🛠️ Linux Command Cheatsheet

📦 Installing Applications

sudo apt-get install <package-name>

Installs a package using APT (Advanced Package Tool).

📄 Create a File

touch hlo.txt

Creates a new empty file named hlo.txt.

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📁 Listing Files

Command	Description
ls	Lists contents of the current directory
ls -l	Lists with detailed information
ls /home	Lists the content of the file from another directory (ls /(directory)_

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⛓️ Aliases

alias hlo="ls"

Creates a custom shortcut command hlo that performs ls.

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📖 Help & Documentation

Command	Description
man <command>	Opens the manual page for a command (use / to search)
<command> --help	Displays brief help for the command
help	Lists built-in shell commands and syntax
apropos <keyword>	Searches command descriptions matching the keyword (e.g. apropos list)
stat <file>	Shows detailed information about a file or filesystem

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🧠 Bash Shortcuts

Shortcut	Action
Ctrl + A	Move cursor to start of the line
Ctrl + E	Move cursor to end of the line
Ctrl + U	Delete everything before the cursor
Ctrl + K	Delete everything after the cursor

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📂 Important System Paths

Path	Description
/etc	Common system configuration files
/bin, /sbin	Essential user/system binaries
/lib	Shared library files

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📁 Navigating Directories

• Use cd to change directory.

• To access a space-separated folder like exercise file, use:

• when file name starts with - we need to use ./ infront of the file name

  cd exercise\ file
  cd ./--spaces\ in\ this\ filename--

• To go back one level:

  cd ..

• To go back two or more levels:

  cd ../../

• To switch between two directories:

  cd -

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🗂️ File Operations

Command	Description
mkdir newfolder	Create a new directory
rmdir newfolder	Remove an empty directory
cp file.txt backup.txt	Copy a file
mv file.txt folder/	Move file to folder
mv oldname.txt newname.txt	Rename a file
mv *.txt folder/	Move all .txt files to a folder
rm -r folder/	Delete a folder and its contents recursively

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🧵 Process Management

Command	Description
ps -u <user>	List processes for a specific user
pgrep <program>	Find process ID of a program
kill <PID>	Kill a process by ID
pkill -9 <name>	Force kill all processes by name
top, htop	Real-time process monitor
Ctrl + Z	Pause current foreground job
bg	Move paused job to background
fg	Bring background job to foreground

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GREP

📘 Regex Expression

🔹 Anchors – Start & End of Line

Symbol	Description	Example	Explanation
^	Start of string/line	^abc	Matches any string that starts with "abc"
$	End of string/line	xyz$	Matches any string that ends with "xyz"

🔹 Character Classes – Match Letters, Digits

Pattern	Description	Example	Explanation
[A-Z]	Uppercase letters	^[A-Z]	Matches strings starting with uppercase
[a-z]	Lowercase letters	[a-z]+	Matches one or more lowercase letters
[0-9]	Digits	[0-9]{3}	Matches exactly 3 digits
[A-Za-z]	Any letter	[A-Za-z]+	Matches one or more letters
[^a-z]	Negation (not lowercase)	[^a-z]	Matches any character except lowercase

🔹 Bracket Expression [] Rules

Bracket expressions match one character from a set:

Pattern	Matches
[abc]	One of 'a', 'b', or 'c'
[a-z]	Any lowercase letter
[A-Z]	Any uppercase letter
[0-9]	Any digit from 0 to 9
[a-zA-Z0-9_]	Any alphanumeric character or underscore
[^0-9]	Any non-digit character

🔹 Quantifiers & Modifiers

Symbol	Description	Example	Explanation
.	Any character except newline	a.c	Matches "abc", "axc", etc.
*	0 or more repetitions	lo*l	Matches "ll", "lol", "lool", etc.
+	1 or more repetitions	lo+l	Matches "lol", "lool" but not "ll"
?	0 or 1 repetition	lo?l	Matches "ll" or "lol"
{n}	Exactly n times	[0-9]{4}	Matches exactly 4 digits
{n,m}	Between n and m times	[a-z]{2,5}	2 to 5 lowercase letters

🧪 Real-world Example

^[A-Z][a-z]{3,8}[0-9]{2}$

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🔒 File Permissions & Ownership

Symbol	Meaning
r	Read
w	Write
x	Execute

Example output from ls -l:

drwxr-xr-x

• Owner: rwx
• Group: r-x
• Others: r-x

Change permissions:

chmod 777 file.txt

Grants read, write, and execute to everyone.

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👥 User & Group Management

adduser aswin         # Create new user
userdel aswin         # Delete user
sudo groupadd family  # Create new group

Important files:

• /etc/passwd → All users
• /etc/shadow → Passwords in hashed form
• /etc/group → Group details

Other Commands:

sudo passwd <username>  # Set password for a user
usermod <options>       # Modify user settings
sudo visudo             # Modify sudoers file for permissions

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🌐 Hosting a Simple Website with Python

Start HTTP server:

python3 -m http.server 8000

• Access via browser: http://localhost:8000
• Change port by replacing 8000

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🌐 Interact with Web Servers from CLI

Use curl to access websites:

curl http://localhost:8080

Download files with curl:

curl -o file.html http://localhost:8080

Inspect response headers:

curl -I http://localhost:8080

Verbose (detailed request info):

curl -v http://localhost:8080

Use wget to download:

wget http://localhost:8080/file.txt

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🧰 Common Hacking Tools

Tool	Purpose
crunch	Generate wordlists
hydra	Password cracking
john	Hash cracking

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🧪 Extracting IPs from Ping Output (Example)

cat ip.txt | grep "64 bytes" | cut -d " " -f 4 | tr -d ":"

Extracts clean IP addresses from ping output in ip.txt

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📂 Move Executable to Global Directory

To make a CLI tool (like caido) accessible from anywhere:

sudo mv caido /usr/local/bin/

🔒 sudo is required because /usr/local/bin/ is a system-wide directory for executables.

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