[FIXED] How do I set an env variable in PowerShell if it doesn't exist?

Issue

I'm surprised that I didn't get the answer for this common scenario after Googling for while...

How can an environment variable in be set in PowerShell if it does not exist?

Solution

The following code defines environment variable FOO for the current process, if it doesn't exist yet.

if ($null -eq $env:FOO) { $env:FOO = 'bar' }

# Alternatively:
if (-not (Test-Path env:FOO)) { $env:FOO = 'bar' }

In PowerShell (Core) 7.1+, which has null-coalescing operators, you can simplify to:

$env:FOO ??= 'bar'

^{Note: Environment variables are strings by definition. If a given environment variable is defined, but has no value, its value is the empty string ('') rather than $null. Thus, comparing to $null can be used to distinguish between an undefined environment variable and one that is defined, but has no value. However, note that assigning to environment variables in PowerShell / .NET makes no distinction between $null and '', and either value results in undefining (removing) the target environment variable.}

Note: If the environment variable is created on demand by the assignment above ($env:FOO = ...), it will exist for the current process and any child processes it creates only ^{Thanks, PetSerAl}.

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^{The following was mostly contributed by Ansgar Wiechers, with a supplement by Mathias R. Jessen:}

On Windows^[*], if you want to define an environment variable persistently, you need to use the static SetEnvironmentVariable() method of the [System.Environment] class:

# user environment
[Environment]::SetEnvironmentVariable('FOO', 'bar', 'User')

# system environment (requires admin privileges)
[Environment]::SetEnvironmentVariable('FOO', 'bar', 'Machine')

Note that these definitions take effect in future sessions (processes), so in order to define the variable for the current process as well, run $env:FOO = 'bar' in addition, which is effectively the same as [Environment]::SetEnvironmentVariable('FOO', 'bar', 'Process').

^{When using [Environment]::SetEnvironmentVariable() with User or Machine, a WM_SETTINGCHANGE message is sent to other applications to notify them of the change (though few applications react to such notifications).
This doesn't apply when targeting Process (or when assigning to $env:FOO), because no other applications (processes) can see the variable anyway.}

See also.

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^{[*] On Unix-like platforms, attempts to target persistent scopes User or Machine are quietly ignored, as of .NET (Core) 5, and this non-support for defining persistent environment variables is unlikely to change, given the lack of a unified mechanism across Unix platforms.}

Answered By - mklement0

Answer Checked By - Mary Flores (PHPFixing Volunteer)