🚀 [Feature]: Support installing prerelease PowerShell versions (#16)

The action now supports installing prerelease PowerShell versions (e.g.
`7.4.0-preview.5`, `7.5.0-rc.1`) on all platforms. You can install a
specific prerelease version by passing the full version string, or
install the latest prerelease by setting the new `Prerelease` input to
`true`.

- Fixes #15

## New `Prerelease` input

A new `Prerelease` boolean input has been added, following the same UX
pattern as `Install-PSResource -Prerelease`. When set to `true` and
`Version` is `latest`, the action resolves to the latest prerelease from
the PowerShell/PowerShell GitHub releases.

| Usage | `Version` | `Prerelease` | Result |
|-------|-----------|-------------|--------|
| Latest stable | `latest` (default) | `false` (default) | Resolves to
latest stable via `/releases/latest` |
| Latest prerelease | `latest` | `true` | Resolves to latest prerelease
via `/releases` API |
| Specific stable | `7.5.0` | — | Installs `7.5.0` directly |
| Specific prerelease | `7.4.0-preview.5` | — | Installs
`7.4.0-preview.5` directly (no flag needed) |

### Installing the latest prerelease

```yaml
- uses: PSModule/Install-PowerShell@v1
  with:
    Prerelease: true
```

### Installing a specific prerelease version

```yaml
- uses: PSModule/Install-PowerShell@v1
  with:
    Version: 7.4.0-preview.5
```

## Linux package conflict handling

On Linux, the PowerShell project publishes multiple `.deb` variants
(`powershell`, `powershell-lts`, `powershell-preview`) that all provide
`/usr/bin/pwsh` and conflict with each other. The action now:

- Uses specific asset name filters (`powershell_` for stable,
`powershell-preview_` for prerelease) instead of a broad regex that
could match the wrong package variant (e.g. `powershell-lts` instead of
`powershell`)
- Removes all existing PowerShell packages before installing to avoid
`dpkg` conflicts when switching versions
- Verifies the installed version matches the requested version after
install, failing early with a clear error if not
- Applies the same fixes for RPM-based distributions

## Windows prerelease handling

On Windows, preview builds install to
`$env:ProgramFiles\PowerShell\7-preview\` instead of
`$env:ProgramFiles\PowerShell\7\`. The action now correctly handles:

- Downgrade detection by stripping the prerelease suffix before
`[version]` comparison (Windows PowerShell 5.1 lacks `[semver]`)
- Uninstall registry filtering to match Preview display names when the
current version is a preview
- Cross-prerelease version changes (same base version, different label)
by forcing an uninstall-then-install cycle

## PATH management

After installation, the action adds the PowerShell install directory to
`GITHUB_PATH` on all platforms. This ensures subsequent workflow steps
using `shell: pwsh` resolve to the newly installed version — including
preview builds whose install directory is not on the runner's default
PATH.

Author: MariusStorhaug

.github/workflows/Action-Test.yml

@@ -22,7 +22,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
-        version: ['latest', '7.4.7', '7.5.0']
+        version: ['latest', 'prerelease', '7.4.7', '7.5.0', '7.4.0-preview.5']
     runs-on: ${{ matrix.os }}
     name: '${{ matrix.os }} - [${{ matrix.version }}]'
     steps:
@@ -34,7 +34,8 @@ jobs:
       - name: Action-Test
         uses: ./
         with:
-          Version: ${{ matrix.version }}
+          Version: ${{ matrix.version == 'prerelease' && 'latest' || matrix.version }}
+          Prerelease: ${{ matrix.version == 'prerelease' && 'true' || 'false' }}
 
       - name: Verify installed version
         shell: pwsh
@@ -44,8 +45,23 @@ jobs:
           # Requested version that came from the matrix
           $requested = '${{ matrix.version }}'
 
+          # When 'prerelease' → resolve to latest prerelease
+          if ($requested.Trim().ToLower() -eq 'prerelease') {
+              $releases = Invoke-RestMethod -Uri 'https://api.github.com/repos/PowerShell/PowerShell/releases' `
+                -Headers @{
+                  'Accept' = 'application/vnd.github+json'
+                  'Authorization' = "Bearer $($env:GITHUB_TOKEN)"
+                  'X-GitHub-Api-Version' = '2022-11-28'
+                }
+              $latestPrerelease = $releases | Where-Object { $_.prerelease -eq $true } | Select-Object -First 1
+              if (-not $latestPrerelease) {
+                  throw "No prerelease releases found for PowerShell/PowerShell."
+              }
+              $requested = $latestPrerelease.tag_name.TrimStart('v')
+              Write-Host "Resolved 'prerelease' → $requested"
+          }
           # When empty / 'null' / 'latest' → resolve to latest stable release
-          if ([string]::IsNullOrWhiteSpace($requested) -or
+          elseif ([string]::IsNullOrWhiteSpace($requested) -or
               $requested.Trim().ToLower() -in @('latest','null')) {
 
               $requested = (
@@ -59,8 +75,24 @@ jobs:
               Write-Host "Resolved 'latest' → $requested"
           }
 
-          # Actual version installed by the action
-          $installed = ($PSVersionTable.PSVersion).ToString()
+          # On Windows, always verify by launching pwsh from the known install directory.
+          # This avoids relying on PATH resolution, which may still point to the pre-installed
+          # version if the runner's environment hasn't refreshed after the MSI install.
+          if ($IsWindows) {
+              $isPrerelease = $requested -match '-'
+              $majorVersion = ($requested -split '[\.-]')[0]
+              $installDir = if ($isPrerelease) { "$majorVersion-preview" } else { $majorVersion }
+              $pwshPath = "$env:ProgramFiles\PowerShell\$installDir\pwsh.exe"
+              Write-Host "Windows: verifying via subprocess at $pwshPath"
+              if (Test-Path $pwshPath) {
+                  $installed = (& $pwshPath -NoLogo -NoProfile -Command '$PSVersionTable.PSVersion.ToString()')
+              } else {
+                  Write-Host "Warning: Expected pwsh not found at $pwshPath, falling back to `$PSVersionTable"
+                  $installed = ($PSVersionTable.PSVersion).ToString()
+              }
+          } else {
+              $installed = ($PSVersionTable.PSVersion).ToString()
+          }
           Write-Host "Installed PowerShell version: $installed"
           Write-Host "Expected  PowerShell version: $requested"
 

README.md

@@ -2,6 +2,7 @@
 
 A cross‑platform GitHub Action that installs a specific **PowerShell Core** version—or the latest stable release—on any GitHub‑hosted runner
 (Linux, macOS, or Windows). The action automatically skips installation when the requested version is already present.
+Prerelease versions (e.g. `7.6.0-preview.6`, `7.5.0-rc.1`) are also supported.
 
 ## Usage
 
@@ -25,11 +26,30 @@ jobs:
           Write-Host "Using PowerShell $($PSVersionTable.PSVersion)"
 ```
 
+### Installing a prerelease version
+
+```yaml
+      - name: Install PowerShell Preview
+        uses: PSModule/install-powershell@v1
+        with:
+          Version: 7.6.0-preview.6
+```
+
+### Installing the latest prerelease
+
+```yaml
+      - name: Install latest PowerShell Preview
+        uses: PSModule/install-powershell@v1
+        with:
+          Prerelease: true
+```
+
 ## Inputs
 
 | Input | Required | Default | Description |
 | ----- | -------- | ------- | ----------- |
-| `Version` | `false` | `latest` | Desired PowerShell Core version (e.g. `7.4.1`). Use `latest` to install the newest stable release. |
+| `Version` | `false` | `latest` | Desired PowerShell Core version (e.g. `7.4.1`, `7.6.0-preview.6`). Use `latest` to install the newest stable release (or newest prerelease when `Prerelease` is `true`). |
+| `Prerelease` | `false` | `false` | Install a prerelease version. When `true` and `Version` is `latest`, resolves to the latest prerelease. Similar to `-Prerelease` on `Install-PSResource`. |
 
 ## Secrets
 
@@ -43,7 +63,8 @@ This action does **not** generate any outputs.
 
 * **Version resolution**
   If `Version` is set to `latest` (case‑insensitive), the action queries the GitHub API for the newest stable release tag in the
-  `PowerShell/PowerShell` repository and substitutes that version.
+  `PowerShell/PowerShell` repository and substitutes that version. When `Prerelease` is `true`, it queries for the latest prerelease
+  instead. Explicit prerelease version strings (e.g. `7.6.0-preview.6`) are passed through directly.
 
 * **Skip logic**
   Before installing, the action checks the current runner to see whether the requested version is already available