CLI & Scripting

The terminal is where real work happens. Scripting is the difference between doing something once and automating it forever.

This section covers Bash fundamentals and enough Python to be dangerous.

Three Skills, In Order

You do not need to be a programmer to be dangerous here. You need three things, stacked in this order — each one makes the next one worth learning.

Shell Fluency & Pipes

The shell is the program that reads what you type and runs it — Bash is the shell almost every Linux system hands you by default. Fluency just means you stop thinking about the syntax and start thinking about the task. The one idea worth learning early is the pipe (|): it takes the output of one command and feeds it straight in as the input to the next, no temporary files, no copy-paste.

ls -la | grep ".txt"

ls -la lists files. grep ".txt" reads that list and keeps only lines containing .txt. Neither command knows about the other — the shell wires one’s output into the other’s input. Small tools, chained together, doing something neither does alone. That is the whole philosophy of the command line. Full reference: Bash Fundamentals.

Bash: Automate One Chore

A script is a list of commands saved in a file so you can run them again without retyping them. That is all “automation” means at this level — you are recording your own hands, not writing software. Pick one thing you do more than once and put it in a file:

#!/bin/bash
for ip in 192.168.1.1 192.168.1.2 192.168.1.3; do
    ping -c 1 "$ip"
done

The first line tells the OS which program runs this file. The for loop repeats the command once per address. Run chmod +x script.sh to make it executable, then ./script.sh runs the whole chore in one shot on your lab VM. You do not learn Bash by reading about loops — you learn it by automating one real, small, annoying chore. Conditionals and more one-liners: Bash Fundamentals.

Python: Glue Tool Output

Bash is great at chaining whole programs together. It gets painful the moment you need to actually parse something — pull one field out of JSON, loop over structured data. That is where Python takes over. Think of it as glue: most security tools already exist and already work, and Python’s job is rarely to reimplement them — it takes the output of one tool, reshapes it, and feeds it into the next step. Read a wordlist file line by line. Pull the open ports out of an nmap scan.

with open("wordlist.txt", "r") as f:
    for line in f:
        word = line.strip()
        print(word)

Open the file, walk it one line at a time, strip the trailing newline off each word. That pattern — open something, loop over it, do a small thing per item — covers a surprising share of real scripting work. Full reference: Python for Security.

Learn shell fluency first — you use it every session. Add Bash scripting the first time retyping something gets annoying. Reach for Python the first time Bash’s text-munging starts fighting you.

Next Bash FundamentalsThe Bash basics needed to automate tasks and chain tools together on the command line.