

Output

echo "text"     # Print text
printf "%s" text # Formatted output

Help

man command     # Manual page
info command    # Info page
help            # Built-in command help

Terminal Utilities

clear       # Clear terminal
reset       # Reset terminal
exit        # Close terminal
history     # Show command history

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📂 Navigating Files and Directories

pwd              # Show current directory
ls               # List directory contents
cd folder        # Change directory
touch file.txt   # Create file
cat file.txt     # View file content

Special Paths:

• ~ = Home
• .. = Parent directory

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📤 Copying, Moving, Deleting

cp file1 path/           # Copy file
cp -r dir path/          # Copy directory
mv file1 path/           # Move/rename file
rm file1                 # Remove file
rm -r dir                # Remove directory

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📁 Directory Management

mkdir new_folder         # Create directory
rmdir empty_folder       # Remove empty directory

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📄 Listing and Creating Files

Using ls Options:

ls -a        # All files (incl. hidden)
ls -l        # Long listing with details
ls -R        # Recursive listing
ls -la       # Combined view

Creating and Editing Files:

touch file.txt       # Empty file
cat > file.txt       # Overwrite file with input
cat >> file.txt      # Append to file
nano file.txt        # Easy text editor
vi file.txt          # Advanced text editor

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✏️ File Content Management

cat file1 > file2     # Copy
cat file1 >> file2    # Append
cat file1 file2 > file3  # Merge files

Viewing Content:

cat file.txt       # Normal
nl file.txt        # With line numbers
tac file.txt       # Reverse

Search & Count:

grep "word" file.txt     # Search
wc file.txt              # Count lines, words, characters
wc -l/-w/-c file.txt     # Specific counts

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🔐 File Permissions

Entities:

• User (owner)
• Group
• Others

Permission Types:

Symbol	Meaning
r	Read
w	Write
x	Execute

Numeric (Octal) Mode:

Value	Permission
7	rwx (full)
6	rw-
5	r-x
4	r--
0	---

chmod 755 file.txt      # Owner: all, others: read/execute

Symbolic Mode:

chmod g+w file.txt      # Add write to group
chmod o-r file.txt      # Remove read from others

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👑 Ownership and Groups

chown user file.txt            # Change owner
chown user:group file.txt      # Change owner & group
chgrp group file.txt           # Change group only
groups                        # View current user groups

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🔍 Advanced File Tools

head & tail

head file.txt             # First 10 lines
head -n 5 file.txt        # First 5 lines
tail file.txt             # Last 10 lines
tail -n +20 file.txt      # From line 20 onward

File Search & Location

which ls                     # Full path
whereis ls                   # Binary + source + man page
locate filename              # Fast search
find /home -name "*.txt"     # Deep search

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🧠 System Info

ps aux                  # Running processes
w                       # Who is logged in
id                      # User & group ID
df -h                   # Disk usage
free -m                 # RAM usage
cat /proc/cpuinfo       # CPU info
cat /proc/meminfo       # RAM details

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🧩 Special Characters

Symbol	Use
\	Escape
/	Path separator
.	Current dir / hidden file
..	Parent directory
~	Home directory
*	Wildcard
>	Redirect output
>>	Append output
<	Input redirection
;	Run multiple commands

echo Hello > file.txt     # Create or overwrite
cat file.txt | grep word # Pipe example
cd ~/Docs; ls -l         # Chained commands

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This guide covers essential Linux basics for navigating, managing, and interacting with the system via the command line. Perfect for beginners and intermediate users!