# 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<br>

***

## Variables

Variables can be set using:

```bash
varname=value
```

Variables can be retrieved using `$varname`:

```bash
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:

```bash
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:**

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

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

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

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

***

## 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. |

```bash
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:

```bash
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:

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

Or you can use a more familiar style:

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

Loop through an array:

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

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

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