DevOps Engineer

PowerShell ships with Windows; bash ships with macOS and Linux. You already have everything you need.

1. 1

   Pick a real chore

   Not a toy problem — something that genuinely annoys you. Sorting a Downloads folder full of four hundred files. Renaming photos to a consistent format. Finding every file over 500MB. It has to be real, or the payoff at the end will not land.

2. 2

   Do it by hand once, and time it

   Actually time it. Write the number down. This is the figure you are going to beat, and it is also how you will one day justify automation work to a manager who wants to know why it is worth your afternoon.

3. 3

   Make a copy to work on

   Duplicate the folder. Everything from here runs against the copy. This is not caution for beginners — it is what people do professionally, because the cost of being wrong is asymmetric.

   Windows

   Copy-Item -Path .\Downloads -Destination .\Downloads-copy -Recurse

   macOS

   cp -R ~/Downloads ~/Downloads-copy

   Linux

   cp -R ~/Downloads ~/Downloads-copy

4. 4

   Ask what it WOULD do, before it does anything

   Run the operation in preview mode first. Nothing changes; it just tells you what it intends. Read that output properly. This habit is the single biggest difference between someone safe to give automation to and someone who is not.

   Windows

   Get-ChildItem .\Downloads-copy -File | Group-Object Extension | Format-Table Name, Count

   macOS

   find ~/Downloads-copy -type f | sed "s/.*\.//" | sort | uniq -c | sort -rn

   Linux

   find ~/Downloads-copy -type f | sed "s/.*\.//" | sort | uniq -c | sort -rn

5. 5

   Now let it run

   Sort every file into a folder named after its extension. Five lines or so. It will not work first time — read the error, change one thing, run it again. That loop, repeated, is the actual job.

   Windows

   Get-ChildItem .\Downloads-copy -File | ForEach-Object { $d = ".\Downloads-copy\" + $_.Extension.TrimStart("."); New-Item -ItemType Directory -Force -Path $d | Out-Null; Move-Item $_.FullName $d }

   macOS

   cd ~/Downloads-copy && for f in *.*; do d="${f##*.}"; mkdir -p "$d"; mv "$f" "$d"/; done

   Linux

   cd ~/Downloads-copy && for f in *.*; do d="${f##*.}"; mkdir -p "$d"; mv "$f" "$d"/; done

6. 6

   Compare it to your number

   You wrote down how long it took by hand. It now takes under a second, and it will take under a second every time from now on. Sit with that for a moment — that gap, multiplied across an organisation, is the entire economic argument for automation as a career.

One free single-file download (Terraform). No account, no cloud provider, no credit card, and nothing here can cost you money — you will be creating files on your own disk, not servers. Delete the binary afterwards if you like.

1. 1

   Get the tool

   Download Terraform from terraform.io — it is a single executable. Put it somewhere on your PATH, or just work in the folder you downloaded it to. Check it runs.

   Any

   terraform -version

2. 2

   Describe something that does not exist yet

   Make a new empty folder and create a file called main.tf. You are not writing instructions here — you are describing a desired end state, and that distinction is the whole idea.

   Any

   terraform {
     required_providers {
       local = { source = "hashicorp/local" }
     }
   }
   
   resource "local_file" "hello" {
     filename = "${path.module}/out/hello.txt"
     content  = "built by terraform\n"
   }

3. 3

   Ask what it WOULD do

   Initialise, then run a plan. Read the output properly. It tells you exactly what will be created, changed or destroyed, and nothing has happened yet. This preview-before-acting habit is most of what separates infrastructure people who are safe from ones who are not.

   Any

   terraform init
   terraform plan

4. 4

   Make it real

   Apply it. Look in the out/ folder — the file exists. Now run plan again: it reports no changes, because reality already matches your description.

   Any

   terraform apply
   terraform plan

5. 5

   Break reality and watch it notice

   Delete the file it made, by hand. Run plan again. It has detected drift — reality no longer matches the description — and it tells you precisely how to get back. Apply again and it repairs itself. This is the moment the idea usually lands.

   Any

   terraform plan
   terraform apply

6. 6

   Add three more, then destroy everything

   Change the resource to create three files instead of one using count = 3. Plan, apply. Then run destroy: everything it made is gone, and only your description remains. You can rebuild it identically, forever, from four lines of text. That property is what the whole cloud industry is built on.

   Any

   terraform destroy

A spreadsheet is enough — Excel, Numbers, or free LibreOffice. There is an optional SQL path at the end if you want it. No account needed.

1. 1

   Get something real and unhelpful

   Download a CSV from a public data portal — data.gov.uk, data.gov, or your national statistics office. Pick a subject you actually care about: crime near you, rainfall, energy prices, hospital waiting times. Deliberately do not pick a tutorial dataset. Real data is messy, and the mess is the job.

2. 2

   Write your question down before you look

   One sentence, on paper, before you open the file. "Has burglary in my area gone up or down over five years?" Doing this first is what stops you finding whatever the data happens to show and calling it a finding afterwards.

3. 3

   Meet the mess

   Open it. Find the problems: blank cells, dates in three formats, a total row hiding at the bottom that will corrupt every average you take, categories renamed halfway through the period. Write a list of every defect you find before you fix any of them.

4. 4

   Clean it, and keep a record of what you changed

   Fix the defects — but write down each decision as you go. "Dropped 214 rows with no date." "Treated blank as zero." Those decisions change the answer, and being unable to say what you did to the data is the difference between analysis and guessing.

5. 5

   Answer the question

   Pivot table, chart, or a formula. Get to a number. Then do the part almost everyone skips: try to disprove it. Is the trend just one anomalous year? Did the collection method change? Would a different but equally reasonable cleaning decision have flipped the result?

   Any

   Optional SQL path — sqlite3 ships with macOS and Linux:
   
   sqlite3 data.db
   .mode csv
   .import yourfile.csv t
   SELECT year, COUNT(*) FROM t GROUP BY year ORDER BY year;

6. 6

   Say it in three sentences

   What you found, how confident you are, and what it does not tell you. Show it to someone who has not seen the data. If they ask a question you cannot answer, that is not a failure — that is you discovering the limit of your own finding, which is the most valuable output of the whole exercise.

A text editor and a browser. No install, no server, no account — the page runs from a file on your desktop.

1. 1

   Build something that works

   Save this as a file called app.html and open it in your browser. It is a greeting page that takes a name. Use it normally first — type your own name, see it work. Everything you build starts here: correct, for the input you imagined.

   Any

   <!doctype html>
   <input id="n" placeholder="your name">
   <button onclick="go()">Greet</button>
   <div id="out"></div>
   <script>
   function go() {
     const name = document.getElementById("n").value;
     document.getElementById("out").innerHTML = "Hello, " + name + "!";
   }
   </script>

2. 2

   Type something that is not a name

   Now put this in the box instead and press the button. Something will happen that the author did not intend and did not anticipate. You have just done the fundamental move of application security: supplying input the designer never pictured.

   Any

   <img src=x onerror="alert('this is now my page')">

3. 3

   Work out precisely why it worked

   The code never asked for code — it asked for a name. But it glued your text into the page as markup rather than as text, so the browser did what browsers do and executed it. Sit with that: the flaw is not the alert box, it is that data and instructions were allowed to occupy the same channel. Nearly every major vulnerability class is a variation of that one sentence.

4. 4

   Escalate it

   An alert box proves nothing to anyone. Change your payload so it reads something real from the page instead — the document title, a cookie you set yourself, the contents of another element. Proving actual impact rather than mere possibility is the difference between a finding a developer fixes and one they close as noise.

   Any

   <img src=x onerror="document.getElementById('out').textContent = document.title">

5. 5

   Fix it — the right way and the wrong way

   First fix it badly: filter out the word "script". Then defeat your own filter — the payload above never used that word. Now fix it properly by changing innerHTML to textContent, so the input is treated as text and can never be instructions. Try every payload again. This contrast, blocklist versus correct handling, is the single most useful lesson in the whole trial.

   Any

   document.getElementById("out").textContent = "Hello, " + name + "!";

6. 6

   Go looking for the same shape elsewhere

   Where else does something you have written take input and put it somewhere with meaning — a filename, a database query, a shell command, a URL? Write down three. You now have the pattern-recognition that application security work actually consists of, and it transfers to every language you will ever use.