How can I use PowerShell to import a file, and replace all existing placeholder variables with ones defined in the script?

Question

I have a number of .txt files that I use as standardized templates for Arista switch deployments, but I want to update them rapidly using PowerShell. I replaced all the necessary values in these templates with $variables and I now am attempting to write a script to replace them.

I found a solution that I liked, but it is not working for me. I am not sure what I am doing wrong here. https://stackoverflow.com/a/9326779/

This is a snippet from the source file :

router bgp $asn
   router-id 10.1.1.1
   bgp listen range 192.168.$id.0/25 peer-group cluster$id remote-as $asn
   neighbor cluster$id peer-group
   neighbor cluster$id update-source Loopback0
   neighbor cluster$id description cluster$id-BGP
   neighbor cluster$id ebgp-multihop 3
   neighbor cluster$id maximum-routes 12000
   network 10.1.1.1/32
exit

Here is a snippet from the powershell script :

$newvars = @{
'$id' = '101'
'$asn' = '12345'
}

$template = '.\Arista\arista.txt'
$destination_file = '.\switchconfig' + $id + '.txt'

Get-Content -Path $template | ForEach-Object {
    $line = $_

    $newvars.GetEnumerator() | ForEach-Object {
        if ($line -match $_.Key)
        {
            $line = $line -replace $_.Key, $_.Value
        }
    }
   $line
} | Set-Content -Path $destination_file

What I want is to have a group of variables defined (upwards of 30), and then replace each instance of that variable in the text file with the value contained in the script. This solution seemed good, since it would avoid doing a "replace" over and over, but it just prints the file as it originally was.

Answer 1

Since powershell uses $ as an identifier for variables (reserved), you have to properly escape that when running your method. Following is a little off but does what you are looking for. update your dictionary with \ before your $ sign to replace text including $.

$newvars = @{
'\$id' = '101'
'\$asn' = '12345'
}

$template = "C:\temp\new.txt"
$destination_file = "C:\temp\replaced.txt"
$data = @()
foreach($line in Get-Content $template) {
    foreach($key in $newvars.Keys) {
        if ($line -match $key) {
            $line = $line -replace $key, $newvars[$key]
        }
    }
    $data += $line
}

$data | Out-File $destination_file

Another thing to note.. in your file you are defining $id as the name of the file. I am not sure where but that variable would always be null as its not defined yet (unless your snippet here is different from your actual code.

If you want to use Invoke-Expression, you can use it in the following way,

$id = '101'
$asn = '12345'

$template = (Get-Content "C:\temp\new.txt") | out-string
$data = Invoke-Expression "`"$template`""
$data | Out-File "C:\Temp\test.txt"

You'll have to make sure your variables ($id, $asn) have a value to replace when evaluating the variables within your text file.