# Bash Scripting

## <mark style="color:yellow;">ABOUT</mark>

**Bash is the scripting language we use to communicate with Unix-based OS and give commands to the system.**

### <mark style="color:blue;">Script Execution - Examples</mark>

```bash
venator17@kali[/kali]$ bash script.sh <optional arguments>
venator17@kali[/kali]$ sh script.sh <optional arguments>
venator17@kali[/kali]$ ./script.sh <optional arguments>
```

### <mark style="color:blue;">Shebang</mark>

The `#!/bin/bash` at the beginning of a Bash script is known as a shebang or hashbang. It serves as a directive to the operating system, indicating which interpreter should be used to execute the script.

## <mark style="color:yellow;">CONDITIONAL EXECUTION</mark>

### <mark style="color:blue;">If-Else</mark>

```bash
if [condition]
then [execution]
else [what would be executed if condition would fail]
fi [closing]
```

### <mark style="color:blue;">If-Only</mark>

```bash
if [condition]
then [execution]
fi [closing]
```

### <mark style="color:blue;">If-Elif-Else</mark>

```bash
if [first condition]
then [execution]
elif [second condition]
then [execution]
else [what would be executed if conditions would fail]
fi [closing]
```

### <mark style="color:blue;">Case</mark>

```bash
case <expression> in
	pattern_1 ) statements ;;
	pattern_2 ) statements ;;
	pattern_3 ) statements ;;
esac
```

**Example:**

```bash
case $opt in 
	"1") network_range ;;
	"2") ping_host ;;
	"3") network_range && ping_host ;;
	"*") exit 0 ;;
esac
```

### <mark style="color:blue;">Comparison Operators</mark>

| Operator | Explanation              |
| -------- | ------------------------ |
| `-eq`    | equal to                 |
| `-ne`    | not equal to             |
| `-lt`    | less than                |
| `-le`    | less than or equal to    |
| `-gt`    | greater than             |
| `-ge`    | greater than or equal to |

### <mark style="color:blue;">String Operators</mark>

| Operator | Description                                 |
| -------- | ------------------------------------------- |
| `==`     | is equal to                                 |
| `!=`     | is not equal to                             |
| `<`      | is less than in ASCII alphabetical order    |
| `>`      | is greater than in ASCII alphabetical order |
| `-z`     | if the string is empty (null)               |
| `-n`     | if the string is not null                   |

### <mark style="color:blue;">File Operators</mark>

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

### <mark style="color:blue;">Special Variables</mark>

|      |                                                      |
| ---- | ---------------------------------------------------- |
| `$#` | Number of arguments passed to the script.            |
| `$@` | List of command-line arguments.                      |
| `$n` | `n` is number of argument                            |
| `$$` | Id of executing process                              |
| `$?` | Success of command. `0` is success, `1` is a failure |

### <mark style="color:blue;">Regular Variables</mark>

```bash
> variable="Declared without an error."
> echo $variable
> Declared without an error.
```

## <mark style="color:yellow;">ARRAYS</mark>

```bash
> domains=(shadow wizard money gang)
> echo ${domains[0]}
> shadow
```

**OR**

```bash
> domains=(shadow wizard "money gang")
> echo ${domains[2]}
> money gang
```

## <mark style="color:yellow;">LOOPS</mark>

### <mark style="color:blue;">For</mark>

```bash
for variable in list
do
    # Commands to be executed for each item in the list
done
```

**Example:**

```bash
fruits=("apple" "orange" "banana")
for fruit in "${fruits[@]}"
do
    echo "I like $fruit"
done
```

### <mark style="color:blue;">While</mark>

```bash
while [ condition ]
do
    # Commands to be executed while the condition is true
done
```

**Example:**

```bash
count=1 # Count from 1 to 5
while [ $count -le 5 ]
do
    echo $count
    ((count++))
done
```

### <mark style="color:blue;">Until</mark>

```bash
until [ condition ]
do
    # Commands to be executed until the condition becomes true
done
```

**Example:**

```bash
count=1 # Count from 1 to 5
until [ $count -gt 5 ]
do
    echo $count
    ((count++))
done
```

### <mark style="color:blue;">Script Termination</mark>

| Exit Status | Explanation              |
| ----------- | ------------------------ |
| `exit 0`    | Succesful execution      |
| `exit 1`    | General error condition  |
| `exit 2`    | Specific error condition |

### <mark style="color:blue;">Wildcards</mark>

In Bash, a **wildcard** refers to a **character or a set of characters** that can be used to represent a group of filenames or strings. Wildcards are often used in commands to perform operations on multiple files or strings that match a specified pattern.

| Wildcard                                 | Example Usage              | Explanation                                                                         |
| ---------------------------------------- | -------------------------- | ----------------------------------------------------------------------------------- |
| `*` (Asterisk)                           | `echo *.txt`               | Matches all files ending with ".txt".                                               |
| `?` (Question Mark)                      | `ls file?.txt`             | Matches files like "file1.txt", "fileA.txt", etc.                                   |
| `[ ]` (Square Brackets)                  | `ls [aeiou]*.txt`          | Matches any file starting with a vowel and ending with ".txt".                      |
| `{ }` (Brace Expansion)                  | `cp file{1,2,3}.txt dest/` | Expands to "file1.txt", "file2.txt", and "file3.txt" and copies to the destination. |
| `!(pattern)` (Extended Pattern Matching) | `ls !(file*.txt)`          | Matches all files except those starting with "file" and ending with ".txt".         |
| `?(pattern)` (Zero or One Occurrence)    | `ls file?(1).txt`          | Matches "file.txt" or "file1.txt".                                                  |
| `+(pattern)` (One or More Occurrences)   | `ls file+(1).txt`          | Matches "file1.txt", "file11.txt", etc.                                             |
| `*(pattern)` (Zero or More Occurrences)  | `ls file*(1).txt`          | Matches "file.txt", "file1.txt", "file11.txt", etc.                                 |

## <mark style="color:yellow;">TIPS & TRICKS</mark>

1. You Could use <mark style="color:green;">`tee`</mark> command for writing output to both standard output and file. If you would use <mark style="color:green;">`tee -a`</mark>, it would
2. Use <mark style="color:green;">`bash -x -v`</mark> to verbose debugging


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