There are a few other variables that the system sets for you to use as well.
$0 - The name of the Bash script.
$1 - $9 - The first 9 arguments to the Bash script. (As mentioned above.)
$# - How many arguments were passed to the Bash script.
$@ - All the arguments supplied to the Bash script.
$? - The exit status of the most recently run process.
$$ - The process ID of the current script.
$USER - The username of the user running the script.
$HOSTNAME - The hostname of the machine the script is running on.
$SECONDS - The number of seconds since the script was started.
$RANDOM - Returns a different random number each time is it referred to.
$LINENO - Returns the current line number in the Bash script.
$HOME - /Users/faymaz
$SHELL -
$LOGNAME -
$USER -

variable=value

read user_input
# Print the entered string
echo "Welcome $user_input"

read num1
read num2

echo "$((num1 + num2))"
echo "$((num1 - num2))"
echo "$((num1 * num2))"
echo "$((num1 / num2))"

#!/bin/bash

# Define two integer variables
num1=5
num2=3

# Add the two integers and store the result in a third variable
result=$((num1 + num2))

# Print the result
echo "The sum of $num1 and $num2 is: $result"

! EXPRESSION 	The EXPRESSION is false.
-n STRING 	The length of STRING is greater than zero.
-z STRING 	The lengh of STRING is zero (ie it is empty).
STRING1 = STRING2 	STRING1 is equal to STRING2
STRING1 != STRING2 	STRING1 is not equal to STRING2
INTEGER1 -eq INTEGER2 	INTEGER1 is numerically equal to INTEGER2
INTEGER1 -gt INTEGER2 	INTEGER1 is numerically greater than INTEGER2
INTEGER1 -lt INTEGER2 	INTEGER1 is numerically less than INTEGER2
-d FILE 	FILE exists and is a directory.
-e FILE 	FILE exists.
-r FILE 	FILE exists and the read permission is granted.
-s FILE 	FILE exists and it's size is greater than zero (ie. it is not empty).
-w FILE 	FILE exists and the write permission is granted.
-x FILE 	FILE exists and the execute permission is granted.
-lt as opposed to -le (less than as opposed to less than or equal).

if [ <some test> ]
then
<commands>
fi

#!/bin/bash
# Basic if statement
if [ $1 -gt 100 ]
then
echo Hey that\'s a large number.
pwd
fi
date

if [ <some test> ]
then
<commands>
elif [ <some test> ]
then
<different commands>
else
<other commands>
fi

#!/bin/bash
# elif statements
if [ $1 -ge 18 ]
then
echo You may go to the party.
elif [ $2 == 'yes' ]
then
echo You may go to the party but be back before midnight.
else
echo You may not go to the party.
fi

#!/bin/bash
# and example
if [ -r $1 ] && [ -s $1 ]
then
echo This file is useful.
fi

#!/bin/bash
# or example
if [ $USER == 'bob' ] || [ $USER == 'andy' ]
then
ls -alh
else
ls
fi

case <variable> in
<pattern 1>)
<commands>
;;
<pattern 2>)
<other commands>
;;
esac

#!/bin/bash
# case example
case $1 in
start)
echo starting
;;
stop)
echo stoping
;;
restart)
echo restarting
;;
*)
echo don\'t know
;;
esac

    #!/bin/bash
    # Print a message about disk useage.
    space_free=$( df -h | awk '{ print $5 }' | sort -n | tail -n 1 | sed 's/%//' )
    case $space_free in
    [1-5]*)
    echo Plenty of disk space available
    ;;
    [6-7]*)
    echo There could be a problem in the near future
    ;;
    8*)
    echo Maybe we should look at clearing out old files
    ;;
    9*)
    echo We could have a serious problem on our hands soon
    ;;
    *)
    echo Something is not quite right here
    ;;
    esac

while [ <some test> ]
do
<commands>
done

#!/bin/bash
# Basic while loop
counter=1
while [ $counter -le 10 ]
do
echo $counter
((counter++))
done
echo All done

#!/bin/bash
# Basic until loop
counter=1
until [ $counter -gt 10 ]
do
echo $counter
((counter++))
done
echo All done

for var in <list>
do
<commands>
done

#!/bin/bash
# Basic range in for loop
for value in {1..5}
do
echo $value
done
echo All done

#!/bin/bash
# Basic for loop
names='Stan Kyle Cartman'
for name in $names
do
echo $name
done
echo All done

#!/bin/bash

# Define the number of odd natural numbers to generate
n=10

# Initialize a variable to keep track of the current number
current=1

# Loop to generate and print odd natural numbers
while [ $current -le $n ]; do
    # Check if the current number is odd
    if [ $((current % 2)) -ne 0 ]; then
        echo $current
    fi

    # Increment the current number
    ((current++))
done

ls /etc | wc -l
read -p 'Username: ' uservar
read -sp 'Password: ' passvar
let <arithmetic expression>
expr item1 operator item2

+, -, \*, / 	addition, subtraction, multiply, divide
var++ 	Increase the variable var by 1
var-- 	Decrease the variable var by 1
% 	Modulus (Return the remainder after division)

#!/bin/bash
# Basic arithmetic using double parentheses
a=$(( 4 + 5 ))
echo $a # 9
a=$((3+5))
echo $a # 8
b=$(( a + 3 ))
echo $b # 11
b=$(( $a + 4 ))
echo $b # 12
(( b++ ))
echo $b # 13
(( b += 3 ))
echo $b # 16
a=$(( 4 * 5 ))
echo $a # 20

${#variable}

cat << EOF
Bla bla
bla balalalals
Check out the README file on GitHub to do 1 quick thing manually:
You can safely close this terminal.
EOF

$ wget -O- https://repository.faymaz.com/deb/faymaz.gpg | sudo tee /usr/share/keyrings/faymaz-archive-keyring.gpg
$ echo "deb     [signed-by=/usr/share/keyrings/faymaz-archive-keyring.gpg] https://repository.faymaz.com/deb/pub all main" | sudo tee /etc/apt/sources.list.d/faymaz.com.list