powershell - Expanding variables in file contents

Question

I have a file template.txt which contains the following:

Hello ${something}

I would like to create a PowerShell script that reads the file and expands the variables in the template, i.e.

$something = "World"
$template = Get-Content template.txt
# replace $something in template file with current value
# of variable in script -> get Hello World

How could I do this?

Answer 1

Another option is to use ExpandString() e.g.:

$expanded = $ExecutionContext.InvokeCommand.ExpandString($template)

Invoke-Expression will also work. However be careful. Both of these options are capable of executing arbitrary code e.g.:

# Contents of file template.txt
"EvilString";$(remove-item -whatif c: -r -force -confirm:$false -ea 0)

$template = gc template.txt
iex $template # could result in a bad day

If you want to have a "safe" string eval without the potential to accidentally run code then you can combine PowerShell jobs and restricted runspaces to do just that e.g.:

PS> $InitSB = {$ExecutionContext.SessionState.Applications.Clear(); $ExecutionContext.SessionState.Scripts.Clear(); Get-Command | %{$_.Visibility = 'Private'}}
PS> $SafeStringEvalSB = {param($str) $str}
PS> $job = Start-Job -Init $InitSB -ScriptBlock $SafeStringEvalSB -ArgumentList '$foo (Notepad.exe) bar'
PS> Wait-Job $job > $null
PS> Receive-Job $job
$foo (Notepad.exe) bar

Now if you attempt to use an expression in the string that uses a cmdlet, this will not execute the command:

PS> $job = Start-Job -Init $InitSB -ScriptBlock $SafeStringEvalSB -ArgumentList '$foo $(Start-Process Notepad.exe) bar'
PS> Wait-Job $job > $null
PS> Receive-Job $job
$foo $(Start-Process Notepad.exe) bar

If you would like to see a failure if a command is attempted, then use $ExecutionContext.InvokeCommand.ExpandString to expand the $str parameter.