PowerShell Functions

Guided practice

Work the same idea seven ways: understand it, predict the result, run it, inspect it, change it, debug it, then write your own. Your progress on this page is remembered on this device.

1. Understand — name the idea once

   The same disk-space check copied into five scripts is five places to update when the rule changes, and five chances to drift apart. A function gives the idea a name (Get-DiskFreePercent), declares its input (-Drive, defaulting to 'C'), and keeps one place to fix. The lesson above showed the shape; the steps below make it stick.

   The idea is clear — continue

2. Predict — say what happens before it happens

   Reading code well means predicting before running. This call runs on a machine where drive C: has 48.2% free space:

   Get-DiskFreePercent -Drive 'C'

   What comes back?

   The number 48.2, as plain text One object with Drive and FreePercent properties Nothing — functions only return a value inside scripts A table of every drive on the machine

3. Run — watch the recorded example

   The lab on this page holds the function defined above, called with -Drive 'C'. Press the button to scroll to the lab and play its saved output. Nothing is executed in your browser — it is the recorded result of this exact example, against fictional volume data.

   Run the recorded example in the lab

4. Inspect — read the result like a debugger

   How many objects did the call return? Check
   Which property would a monitoring script test to page someone about a nearly-full disk? Check

5. Modify — change the input, predict the change

   Same function, different drive. Both outputs below are recorded examples — sample output, not executed here.

   Drive C

   Get-DiskFreePercent -Drive 'C'

   Recorded output

   Drive  FreePercent
   -----  -----------
   C      48.2

   Drive D

   Get-DiskFreePercent -Drive 'D'

   Recorded output

   Drive  FreePercent
   -----  -----------
   D      71.9

   You change 'C' to 'D' and re-run. Select every true statement, then check.

   The Drive property now reads 'D' FreePercent is recalculated for D:'s volume The function now returns two objects The function must be redefined before calling it with 'D'

   Check my selection

6. Solve independently — find the accidental output

   Someone edited the function and the Pester test started failing. Read it the way you would in a real review — no hints highlighted:

    1function Get-DiskFreePercent { 2    param([string] $Drive = 'C') 3    $v = Get-Volume -DriveLetter $Drive 4    $v.SizeRemaining 5    [pscustomobject]@{ 6        Drive       = $Drive 7        FreePercent = [math]::Round($v.SizeRemaining / $v.Size * 100, 1) 8    }

   First, without clicking above: how many objects does Get-DiskFreePercent -Drive 'C' return now? Check

   Show the fix

7. Explain — write your own

   Write Get-VolumeLabel: a function that takes a drive letter (default 'C') and returns one object with Drive and Label properties. Assume Get-Volume -DriveLetter $Drive gives you .DriveLetter and .FileSystemLabel. Your draft stays on this page — nothing is sent anywhere.

   Your Get-VolumeLabel draft

   Compare with a model answer

   Tick every box that is true of your draft. Be honest — this checklist is the review:

   It declares a param block with a default drive of 'C' It returns one [pscustomobject] with Drive and Label properties No Write-Host, and no value sits uncaptured on its own line

   Keep going: the Pester lesson shows how to test a function like the one you just wrote — that is how you defend it in a review.

   Ready to prove it? Take the Functions capstone assessment: six checks, one attempt each, pass mark 5 of 6.