Access Environment Variables

1. Problem

You want to use an environment variable (such as the system path, or current user’s name) in your script or interactive session.

2. Solution

PowerShell offers several ways to access environment variables.

To list all environment variables, list the children of the env drive:

        Get-ChildItem env:

To get an environment variable using a more concise syntax, precede its name with $env:

        $env:variablename

i.e.: $env:username

To get an environment variable using its Provider path, supply env: or Environment:: to the Get-ChildItem cmdlet:

        Get-ChildItem env:variablename

        Get-ChildItem Environment::variablename

3. Discussion

PowerShell provides access to environment variables through its environment provider. Providers let you work with data stores (such as the registry, environment variables, and aliases) much as you would access the filesystem.

By default, PowerShell creates a drive (called env) that works with the environment provider to let you access environment variables. The environment provider lets you access items in the env: drive as you would any other drive: dir env:\variablename or dir env:variablename. If you want to access the provider directly (rather than go through its drive), you can also type dir Environment::variablename.

However, the most common (and easiest) way to work with environment variables is by typing $env:variablename. This works with any provider but is most typically used with environment variables.

This is because the environment provider shares something in common with several other providers—namely support for the *-Content set of core cmdlets (see Example 3-1).

Example 3-1. Working with content on different providers

PS >"hello world" > test

PS >Get-Content c:test

hello world

PS >Get-Content variable:ErrorActionPreference

Continue

PS >Get-Content function:more

param([string[]]$paths); if(($paths -ne $null) -and ($paths.length -ne 0)) { ...

       Get-Content $local:file | Out-Host -p } } else { $input | Out-Host ...

PS >Get-Content env:systemroot

C:\WINDOWS

                                        

For providers that support the content cmdlets, PowerShell lets you interact with this content through a special variable syntax (see Example 3-2).

Example 3-2. Using PowerShell’s special variable syntax to access content

PS >$function:more

param([string[]]$paths); if(($paths -ne $null) -and ($paths.length -ne 0)) { ...

       Get-Content $local:file | Out-Host -p } } else { $input | Out-Host ...

PS >$variable:ErrorActionPreference

Continue

PS >$c:test

hello world

PS >$env:systemroot

C:\WINDOWS

                                        

This variable syntax for content management lets you to both get and set content:

        PS >$function:more = { $input | less.exe }

        PS >$function:more

        $input | less.exe

Now, when it comes to accessing complex provider paths using this method, you’ll quickly run into naming issues (even if the underlying file exists):

        PS >$c:\temp\test.txt

        Unexpected token '\temp\test.txt' in expression or statement.

        At line:1 char:17

        + $c:\temp\test.txt <<<<

The solution to that lies in PowerShell’s escaping support for complex variable names. To define a complex variable name, enclose it in braces:

        PS >${1234123!@#$!@#$12$!@#$@!} = "Crazy Variable!"

        PS >${1234123!@#$!@#$12$!@#$@!}

        Crazy Variable!

        PS >dir variable:\1*

        Name                              Value

        ----                              -----

        1234123!@#$!@#$12$!@#$@!          Crazy Variable!

… and the content equivalent (assuming that the file exists):

        PS >${c:\temp\test.txt}

        hello world

Since environment variable names do not contain special characters, this Get-Content variable syntax is the best (and easiest) way to access environment variables.