Linux Basics¶

1. What is Linux?¶

Linux is an open-source operating system developed by Linus Torvalds in 1991.

┌─────────────────────────────────────────────────────────┐
│                   Applications                          │
│          (web servers, databases, dev tools)            │
├─────────────────────────────────────────────────────────┤
│                        Shell                            │
│                  (bash, zsh, sh)                        │
├─────────────────────────────────────────────────────────┤
│                    Linux Kernel                         │
│      (processes, memory, filesystem, networking)        │
├─────────────────────────────────────────────────────────┤
│                      Hardware                           │
│          (CPU, memory, disk, network)                   │
└─────────────────────────────────────────────────────────┘

Linux Characteristics¶

Feature	Description
Open Source	Source code public, free to use
Stability	High stability suitable for server operations
Security	Permission-based security model
Multi-user	Support for multiple simultaneous user connections
Multi-tasking	Multiple processes run concurrently
Portability	Runs on various hardware platforms

2. Linux Distributions¶

Various distributions exist based on the Linux kernel.

              ┌──────────────────┐
              │   Linux Kernel   │
              └────────┬─────────┘
                       │
       ┌───────────────┼───────────────┐
       │               │               │
       ▼               ▼               ▼
┌──────────────┐ ┌──────────────┐ ┌──────────────┐
│    Debian    │ │    RHEL      │ │    Arch      │
└──────┬───────┘ └──────┬───────┘ └──────────────┘
       │                │
       ▼                ▼
┌──────────────┐ ┌──────────────┐
│   Ubuntu     │ │   CentOS     │
│   Mint       │ │   Rocky      │
└──────────────┘ │   Fedora     │
                 └──────────────┘

Major Distribution Comparison¶

Distribution	Base	Features	Use Cases
Ubuntu	Debian	Ease of use, large community	Beginners, desktop, server
Debian	-	Stability, strict package policy	Server
CentOS/Rocky	RHEL	Enterprise-grade stability	Corporate servers
Fedora	RHEL	Latest technology, RHEL testbed	Developers
Alpine	-	Lightweight (5MB), security	Containers
Arch	-	Latest packages, DIY philosophy	Advanced users

3. Terminal and Shell¶

Terminal¶

The terminal is a text-based interface for interacting with the computer.

┌─────────────────────────────────────────────────────────┐
│ user@hostname:~$                                        │
│                                                         │
│  ← Space to enter commands and view results            │
│                                                         │
└─────────────────────────────────────────────────────────┘

Shell¶

The shell is a program that interprets user commands and passes them to the kernel.

Shell	Description
bash	Default shell for most Linux systems (Bourne Again Shell)
zsh	Enhanced features, macOS default shell
sh	Original shell (Bourne Shell)
fish	User-friendly shell

# Check current shell
echo $SHELL

Output:

/bin/bash

4. Command Structure Basics¶

command [options] [arguments]
   │        │          │
   │        │          └── Target of command (files, directories, etc.)
   │        └── Modify command behavior (-a, --all, etc.)
   └── Command to execute

Examples¶

# Basic form
ls

# With option
ls -l

# Option + argument
ls -l /home

# Multiple options
ls -la /home

# Long options
ls --all --human-readable

Option Formats¶

Format	Example	Description
Short option	`-l`	Dash + single letter
Combined options	`-la`	Multiple short options combined
Long option	`--all`	Double dash + word

5. Using Help¶

man (Manual Pages)¶

# View command manual
man ls
man cp
man chmod

# Search by keyword
man -k "copy file"

Navigating man pages: | Key | Action | |-----|--------| | Space / f | Next page | | b | Previous page | | /search_term | Search | | n | Next search result | | q | Quit |

--help Option¶

# Quick help
ls --help
cp --help

info¶

# Detailed information (GNU commands)
info ls

6. Basic Commands¶

whoami - Current User¶

whoami

Output:

ubuntu

hostname - System Name¶

hostname

Output:

my-server

date - Current Date/Time¶

# Current date/time
date

# Specify format
date "+%Y-%m-%d %H:%M:%S"

Output:

Tue Jan 23 14:30:00 KST 2024
2024-01-23 14:30:00

cal - Calendar¶

# Current month
cal

# Specific year
cal 2024

# Specific month
cal 3 2024

clear - Clear Screen¶

clear
# Or Ctrl + L

echo - Print Text¶

echo "Hello, Linux!"
echo $HOME
echo "Current path: $(pwd)"

7. Command History¶

history Command¶

# View history
history

# Last 10 commands only
history 10

# Search history
history | grep "apt"

Using History¶

Command	Description
`!!`	Execute previous command
`!n`	Execute nth history command
`!string`	Execute most recent command starting with string
`Ctrl + R`	Reverse history search

# Re-execute previous command
!!

# Re-execute with sudo
sudo !!

# Execute command 123
!123

# Most recent command starting with ls
!ls

Ctrl + R (Reverse Search)¶

# Press Ctrl + R then enter search term
(reverse-i-search)`apt': apt update

# Enter: execute
# Ctrl + R: next result
# Ctrl + G: cancel

8. Keyboard Shortcuts¶

Cursor Movement¶

Shortcut	Action
`Ctrl + A`	Beginning of line
`Ctrl + E`	End of line
`Ctrl + ←/→`	Move by word

Editing¶

Shortcut	Action
`Ctrl + U`	Delete before cursor
`Ctrl + K`	Delete after cursor
`Ctrl + W`	Delete word before cursor
`Ctrl + Y`	Paste deleted content

Control¶

Shortcut	Action
`Ctrl + C`	Interrupt running command
`Ctrl + D`	End input (EOF) / logout
`Ctrl + Z`	Suspend process
`Ctrl + L`	Clear screen

9. Tab Auto-completion¶

Using the Tab key enables auto-completion of commands and filenames.

# Command auto-completion
sys[Tab]     → systemctl

# Filename auto-completion
cd /ho[Tab]  → cd /home/

# When multiple candidates exist
cd /[Tab][Tab]  → Display possible list

10. Practice Exercises¶

Exercise 1: Check System Information¶

# Check current user
whoami

# Check hostname
hostname

# Current date/time
date

# System uptime
uptime

# Kernel information
uname -a

Exercise 2: Using Help¶

# Check ls manual
man ls

# Check cp help
cp --help

# Search manuals by keyword
man -k "disk space"

Exercise 3: Using History¶

# Check history
history

# Re-execute previous command
!!

# Search previous commands with Ctrl + R
# (Press Ctrl + R and enter search term)

Next Steps¶

Learn about Linux directory structure and navigation commands in 02_Filesystem_Navigation.md!