Introduction to Linux Operating System

What is Linux?

• Free, open-source OS based on Unix
• Created by Linus Torvalds (1991)
• Runs on servers, desktops, mobiles, embedded systems
• Known for stability, security, flexibility

Open Source

• Source code freely available
• Anyone can view, modify, distribute
• Global community contributes to development

Interfaces

• Command-line interface (CLI)
• Graphical interfaces: GNOME, KDE, Xfce
• Supports wide range of applications

Key Features

• Multi-user
• Multi-tasking
• CLI + GUI
• Hardware compatibility
• Strong security
• High stability
• Highly customizable
• Community-driven

Linux System Architecture

[ User Applications ]
↓
[ System Libraries ]
↓
[ Linux Kernel ]
↓
[ Hardware ]

Architecture Overview

• Kernel manages CPU, memory, devices
• User-space programs interact via system calls
• Runs on many hardware platforms
• Uses hierarchical filesystem

passwd

• Change a user's password
• Root can change any user’s password
• Options:
  • -l lock account
  • -u unlock
  • -d delete password

who

• Shows currently logged-in users
• Displays username, terminal, login time, host
• Options:
  • -a all info
  • -b last boot
  • -u user details

w

Shows:

• System time, uptime
• Logged-in users count
• Load averages
• USER, TTY, FROM, LOGIN@, IDLE, JCPU, PCPU

tty

• Shows current terminal device
• Example: /dev/pts/0
• tty -s checks if session is a terminal

lock

• Locks a terminal session
• Prompts for confirmation
• Requires password to unlock
• Not available on all Linux distros

stty

• Views or modifies terminal I/O settings
• Controls echo, speed, line discipline
• Example: stty -a shows current settings

script

• Records all terminal activity
• Saves output to typescript by default
• Useful for logs, installations, debugging
• Start: script filename
• Stop: exit

clear

• Clears the terminal screen
• Simply run: clear

uname

• Prints system information
• Options:
  • -a all info
  • -m machine type
  • -n hostname
  • -r kernel release
  • -s OS name
  • -v OS version

date

• Shows or sets system date/time
• Custom format with +FORMAT
• Example: date "+%Y-%m-%d %H:%M:%S"
• Set date: date -s "2025-01-01 10:00:00"

cal

• Shows a calendar
• Default: current month
• Example: cal 6 2022

calendar

• Reads reminders from ~/.calendar
• Shows events for current date (if configured)

bc

• Precision calculator language
• Used in scripts for math
• Example: echo "5*7" | bc

echo

• Prints text or variable values
• Example: echo "Hello"
• Example: echo $USER

cd

• Changes directory
• Examples:
  • cd or cd ~ → home
  • cd .. → parent
  • cd /path/to/dir
  • cd - → previous directory

• Options:
  • -P follow symlinks
  • -L don’t follow symlinks

mkdir

• Creates directories
• Syntax: mkdir [options] name
• Options:
  • -p create parent dirs
  • -m set permissions
• Example: mkdir -p -m 755 /path/to/dir

rmdir

• Removes an empty directory
• Fails if the directory is not empty
• Options:
  • -p remove parent dirs if they become empty
  • --ignore-fail-on-non-empty ignore errors
• Example: rmdir mydirectory

cp

• Copies files or directories
• Syntax: cp [options] source destination
• Options:
  • -r recursive copy (directories)
  • -i prompt before overwrite
  • -v verbose
  • -p preserve attributes
• Example: cp myfile.txt mydir/

rm

• Deletes files or directories
• Options:
  • -r delete directories recursively
  • -f force delete without prompts
  • -i confirm before delete
  • -v verbose
• Dangerous: deletion is permanent
• Example: rm -r foldername

mv

• Moves or renames files/directories
• Options:
  • -v verbose
  • -i prompt before overwrite
  • -u move only if newer
  • -f force overwrite

• Examples:
  • mv file1.txt dir1/
  • mv file1.txt file2.txt
  • mv dir1/*.txt dir2/

cat

• Displays file content or concatenates files
• Options:
  • -n number all lines
  • -b number non-blank lines
  • -s squeeze blank lines
  • -v show non-printable characters

• Examples:
  • cat file1.txt
  • cat file1.txt file2.txt
  • cat -n file1.txt

Linux File Naming

• Case-sensitive
• Max length ~255 chars
• Avoid spaces; use _ or -
• Cannot use / or \
• Prefer lowercase, descriptive names

Hierarchical Directory Structure

• Follows FHS (Filesystem Hierarchy Standard)
• Root at /
• Organized like an inverted tree
• Key dirs: /bin, /etc, /home, /var, /usr, /tmp, /root

Absolute vs Relative Path

Absolute Path

• Starts from /
• Full location of a file
• Examples:
  • /home/user/documents/file.txt
  • /var/log/syslog
  • /usr/bin/python3

Relative Path

• Based on current directory
• Does not start with /
• Examples:
  • ./file.txt
  • ../folder/file.txt
  • ../../dir/file.txt

ls

• Lists directory contents

ls -l

• Shows detailed listing
• Includes permissions, owner, group, size, date, filename
• Example entry:
  -rw-r--r-- 1 user group 4096 Jan 1 00:00 file1.txt

ls -d

• Shows directory names only, not contents
• Example:
• ls -d directory1

chmod
Used to change file/directory permissions.
Syntax: chmod [options] mode file
Modes:

• Numeric (e.g., 644 → owner RW, group R, others R)
• Symbolic (e.g., u+w, g-r, o=x)

Examples:
chmod 644 file1.txt
chmod 755 dir1
chmod u-w file1.txt
chmod go+x dir1

pwd

Shows the current working directory.

cd

Changes directory.
cd /path → go to a specific directory.

cd (home)

cd ~ takes you to your home directory.

cd (parent)

cd .. moves one level up.

ls

Lists files in the current directory.

ls (options)

Common options:
-l → long list
-t → sort by time
-S → sort by size
-h → human-readable sizes
-r → reverse order
Combine: ls -ltr

ls -ltr

Long listing sorted by time, reversed.

Wildcard (*)

* matches any set of characters in filenames.

mkdir

Creates a new directory.

rmdir

Removes an empty directory.

Displaying Files

Tools: cat, less, head, tail.

cat

Shows entire file on screen.
Good for short files.

less

Scrollable file viewer.
Enter → scroll line
Space → scroll page
y → back one line
b → back one page
/text → search
q → quit.

head

Shows top of a file.
Default: first 10 lines.
head -n50 file.txt → first 50 lines.

tail

Same as head, but shows the bottom lines.

cp

Copies a file.

mv

Moves a file to another location.

mv (rename)

Also renames files.

rm

Removes a file.

rm -r

Removes directories and files recursively.
Dangerous—permanent deletion.

File Permissions

Each file has read, write, execute permissions.
ls -l file shows permissions.

Permission Types

r → read
w → write
x → execute (for directories: list contents)

Permission Groups

User → owner
Group → group members
World → everyone else

chmod

Changes permissions (if you own the file).
Format: chmod [u/g/o/a]+[r/w/x] file
Example: give execute to all users.

ps

Shows processes you are running.

top

Shows CPU usage of all processes.

kill

Terminates a process.

I/O Redirection & Piping

a | b → output of a → input of b
a > out.txt → write output to file
a < in.txt → read input from file.

Piping Examples

Combine commands using | to filter or transform output.

wc

Counts lines, words, characters.
Output columns: lines, words, chars.

umask
Sets default permissions for newly created files/directories.
Syntax: umask [mode]
Mode is octal (e.g., 022 prevents write for group/others).

Examples:
umask 022
umask -S
umask -p

chown
Changes owner/group of files.
Syntax: chown owner:group file

Examples:
chown new_owner file1.txt
chown :new_group dir1
chown new_owner:new_group file1.txt
chown -R new_owner:new_group dir1

chgrp
Changes group ownership of files/directories.
Syntax: chgrp new_group file

Examples:
chgrp new_group file1.txt
chgrp new_group dir1
chgrp -R new_group dir1

touch
Creates an empty file or updates timestamps.
Syntax: touch [options] file

Options:

• -a update access time
• -m update modification time
• -c don’t create if missing
• -r copy timestamps from another file

Examples:
touch file1.txt
touch -m file1.txt
touch -r file2.txt file1.txt

more
Displays file content page by page.
Syntax: more [options] file

Options:

• -n show line numbers

• -p show given percentage per page

• -c clear screen before each page

• -d show “more” prompt message

Examples:
more file1.txt
more -n file1.txt
more -p 25 file1.txt
more -c file1.txt

find

Searches files/directories based on conditions.
Syntax: find path [options] [expression]

Common:

• -name match file name
• -type file type (f, d)
• -mtime modified time
• -exec run command on results

Examples:
find . -name file1.txt
find . -type d -name dir1
find . -type f -mtime -7
find . -name file1.txt -exec rm {} \;

inode

Every file/dir has an inode storing metadata.

Contains:

• File type
• Permissions
• Owner + group
• Timestamps (access/modify/change)

• File size
• Disk block pointers
• Hard link count

Inodes identify files; filenames are just links pointing to inodes.

Hard Link

A hard link points directly to an inode.
Two filenames → same inode, same data.
Created with: ln file1 file2

Notes:

• Exists only within same filesystem
• No extra disk space
• Deleting one link doesn’t delete data until link count = 0

Example:
ln file1.txt file2.txt

Soft Link (Symbolic Link)

A soft link is a separate file storing a path to another file.
Created with: ln -s target linkname

Notes:

• Works across filesystems
• Breaks if original is deleted
• Uses extra space (stores path)

Example:
ln -s /path/original.txt link.txt

Hard Link vs Soft Link

Creation

• Hard link → ln file1 file2
• Soft link → ln -s target link

File System Relationship

• Hard link: points to the same inode; same actual file
• Soft link: separate file containing path to original

Effect of Deleting Original

• Hard link: still works; file data remains
• Soft link: breaks; points to nothing

Compatibility

• Hard link: same filesystem only
• Soft link: works across filesystems/partitions

Space Usage

• Hard link: no extra space (same inode)
• Soft link: needs extra space (stores path)