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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. Check git status — nothing.

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.

So each It above got the same drive but wrote a distinct filename. They could safely have reused one name — but distinct names make a failure easier to read, since the filename tells you which test wrote it.

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.

The practical rule: create files in the block that uses them. Sharing a mutable file down the tree is how you get tests that pass alone and fail together, or that depend on execution order.

What this bought you​

Compare against the alternatives from the previous page. No AfterEach to forget. No fixed paths to collide. No cleanup skipped because a test threw halfway. Nothing left behind when a test fails — and a failing test is precisely when stale files do the most damage.

The parallel-safety is not theoretical either: the directory name is randomised specifically so several Pester processes can run at once, which is what your CI will do the moment the suite gets slow enough to shard.

Before you move on

0/5

Next module: finding out which parts of the module your tests never touch.