Chapter 6: Basic Ubuntu Linux Commands 🐧¶

Mentor's Note: In Linux, the Terminal is your magic wand. While most people use a mouse to click icons (GUI), power users and developers use the Command Line (CLI) to talk directly to the computer's soul. It's faster, more powerful, and once you master it, you'll feel like a wizard! 🧙‍♂️

🌟 The Scenario: The Universal Translator 🗣️¶

Imagine you are visiting a foreign country. You can point at things (GUI), but it's much more efficient if you speak the local language (CLI).

The Shell: This is your Translator. You give it a command in English-like words, and it translates them into "0s and 1s" for the computer's CPU.
The Terminal: This is the Microphone you use to speak to the translator.
The Result: Total control over the computer without ever needing a mouse! ✅

📖 Main Topics Covered¶

Starting Up the Terminal
Shells in Linux
Command Syntax
General Purpose Commands
Working with Directories
Working with Files
Manipulating Files and Directories
File Permissions
I/O Redirection and Piping
Filters

🎨 Visual Logic: The Shell Architecture¶

graph TD
    A[User 🧑‍💻] -- Commands --> B[Shell 🐚]
    B -- Translation --> C[Kernel ⚙️]
    C -- Control --> D[Hardware 🖥️]
    D -- Result --> C
    C -- Response --> B
    B -- Output --> A

🐚 Understanding Shells¶

A Shell is a command-line interpreter. - Available Shells: Use cat /etc/shells to see all shells installed. - Default Shell: Use echo $SHELL to see your default (usually /bin/bash). - Switching Shells: Type csh to enter C Shell, exit to return. - Internal vs External: Use type [command] to check if a command is built-in or a separate program.

🛠️ Complete Command Reference¶

📅 1. General Purpose Commands¶

Command	Purpose	Logic/Example
`cal`	Display calendar	`cal 01 2024` (Specific month)
`date`	Display system date	`date +%D` (Date), `date +%T` (Time)
`bc`	Command line calculator	Use `bc -l` for precision math
`echo`	Display message/variable	`echo "Hello $USER"`
`passwd`	Change password	`passwd` (Self) or `passwd user`
`clear`	Clear terminal screen	Wipes previous output

📂 2. Help & Help Systems¶

Command	Purpose	Example
`--help`	Brief help flag	`mv --help`
`man`	Full manual pages	`man ls`
`whatis`	One-line description	`whatis cat`
`apropos`	Search by keyword	`apropos copy`

📂 3. Directory Management¶

Command	Purpose	Example
`pwd`	Print working directory	Shows absolute path 📍
`mkdir`	Create directory	`mkdir -p a/b/c` (Nested)
`cd`	Change directory	`cd ~` (Home), `cd -` (Back)
`rmdir`	Remove empty folder	`rmdir old_docs`

📄 4. File Operations¶

Command	Purpose	Details
`cat`	Create/View/Concat	`cat > f` (New), `cat >> f` (Append)
`rm`	Remove file/folder	`-i` (Ask), `-r` (Recursive), `-f` (Force)
`ls`	List contents	`-a` (Hidden), `-l` (Long), `-t` (Time)
`cp`	Copy data	`cp file1 file2`
`mv`	Move or Rename	`mv old.txt new.txt`
`more`	Paging	View files one page at a time
`wc`	Word count	`-l` (Lines), `-w` (Words), `-c` (Chars)

🔍 5. File Analysis & Comparison¶

Command	Purpose	Example
`cmp`	Compare two files	Finds first byte of difference
`diff`	Show differences	Shows exactly what lines differ
`grep`	Pattern matching	`grep "Surat" address.txt`

🔐 File Permissions¶

Linux uses a 3-part system: User (u), Group (g), and Others (o).

Octal Values¶

4: Read (r)
2: Write (w)
1: Execute (x)
0: None (-)

Common Modes: - 777: Everyone can do everything (Insecure! ⚠️) - 755: Owner can write, everyone else can only read/run. - 644: Owner can write, everyone else read only.

🔍 Wildcard Characters (The Search Shortcuts)¶

* : Any number of characters (including none).
? : Exactly one character.
[abc] : Any one of a, b, or c.
[!abc] : Any character except a, b, or c.
[p-s] : Any character in the range p to s.

⚡ I/O Redirection & Piping¶

Symbol	Action	Analogy
`>`	Redirection (Overwrite)	Pouring water into a new bucket 🪣
`>>`	Redirection (Append)	Adding water to an existing bucket 🚰
`<`	Input Redirection	Sucking water from a bucket 🥤
`\\|`	Pipe	Connecting two pipes together 🔗

🧪 Filter Commands (The Sifters)¶

Filters take input, process it, and output the result. - head -n: Show first n lines. - tail -n: Show last n lines. - cut: Vertical slicing. cut -d "," -f 1 (Get 1^st field of CSV). - paste: Join files side-by-side. - sort: Sort lines (use -r for reverse). - tr: Translate characters. cat f | tr 'a' 'A'.

🎯 Practice & Implementation¶

The best way to learn Linux is by doing. We have prepared a comprehensive 50-problem practice set for you.

Go to Chapter 6 Practice Sheet →

💡 Interview & Board Tip 👔¶

"Board examiners love asking about Case Sensitivity. Remember: MyFile.txt and myfile.txt are two different files in Linux! Also, always know the difference between rm (files) and rmdir (folders)."

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