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Isolating files with TestDrive

Pester gives every test file its own temporary directory, cleans it up afterwards, and names it randomly so parallel runs cannot collide. It is called TestDrive, and it needs no setup at all — it is simply there.

Two ways to reach it​

  • TestDrive:\ — a PowerShell drive, for use with PowerShell commands.
  • $TestDrive — the same location as a plain filesystem path.

Prefer $TestDrive with Join-Path. TestDrive:\ only exists inside PowerShell, so the moment a path reaches a .NET method or an external executable it breaks — and Export-PlanetReport hands its path to Set-Content, which is fine, right up until someone changes it to [System.IO.File]::WriteAllLines.

Testing the report​

Planetarium/Public/Export-PlanetReport.Tests.ps1
BeforeAll {
    Import-Module "$PSScriptRoot/../Planetarium.psd1" -Force
}

Describe 'Export-PlanetReport' {
    It 'Creates the report file' {
        $path = Join-Path $TestDrive 'report.txt'

        Test-Path -Path $path | Should-BeFalse
        Export-PlanetReport -Path $path
        Test-Path -Path $path | Should-BeTrue
    }

    It 'Writes one line per planet' {
        $path = Join-Path $TestDrive 'all.txt'

        Export-PlanetReport -Path $path

        (Get-Content -Path $path).Count | Should-Be 8
    }

    It 'Writes the name and the distance in astronomical units' {
        $path = Join-Path $TestDrive 'earth.txt'

        Export-PlanetReport -Path $path -Name 'Earth'

        Get-Content -Path $path | Should-Be 'Earth    1 AU'
    }

    It 'Throws when no planet matches' {
        $path = Join-Path $TestDrive 'nothing.txt'

        { Export-PlanetReport -Path $path -Name 'Pluto' } |
            Should-Throw -ExceptionMessage "No planets matched 'Pluto'."
    }
}
Invoke-Pester -Path ./Planetarium/Public/Export-PlanetReport.Tests.ps1 -Output Detailed
Describing Export-PlanetReport
  [+] Creates the report file 45ms
  [+] Writes one line per planet 8ms
  [+] Writes the name and the distance in astronomical units 4ms
  [+] Throws when no planet matches 24ms
Tests completed in 362ms
Tests Passed: 4, Failed: 0, Skipped: 0, Inconclusive: 0, NotRun: 0

Four real files were written and four are already gone. Look in your working folder — no report files anywhere.

The first test asserts the file is absent before the call as well as present after. Without that, a leftover file from an earlier run could make the test pass on its own.

Scoping​

TestDrive is not one directory for the whole run. The rules that matter day to day:

  1. A clean drive is created per test file, at the first top-level Describe or Context.
  2. Files made in a block are visible to everything nested inside it.
  3. On leaving a block, files created during that block are removed.
  4. When the file finishes, the whole drive goes.

Each It test above generated unique filenames but in the same folder. Once the last test in the Describe block was done, all four files were cleaned up.

Modifications to inherited files are not undone

Cleanup works by tracking which paths existed when a block was entered. Create a file in Describe, change it inside a Context, and the change survives after the Context ends — the file already existed, so it is excluded from that block's cleanup.

Create files in the block or test that needs it and avoid reuse when possible. You don't want tests to depend on execution order, where one test might break another.

What this bought you​

Compare against the alternatives from the previous page, point by point. Nothing lands in your source folder. The randomised directory name means two tests — or two whole Pester runs at once — never collide. And cleanup is Pester's job rather than bookkeeping in your AfterEach, so nothing is left behind even when a run fails or is interrupted.

Before you move on

0/5

Next module: finding out which parts of the module you're not testing.