Adds or updates text between two markers in a text file.

AddOrUpdateInTextFile.ps1

function AddOrUpdateTextInFile() {
Param(
    $File,
    [string]$StartString,
    [string]$EndString,
    [string]$ReplacementString
    )
    
    $fileContent = Get-Content $File -ReadCount 512
    
    if ($fileContent -match [System.Text.RegularExpressions.Regex]::Escape($StartString)) {
        $numberOflines = $fileContent.Length
        $FoundString = $false
        
        $startLineNumber = $null
        $endLineNumber = $null

        for ($i=0; $i -lt $numberOflines; $i++) {
            if (-not ($startLineNumber) -and ($fileContent[$i].ToString().Trim() -eq $StartString)) {
                $startLineNumber = $i
            } elseif (-not ($endLineNumber) -and ($fileContent[$i].ToString().Trim() -eq $EndString)) {
                $endLineNumber = $i
            }   
        }

        $FileStartPart = $fileContent[0..($startLineNumber-1)]
        $FileEndPart = $fileContent[($endLineNumber+1)..$numberOflines]
        
        $NewContent = ($FileStartPart +  $ReplacementString + $FileEndPart) -join "`r`n"

        $NewContent | Out-File $File -Encoding ascii

    } else {
        # Could not find the start string, append to the end
        Add-Content -Path $File -Value "`r`n" -Encoding ascii
        Add-Content -Path $file -Value $ReplacementString -Encoding ascii

    }

}