Replace the name of environment variable by it's actual value from environment in a text file

Question

I am trying to automate Active Directory installation on Windows Server 2008 using windows powershell. I created a text file with .tmpl extension and added:

[DCINSTALL]
ReplicaOrNewDomain=_ReplicaOrNewDomain__

Then I created an answer file in a text format:

[DCINSTALL]
ReplicaOrNewDomain=$env:ReplicaOrNewDomain

Now I want to be able to write a script in PowerShell which will use the template file to get the value of variable ReplicaOrNewDomain from environment and replace $env:ReplicaOrNewDomain by that value in the text file so that I can use that answer file for AD installation.

Answer 1

You can do that with a simple replacement like this:

$f = 'C:\path\to\your.txt'
(Get-Content $f -Raw) -replace '\$env:ReplicaOrNewDomain', $env:ReplicaOrNewDomain |
  Set-Content $f

or like this:

$f = 'C:\path\to\your.txt'
(Get-Content $f -Raw).Replace('$env:ReplicaOrNewDomain', $env:ReplicaOrNewDomain) |
  Set-Content $f

Note that when using the -replace operator you need to escape the $ (because otherwise it'd have the special meaning "end of string"). When using the Replace() method you just need to use single quotes to prevent expansion of the variable in the search string.

However, why the intermediate step of replacing the template parameter _ReplicaOrNewDomain__ with a different template parameter $env:ReplicaOrNewDomain? You would make your life easier if you just kept the former and replaced that with the value of the environment variable ReplicaOrNewDomain.

Answer 2

You have a few options to do this. One is Environment.ExpandEnvironmentVariables. This uses a %variable% syntax (instead of $env:variable), so it would be simpler if you only want to substitute environment variables:

gc input.tmpl | foreach { [Environment]::ExpandEnvironmentVariables($_) } | sc out.ini

A more complete expansion of PowerShell expressions can be achieve via ExpandString. This is more useful if you want to insert actual PowerShell expressions into the template:

gc input.tmpl | foreach { $ExecutionContext.InvokeCommand.ExpandString($_) } | sc out.ini

A third option would be something like a customized templating scheme that uses Invoke-Expression, which I implemented here.

Answer 3

One thing that I like to do with my template files is something like this.

[DCINSTALL]
ReplicaOrNewDomain={0}
OtherVariable={1}

Then in my code I can use the format operator -f to make the changes.

$pathtofile = "C:\temp\test.txt"
(Get-Content $pathtofile -Raw) -f $env:ReplicaOrNewDomain, "FooBar" | Set-Content $pathtofile

It can help if you have multiple things that you need to update at once. Update your file with as many place holders as you need. You can use the same one multiple times if need be in the file.

[DCINSTALL]
ReplicaOrNewDomain={0}
SimilarVariable={0}

Caveat

If your actual file is supposed to contain curly braces you need to double them up to the are escaped.

Answer 4

You can use the ExpandString function, like this:

$ExecutionContext.InvokeCommand.ExpandString($TemplVal)

(assuming $TemplVal has the template string).