Bash Scripting
Automate the Boring Parts

Variables and Input

Shell scripting is how you stop repeating yourself. Anything you type more than twice should probably be a script. For security work this means: automating host discovery, processing wordlists, parsing tool output, and running the same recon steps against multiple targets without the manual overhead.

Variables in bash are set without spaces around the equals sign. They're accessed with a dollar sign. Command substitution wraps a command in $() and uses its output as a value. $1, $2, etc. are positional parameters — arguments passed to the script.

#!/bin/bash
TARGET=$1
PORT=${2:-80}
DATE=$(date +%Y%m%d)

echo "Scanning $TARGET on port $PORT"
echo "Started: $DATE"

Conditionals and Loops

If statements in bash check exit codes. A command that succeeds returns 0 (true). Failure returns non-zero. The test command (or [ ]) checks conditions: -f tests if a file exists, -z tests for empty string, -eq compares integers. For loops iterate over lists. While loops run until a condition changes.

for ip in $(seq 1 254); do
  ping -c 1 -W 1 192.168.1.$ip &>/dev/null && \
    echo "192.168.1.$ip is up"
done

while IFS= read -r subdomain; do
  result=$(host "$subdomain.target.com" 2>/dev/null)
  [[ "$result" == *"has address"* ]] && echo "$subdomain.target.com found"
done < subdomains.txt

Security Automation Examples

A recon script that runs against a target, collects all open ports, and then automatically runs targeted scripts against each one saves hours on every engagement. The key is making it idempotent — running it twice shouldn't break anything — and making it output structured, parseable results rather than just terminal output.

#!/bin/bash
TARGET=$1
OUTDIR="recon_$(date +%Y%m%d_%H%M%S)"
mkdir -p "$OUTDIR"

echo "[*] All ports scan..."
nmap -sS -p- --min-rate 5000 "$TARGET" -oN "$OUTDIR/allports.txt"

PORTS=$(grep "^[0-9]" "$OUTDIR/allports.txt" | cut -d'/' -f1 | tr '\n' ',' | sed 's/,$//')

echo "[*] Targeted scan on: $PORTS"
nmap -sV -sC -p "$PORTS" "$TARGET" -oN "$OUTDIR/targeted.txt"
echo "[+] Done. Results in $OUTDIR/"