Bash Scripting Basics: Automate Tasks Like a Pro

Why Learn Bash Scripting?

Bash scripting automates repetitive tasks on Linux. Instead of typing commands manually, scripts execute sequences automatically. For cybersecurity, Bash scripting automates reconnaissance, log analysis, tool execution, and reporting.

Prerequisites

Basic Linux commands (ls,cd, grep, awk, sed), terminal navigation, and a text editor.

Script Structure

Every script starts with a shebang and can include comments:

#!/bin/bash
# This is a comment
echo "Hello, World!"

Making Scripts Executable

chmod +x script.sh
./script.sh

Variables

#!/bin/bash
name="KaliGo"
echo "Hello, $name!"
current_date=$(date)
echo "Today is: $current_date"
count=$((5 * 10))
echo "Result: $count"

Arrays

tools=("nmap" "wireshark" "metasploit")
echo ${tools[0]}         # nmap
echo ${tools[@]}         # All elements
echo ${#tools[@]}        # Array length

User Input

read -p "Enter target IP: " target
echo "Scanning $target..."
read -s -p "Enter password: " password

Conditionals

if [ "$num" -gt 10 ]; then
    echo "Greater than 10"
elif [ "$num" -eq 10 ]; then
    echo "Exactly 10"
else
    echo "Less than 10"
fi

File Tests

if [ -f "$file" ]; then echo "File exists"; fi
if [ -d "$dir" ]; then echo "Directory exists"; fi
if [ -x "$file" ]; then echo "File is executable"; fi

Loops

# For loop
for tool in nmap wireshark metasploit; do
    echo "Tool: $tool"
done

# Number range
for i in {1..5}; do
    echo "Iteration: $i"
done

# While loop
count=1
while [ "$count" -le 5 ]; do
    echo "Count: $count"
    ((count++))
done

# Read file line by line
while IFS= read -r line; do
    echo "Line: $line"
done < targets.txt

Functions

scan_port() {
    local host=$1
    local port=$2
    nc -zv "$host" "$port" 2>/dev/null
    if [ $? -eq 0 ]; then
        echo "Port $port is OPEN on $host"
    else
        echo "Port $port is CLOSED on $host"
    fi
}
scan_port "192.168.1.1" 80

Error Handling

set -euo pipefail    # Exit on error, undefined vars, pipe failures

Practical Automation

Network Scan Script

#!/bin/bash
set -euo pipefail
TARGET="$1"
OUTPUT_DIR="scan_results"
mkdir -p "$OUTPUT_DIR"
nmap -sn "$TARGET" -oG "$OUTPUT_DIR/ping_scan.txt"
nmap -sV -sC "$TARGET" -oN "$OUTPUT_DIR/port_scan.txt"
nmap --script vuln "$TARGET" -oN "$OUTPUT_DIR/vuln_scan.txt"
echo "Scan complete. Check $OUTPUT_DIR."

Log Analysis Script

#!/bin/bash
LOG_FILE="/var/log/auth.log"
echo "=== SSH Failed Logins ==="
grep "Failed password" "$LOG_FILE" | awk '{print $1, $2, $11}' | sort | uniq -c | sort -rn | head -10

Real-World Examples

Automated reconnaissance running theHarvester, DNS enumeration, and Nmap. Backup automation with cron. Phishing URL checker against multiple APIs.

Common Mistakes

Missing shebang, spaces in variable assignment (name = "value" is wrong), not quoting variables, not checking exit codes.

Best Practices

Use set -euo pipefail. Quote all variable expansions. Use meaningful names. Validate input. Handle errors gracefully. Use trap for cleanup. Lint with shellcheck.

Related Tools

shellcheck — Static analysis. bashdb — Debugger. shfmt — Formatter. expect — Interactive automation. parallel — Execute in parallel.

Related Articles

Summary

Bash scripting automates tasks using variables, conditionals, loops, functions, and error handling. Key practices: shebang line, proper quoting, exit codes ($?), and set -euo pipefail. Practical uses include automated scanning, log analysis, and tool orchestration.

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