Bash

## ------------------| Environment Variables
env                    # List all envs
echo $PATH             # List path env
echo $USER             # Current user
echo $HOME             # Current user's home
echo $PWD              # Current workind dir
echo $HISTSIZE         # Number of commnds (in memory/current session)
echo $HISTFILESIZE     # How many commands in ~./history
export ip="0.0.0.0"    # Make ip as global variable

#!/bin/bash

array=(hello this is an array 123)
echo ${array[2]}

## ------------------| Maximam 9 ($0-$9)
./script.sh ARG1 ARG2 ARG3 ... ARG9
     $0      $1   $2   $3  ...  $9

## ------------------| Special Variables / Internal Field Separator (IFS)
$#  Number of arguments passed to the script.
$@  Retrieve the list of command-line arguments.
$$  The process ID of the currently executing process.
$?  The exit status of the script. ( 0 <= SUCC , 1 <= FAIL).

==  is equal to [EX: "$1" == "HackTheBox"]
!=  is not equal to
<  is less than in ASCII alphabetical order & works only within the double square brackets [[ <condition> ]]
>  is greater than in ASCII alphabetical order & works only within the double square brackets [[ <condition> ]]
-z  if the string is empty (null) (EX: [[ -z $1 ]])
-n  if the string is not null

-eq  is equal to
-ne  is not equal to
-lt  is less than
-le  is less than or equal to
-gt  is greater than
-ge  is greater than or equal to

-e  file exist ?
-f  a file ?
-d  a directory ?
-L  a symbolic link ?
-O  current user owns the file ?
-r  the file has read permission ?
-w  the file has write permission ?
-x  the file has execute permission ?
-s  the file has a size greater than 0 ?
-G  the file’s group id matches the current user’s ?
-N  checks if the file was modified after it was last read

&&  logical AND (Ex: [[ -e "$1" && -r "$1" ]])
!  logical negotation NOT
||  logical OR

+          Addition
-          Substraction
*          Multiplication
/          Division
%          Modulus
variable++  Increase the value of the variable by 1
variable--  Decrease the value of the variable by 1
${#variable}    Length of the variable

read -p "Select your option: " option
echo $option

-p    Input remains on the same line

..... | tee -a filename.txt

-a / --append

## ------------------| With Numbers
for $i in 1 2 3 4
do
  echo $i
done

## ------------------| With Files
for $words in file1 file2 file3
do
  echo $words
done

## ------------------| One line
for i in {1..5};do echo $i;done

## A statement is executed as long as a condition is fulfilled (true).
while [ <CONDITION> ]
do
  <TASK>
done

## The code inside a until loop is executed as long as the particular condition is false.
counter=0
until [ $counter -eq 10 ]
do
  # Increase $counter by 1
  ((counter++))
  echo "Counter: $counter"
done

if [ <CONDITION> ]
then
  <TASK-I>
elif [ CONDITION ]
then
  <TASK-II>
else
  <TASK-FINAL>
fi

case <expression> in
  pattern_1 ) statements ;;
  pattern_2 ) statements ;;
  "*") exit 0 ;;
esac

## ------------------| Method 1
function name {
  <commands>
}

## ------------------| Method 2
name() {
  <commands>
}

function print {
  echo $1 $2 $3
}

two="Second"

print_pars "First" "$two" "3"

1  General errors
2  Misuse of shell builtins
126  Command invoked cannot execute
127  Command not found
128  Invalid argument to exit
128+n  Fatal error signal "n"
130  Script terminated by Control-C
255\*  Exit status out of range