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

Question

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?

Answer 1

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.}

Answer 2

Code

function Set-LocalEnvironmentVariable {     param (         [Parameter()]         [System.String]         $Name,          [Parameter()]         [System.String]         $Value,          [Parameter()]         [Switch]         $Append     )     if($Append.IsPresent)     {         if(Test-Path "env:$Name")         {             $Value = (Get-Item "env:$Name").Value + $Value         }     }     Set-Item env:$Name -Value "$value" | Out-Null }  function Set-PersistentEnvironmentVariable {     param (         [Parameter()]         [System.String]         $Name,              [Parameter()]         [System.String]         $Value,              [Parameter()]         [Switch]         $Append             )          Set-LocalEnvironmentVariable -Name $Name -Value $Value -Append:$Append     if ($Append.IsPresent) {         $value = (Get-Item "env:$Name").Value     }          if ($IsWindows) {         setx "$Name" "$Value" | Out-Null         return     }     $pattern = "\s*export[ \t]+$Name=[\w]*[ \t]*>[ \t]*\/dev\/null[ \t]*;[ \t]*#[ \t]*$Name\s*"          if ($IsLinux) {         $file = "~/.bash_profile"         $content = (Get-Content "$file" -ErrorAction Ignore -Raw) + [System.String]::Empty         $content = [System.Text.RegularExpressions.Regex]::Replace($content, $pattern, [String]::Empty);         $content += [System.Environment]::NewLine + [System.Environment]::NewLine + "export $Name=$Value > /dev/null ;  # $Name"         Set-Content "$file" -Value $content -Force         return     }     if ($IsMacOS) {         $file = "~/.zprofile"         $content = (Get-Content "$file" -ErrorAction Ignore -Raw) + [System.String]::Empty         $content = [System.Text.RegularExpressions.Regex]::Replace($content, $pattern, [String]::Empty);         $content += [System.Environment]::NewLine + [System.Environment]::NewLine + "export $Name=$Value > /dev/null ;  # $Name"         Set-Content "$file" -Value $content -Force         return     }     throw "Invalid platform." } 

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• function Set-PersistentEnvironmentVariable Set a variable/value in actual process and system. This function calls Set-LocalEnvironmentVariable function to set process scope variables and perform task for set variables in machine scope.

On Windows you can use:

• [Environment]::SetEnvironmentVariable with machine scope, user or machine don't work on Linux or MacOS
• setx command

On Linux we can add export VARIABLE_NAME=Variable value to file ~/.bash_profile. For a new bash terminal the process execute these instructions located in ~/.bash_profile.

On MacOS similiar to Linux but if you have zsh terminal the file is .zprofile, if the default terminal is bash, the file is .bash_profile. In my function code we need to add detection of default terminal if you wish. I assume that default terminal is zsh.

• function Set-LocalEnvironmentVariable Set a variable/value in actual process. Using Drive env:.

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Examples

#Set "Jo" value to variable "NameX", this value is accesible in current process and subprocesses, this value is accessible in new opened terminal. Set-PersistentEnvironmentVariable -Name "NameX" -Value "Jo"; Write-Host $env:NameX  #Append value "ma" to current value of variable "NameX", this value is accesible in current process and subprocesses, this value is accessible in new opened terminal. Set-PersistentEnvironmentVariable -Name "NameX" -Value "ma" -Append; Write-Host $env:NameX  #Set ".JomaProfile" value to variable "ProfileX", this value is accesible in current process/subprocess. Set-LocalEnvironmentVariable "ProfileX" ".JomaProfile"; Write-Host $env:ProfileX   

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Output

Windows 10

Ubuntu WSL

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References

Check About Environment Variables
Shell initialization files
ZSH: .zprofile, .zshrc, .zlogin - What goes where?