Shell scripting in Kali Linux: automate repeated tasks with a safe lab script

Key takeaways

• A Bash script is a text file of commands, made executable and run in a shell.
• Use variables, loops and exit codes to make scripts reliable.
• Build scope checks in so the script only touches approved lab hosts.
• Quote variables, use set -euo pipefail, and check scripts with shellcheck.
• Automate only what you understand when done by hand.

Why script in Kali?

Testers repeat the same steps: scan, save output, sort results. A script does them identically each time and records what was run. For the basics of Bash, read what shell scripting is in Linux first.

What are the building blocks?

#!/usr/bin/env bash
NAME="lab" # variable
for host in a b c; do # loop
 echo "$NAME $host"
done
if [ -f targets.txt ]; then echo "found"; fi # condition

• The first line, the shebang, says which interpreter runs the script.
• "$VAR" in double quotes avoids breaking on spaces.
• $? holds the exit status of the last command.
• chmod +x script.sh makes it executable.

What does a safe lab scan script look like?

This script reads a file of lab hosts you own, refuses to run if it is empty, and saves one result file per host. Run it only on your own virtual lab network.

#!/usr/bin/env bash
set -euo pipefail

TARGETS="targets.txt" # one lab IP per line
OUT="scans_$(date +%Y%m%d_%H%M)"

if [! -s "$TARGETS" ]; then
 echo "No targets listed in $TARGETS" >&2
 exit 1
fi

echo "Scanning ONLY the hosts in $TARGETS."
read -r -p "Do you have written permission for all of them? (yes/no) " ok
[ "$ok" = "yes" ] || { echo "Stopped."; exit 1; }

mkdir -p "$OUT"
while read -r host; do
 [ -z "$host" ] && continue
 echo "Scanning $host"
 nmap -sV -oN "$OUT/$host.txt" "$host"
done < "$TARGETS"

echo "Done. Results in $OUT"

How does the script work?

• set -euo pipefail stops on errors, unset variables and pipe failures.
• $(date...) builds a unique folder name.
• [! -s file ] checks that the file exists and is not empty.
• read -r -p asks for confirmation. The prompt is a reminder that scope matters.
• while read loops over each line of the targets file.
• nmap -sV -oN detects service versions and writes a normal-format file. The Nmap reference guide explains each option.

A target file might contain 192.168.56.101, your own Metasploitable VM.

What mistakes are common?

• Forgetting to quote variables, which breaks on spaces.
• Running scripts as root when not needed.
• Hard-coding passwords or IP ranges that are not yours.
• Not testing on one host before looping over many.
• Ignoring exit codes.

How do you check your script?

Run bash -n script.sh to check syntax and install shellcheck to catch common errors. Test with a single lab host first.

What else can you automate?

Reconnaissance logs, report folder setup, package updates, backups of your notes and routine tool launches. Related reading: mastering Bash scripting with practical examples and command-line shells in Kali.

Legal note

Scanning systems without permission is illegal under India's IT Act. The script's prompt is not a legal safeguard. Written authorisation is.

Next steps

To learn Kali tooling with a trainer, see the KLCP Kali Linux course. For the Linux base, see the Linux course.