In Powershell, how to set variable values within a function and have that value available in the parent scope?

The variables are created in your function's local-scope. Those variables are deleted when your function is done.

Global:     The scope that is in effect when Windows PowerShell    starts. Variables and functions that are present when    Windows PowerShell starts have been created in the    global scope. This includes automatic variables and    preference variables. This also includes the variables, aliases,    and functions that are in your Windows PowerShell    profiles.Local:      The current scope. The local scope can be the global     scope or any other scope. Script:     The scope that is created while a script file runs. Only    the commands in the script run in the script scope. To    the commands in a script, the script scope is the local    scope.

Source: about_Scopes

If you need the variables to be available for the script, then write them to the script scope.

$BackupFile = $null$TaskSequenceID = $null$OSDComputerName = $null$capturedWimPath = $nullFunction Set-OsToBuild {  switch ($OsToBuild)  {    "Win7x64"        {             $script:BackupFile = "Win7x64-SP1.wim"            $script:TaskSequenceID = "WIN7X64BC"            $script:OSDComputerName = "Ref-Win7x64"            $script:capturedWimPath = "$($PathToMdtShare)\Captures\$BackupFile"        }  }}

If you would like to keep the values for whole sessions (until you close the powershell-process), then you should use the global scope.

$global:BackupFile = $null$global:TaskSequenceID = $null$global:OSDComputerName = $null$global:capturedWimPath = $nullFunction Set-OsToBuild {  switch ($OsToBuild)  {    "Win7x64"        {             $global:BackupFile = "Win7x64-SP1.wim"            $global:TaskSequenceID = "WIN7X64BC"            $global:OSDComputerName = "Ref-Win7x64"            $global:capturedWimPath = "$($PathToMdtShare)\Captures\$BackupFile"        }  }}

The powershell about_scope help document is what you want to read for this.

Specifically this section:

Windows PowerShell Scopes

Scopes in Windows PowerShell have both names and numbers. The namedscopes specify an absolute scope. The numbers are relative and reflectthe relationship between scopes.Global:     The scope that is in effect when Windows PowerShell    starts. Variables and functions that are present when    Windows PowerShell starts have been created in the    global scope. This includes automatic variables and    preference variables. This also includes the variables, aliases,    and functions that are in your Windows PowerShell    profiles. Local:      The current scope. The local scope can be the global     scope or any other scope. Script:     The scope that is created while a script file runs. Only    the commands in the script run in the script scope. To    the commands in a script, the script scope is the local    scope.Private:    Items in private scope cannot be seen outside of the current    scope. You can use private scope to create a private version    of an item with the same name in another scope.        Numbered Scopes:    You can refer to scopes by name or by a number that    describes the relative position of one scope to another.    Scope 0 represents the current, or local, scope. Scope 1    indicates the immediate parent scope. Scope 2 indicates the    parent of the parent scope, and so on. Numbered scopes    are useful if you have created many recursive    scopes.

So depending on your exact needs you could use any one of the following I believe.

1. $global:BackupFile = "Win7x64-SP1.wim"
2. $script:BackupFile = "Win7x64-SP1.wim"
3. $1:BackupFile = "Win7x64-SP1.wim"