# Exercise: Rename the Telemetry

Write three small tools for Mission Control, a script that renames every .tmp file in a folder, one that pulls the warnings out of a flight log and a launch countdown, then run them on the real files while a Pester check tells you what's done.

Apprentice · PowerShell · Updated November 10, 2026

It's launch day at Mission Control, and the ground station needs three small tools. The satellite's 30 telemetry files came in ending in `.tmp` instead of `.log`, the flight director wants every warning from last night's flight log in one short report, and the launch needs a countdown. You'll write each one as a script, a small application that works on whatever folder, file or number it's given, then run the first two on the real files. A Pester check tries each script on files of its own and tells you what's done.

## Get the exercise code

Open this lesson's GitHub repository, [github.com/ask-codey/exercise-powershell-rename-the-telemetry](https://github.com/ask-codey/exercise-powershell-rename-the-telemetry), also linked from the **Exercise** box beside the lesson. Click **Code**, then **Download ZIP**, and unzip it into your `terminal-practice` folder. Or clone it with Git.

The folder holds:

- `Rename-Telemetry.ps1`, `Get-Warnings.ps1` and `Start-Countdown.ps1`: the three scripts. Each starts with a comment that says exactly what it must do, and its `param()` line is already written. These are the files you finish.
- `telemetry`: the 30 telemetry files, all ending in `.tmp`. One has a space in its name.
- `flight-log.txt`: last night's flight log, with notes from the night shift and a few lines after the end.
- `README.md`: the four jobs, and tips.
- `Check.Tests.ps1`: the check. You don't need to change it.

## What to do

Open the exercise folder in VS Code (*Writing Scripts in VS Code*), or open PowerShell 7 in it, and run every command from there. Then do the four jobs in the folder's `README.md`:

1. **`Rename-Telemetry.ps1 -Path FOLDER`** renames every `.tmp` file in the folder it's given so it ends in `.log`, with a `foreach` loop. It outputs a line for each file, such as `Renamed orbit-01.tmp to orbit-01.log`, and ends with `Files renamed: 30` (or however many it renamed). Run a second time, with nothing left to rename, it outputs `Files renamed: 0` and no error.
2. **`Get-Warnings.ps1 -Path LOGFILE`** reads the log one line at a time with a `foreach` loop, skips the notes that start with `#` with `continue`, stops at the line `END OF FLIGHT` with `break`, outputs every other line with `WARNING` in it exactly as written, and ends with `Warnings found: 3` (or however many it found).
3. **`Start-Countdown.ps1 -From NUMBER`** counts down with a `for` loop, from `T-minus` and the number it's given down to `T-minus 1`, then outputs `Liftoff!`.
4. **Use them.** Try `Rename-Telemetry.ps1` on a copy first (`Copy-Item telemetry practice -Recurse`, then `.\Rename-Telemetry.ps1 -Path practice`, then `Remove-Item practice -Recurse`). When the checks for a script pass, run it on the real files:

   ```powershell
   .\Rename-Telemetry.ps1 -Path telemetry
   .\Get-Warnings.ps1 -Path flight-log.txt | Set-Content warnings.txt
   ```

Each script works on its `-Path` or `-From` (*Parameters: `param()`*), never on a folder or file name written into the script. Here is what the countdown should say when it's done:

```powershell
.\Start-Countdown.ps1 -From 5
```

```text
T-minus 5
T-minus 4
T-minus 3
T-minus 2
T-minus 1
Liftoff!
```

## Check your work

The check uses Pester, PowerShell's test tool. If you haven't installed it yet, install it once:

```powershell
Install-PSResource Pester
```

Then, from the exercise folder:

```powershell
Invoke-Pester
```

At first almost every test fails. That's expected: each job you finish turns its tests green. The check tries your scripts on made-up files of its own, in a throwaway folder, and only the last two tests look at your `telemetry` folder and `warnings.txt`. A failing test says what to look at, for example *Expected strings to be the same, because the check's log has two warnings, so the last line is Warnings found: 2. Count each warning you output with ++, and output the count after the loop, but they were different.*

If PowerShell says a script can't be loaded because running scripts is disabled or the file is not digitally signed, Windows has marked the downloaded files as coming from the internet. From the exercise folder, run `Get-ChildItem -Recurse | Unblock-File` once, then try again (*Why a Script Won't Run: The Execution Policy*).

## Hints

- **The renaming loop** is the one from *For Each Thing: `foreach`*, with `Get-ChildItem -Path $Path -Filter *.tmp` as its list, so it works on any folder it's given.
- **Look before you rename:** put `-WhatIf` on `Rename-Item` and run the script on your `practice` copy.
- **The counts** start at 0 before the loop and grow with `++` inside it (*Stopping Early: `break` and `continue`*). A loop over an empty list makes no passes, which is what makes a second run safe.
- **Skip, then stop:** in `Get-Warnings.ps1`, check for `#` with `continue` first, then for `END OF FLIGHT` with `break`, then for `WARNING` with `-like '*WARNING*'`.
- **A countdown's three parts** go in the `for` loop's parentheses: a start, a test and a step, as in *Until Something Is True: `while`, `do` and `for`*.

## When you're done

Every test is green, and Pester's last line says:

```text
Tests Passed: 14, Failed: 0, Skipped: 0, Inconclusive: 0, NotRun: 0
```

Now look back at `Rename-Telemetry.ps1`. It turns `.tmp` into `.log` and nothing else. What would you add to its `param()` so the same script could turn `.JPG` into `.jpg` in your photos folder, without writing a new one?

Want the whole mission? Play Mission Control from the start in Terminal Quest: [play it on the game's page](https://www.ask-codey.com/games/terminal-quest/mission-control/powershell/step). It has the telemetry and the sensor check from this chapter, a beacon script that takes a callsign with `param()`, and the launch itself. One goal stops a runaway program with `Get-Process` and `Stop-Process`, which are new here; the Step by step hints show you how. The game keeps your place in this browser, so you can stop and come back.

## Submit your exercise

The exercise is how you finish this lesson. When your work is done, put it on GitHub in a repo of your own, then submit that repo's address here.

1. On GitHub, click **+**, then **New repository**. Name it (for example `exercise-powershell-rename-the-telemetry`), choose **Public**, and click **Create repository**. Leave out the README: your folder already has one.
2. In a terminal in your exercise folder (PowerShell works), send your work to your new repo. If you downloaded the ZIP:

   ```powershell
   git init -b main
   git add .
   git commit -m "My solution"
   git remote add origin https://github.com/your-name/exercise-powershell-rename-the-telemetry.git
   git push -u origin main
   ```

   If you cloned the starter instead, point your copy at your own repo, then commit and push:

   ```powershell
   git remote set-url origin https://github.com/your-name/exercise-powershell-rename-the-telemetry.git
   git add .
   git commit -m "My solution"
   git push -u origin main
   ```

3. Paste your repo's address, such as `https://github.com/your-name/exercise-powershell-rename-the-telemetry`, into **Submit your exercise** on this page. The site checks that the repo is public and holds your commits, and then the lesson is complete.

Changed something after you submitted? Push it, and submit again: your latest submission is the one that counts.

## So what

You wrote three tools that do in a second what would take an hour by hand, on 30 files or 3,000, and they're safe to run again tomorrow: that's what a script that's an application looks like.

## Key terms

- [foreach Loop](https://www.ask-codey.com/glossary/foreach-loop-powershell)
- [for Loop](https://www.ask-codey.com/glossary/for-loop-powershell)
- [continue](https://www.ask-codey.com/glossary/continue-powershell)
- [break](https://www.ask-codey.com/glossary/break-powershell)
- [Loop Counter](https://www.ask-codey.com/glossary/loop-counter-powershell)
- [Rename-Item](https://www.ask-codey.com/glossary/rename-item)

## Related

- [For Each Thing: \`foreach\`](https://www.ask-codey.com/lessons/powershell-foreach-loop)
- [Until Something Is True: \`while\`, \`do\` and \`for\`](https://www.ask-codey.com/lessons/powershell-while-and-for)
- [Stopping Early: \`break\` and \`continue\`](https://www.ask-codey.com/lessons/powershell-loop-control)
- [Parameters: \`param()\`](https://www.ask-codey.com/lessons/powershell-param)

Source: https://www.ask-codey.com/lessons/exercise-powershell-rename-the-telemetry