Bash scripting basics

In a nutshell A bash script is just a file of shell commands run top to bottom. The moment you're doing the same commands twice, a script does them for you — sweeping a range of hosts, trying a list of things, automating a check. For a security person it's the glue that turns manual steps into a repeatable tool.

From typing commands to saving them

#!/bin/bash              # the shebang: which interpreter runs this file
echo "Starting scan..."
nmap -sV 10.10.10.5
echo "Done."

chmod +x scan.sh    # make it runnable
./scan.sh           # run it

Analogy — a recipe card. Instead of remembering each step every time you cook, you write the recipe once and follow it exactly, without forgetting a step.

The building blocks

target="10.10.10.5"           # variable; $target reads it
echo "Scanning $target"

echo "Pinging $1"             # $1 = the first argument you pass in
ping -c 1 "$1"                # ./ping.sh 10.10.10.5

if ping -c 1 "$1" &>/dev/null; then   # conditional
  echo "$1 is up"
fi

The real power for security — loops

# ping-sweep a whole /24 in one loop
for i in $(seq 1 254); do
  ping -c 1 -W 1 10.10.10.$i &>/dev/null && echo "10.10.10.$i is up"
done

# try each word from a file
while read word; do
  echo "trying $word"
done < wordlist.txt

The pipe — the most important operator

The pipe | sends one command's output into the next, building complex processing from simple parts in one line:

Why it matters in security

• Automate the boring, repeatable parts — sweep a network, hit many URLs, parse tool output.
• Read what you find — you will meet bash scripts on targets (cron jobs, backup scripts). Reading them spots a privesc path — a root cron running a writable script (see Linux privilege escalation).
• Glue tools together — pipe one into another, loop over a list, and you've built a small custom tool.