Bash Basics

General Tips

Always start script with #!/bin/bash Give the script run permissions using chmod +x script.sh Each line is a new command that will be ran

---

Variables

Variables can be set using:

varname=value

Variables can be retrieved using $varname:

name="f1shh"
echo "My name is $name"

Script parameters

Parameters are stored in variables $1, $2, $3, ... To get the number of parameters, use $#

User Input

User input can be taken using the read varname command and the value taken can be retrieved like any other variable using $varname. EX:

echo "What is your favorite food"
read food
echo "You said you like: $food"

Arrays

Arrays in Bash are 0 indexed. You can create an array using arrayname=('elm1' 'elm2' 'elm3' 'elm4') You can then access an element of the array with $arrayname[index]

If you want to print out all elements of the array, you can do so with: echo "${arrayname[@]}"

if we want to remove or update a element in the array, we can do unset arrayname[index] or arrayname[index]='value'

Array Cheatsheet:

arrayname=('elm1' 'elm2' 'elm3' 'elm4')

echo $arrayname[1]
echo "{$arrayname[@]}"

arrayname[2]="UpdatedValue"
echo $arrayname[2]

unset arrayname[3]
echo "{$arrayname[@]}"

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Conditionals

Stolen From TryHackMe: https://tryhackme.com/room/bashscripting

Operator	Description
-eq or =	Checks if the value of two operands are equal or not; if yes, then the condition becomes true.
-ne	Checks if the value of two operands are equal or not; if values are not equal, then the condition becomes true.
-gt	Checks if the value of left operand is greater than the value of right operand; if yes, then the condition becomes true.
-lt	Checks if the value of left operand is less than the value of right operand; if yes, then the condition becomes true.
-ge	Checks if the value of left operand is greater than or equal to the value of right operand; if yes, then the condition becomes true.

number = 100
if [$number -ge 50]
then
	echo 'The number is greater then 50'
else
	echo 'the number is less then 50'
fi

You can check multiple conditions using:

number = 100
if [$number = 25] || [$number -gt 50]
then
	echo 'The number is 25 or greater then 50'
else
	echo 'the number is not 25 or is less then 50'
fi

---

For Loops

Loop through an range of values with:

# Prints out values 1-5
for n in {1..5}; 
do
    echo $n
done

Or you can use a more familiar style:

n=25
for (( i=1 ; i<=$n ; i++ )); 
do
    echo $i
done

Loop through an array:

arrayname=("elm1" "elm2" "elm3") 
for n in ${arrayname[@]}; 
do
    echo $n
done

Infinite Loop taken from: https://www.geeksforgeeks.org/bash-scripting-for-loop/

n=4
for (( ; ; )); 
do
    if [ $n -eq 9 ];then
        break
    fi
    echo $n
    ((n=n+1))
done