Determine the OS version, Linux and Windows from Powershell

Since the PowerShell versions 6.1 on Windows/Linux/OSX went to GA you can use the new properties of $PSVersionTable, OS, Platform and GitCommitId

Update In v6.0.0-beta.3 there are some breaking changes:

• Change positional parameter for powershell.exe from -Command to -File

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$PSVersionTable on :

Platform Win32NT OS Microsoft Windows 10.0.15063

PS C:\Users\LotPings> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      6.1.0
PSEdition                      Core
GitCommitId                    6.1.0
OS                             Microsoft Windows 10.0.17134
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Platform Unix OS Linux (ubuntu)

PS /home/LotPings> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      6.1.0
PSEdition                      Core
GitCommitId                    6.1.0
OS                             Linux 4.15.0-34-generic #37-Ubuntu SMP Mon Aug 27 15:21:48 UTC 2018
Platform                       Unix
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Platform Unix OS Darwin

PS /Users/LotPings> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      6.1.0
PSEdition                      Core
GitCommitId                    6.1.0
OS                             Darwin 17.7.0 Darwin Kernel Version 17.7.0: Thu Jun 21 22:53:14 PDT 2018; root:xnu-4570.71.2~1/RE...
Platform                       Unix
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Actually, there should be global variables added by the PowerShell console itself--they're not considered environment variables though, which is why they wouldn't show up when using dir env: to get a list.The OS-specific ones I see for now are $IsLinux, IsMacOS and $IsWindows. This is of at least PowerShell version 6.0.0-rc and above for Mac/Linux.

You can see a list of what's available by using just Get-Variable (in a fresh session without loading your profile, if you just want what comes build-in by default).

For PowerShell Core (Powershell Version 6.0+), you can use Automatic Variables: $IsLinux, $IsMacOS and $IsWindows.

For example,

if ($IsLinux) {
    Write-Host "Linux"
}
elseif ($IsMacOS) {
    Write-Host "macOS"
}
elseif ($IsWindows) {
    Write-Host "Windows"
}

Aren't there environment variables you can view on the other platforms for the OS?

Get-ChildItem -Path Env:

Particularly, on Windows at least, there's an OS environment variable, so you should be able to accomplish this by using $Env:OS.

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Since some time has passed and the PowerShell Core (v6) product is GA now (the Core branding has been dropped as of v7), you can more accurately determine your platform based on the following automatic boolean variables:

$IsMacOS
$IsLinux
$IsWindows