RecentStringMatch.ps1

#
# Installed as a PowerShell script monitor, this will look at a file and
# determine how many times a string has appeared since the last polling
# interval.
#
# The Scripts Arguments line should read:
#
# \\path\to\filename,"string to match"
#

# Functions

# Get-StringHash by Jon Gurgul (https://gallery.technet.microsoft.com/scriptcenter/Get-StringHash-aa843f71)
function Get-StringHash() {
    Param(
        [String]
        $String
    )

    $StringBuilder = New-Object System.Text.StringBuilder
    [System.Security.Cryptography.HashAlgorithm]::Create("MD5").ComputeHash([System.Text.Encoding]::UTF8.GetBytes($String))|%{
        [Void]$StringBuilder.Append($_.ToString("x2"))
    }

    $StringBuilder.ToString()
}

# Script

# $filePath = "\\path\to\some\file\here"
# $matchString = "some string here"
$filePath = $args.Get(0)
$matchString = $args.Get(1)

# to avoid overlap on the temp file name, create a hash made up of the
# path that's being monitored and the string that's being watched
$hash = Get-StringHash -String $filePath + $matchString
$tempFilePath = Join-Path -Path $env:TEMP -ChildPath $hash

if (Test-Path $tempFilePath) {
  $seenRows = Get-Content $tempFilePath
} else {
  $seenRows = 0
}

# fail with a message if the file path is unavailable
if (-not (Test-Path $filePath)) {
    Write-Output "Static: -1"
    Write-Output "Message: Could not find file at path $($filePath)."
    Exit 1
}

# grab file and get linecount
$reader = [System.IO.File]::OpenText($filePath)
while($null -ne ($reader.ReadLine())) {
  $lineCount ++
}

# either we've never seen this file or it's been refreshed, start over
if ($seenRows -eq 0 -or $seenRows -gt $lineCount) {
  # starting point will be line 1
  $seenRows = 0
} elseif ($seenRows -eq $lineCount) {
  $reader.Close()
  Exit
}

# set the current line count in the temp file
Set-Content -Path $tempFilePath -Value $lineCount

# search the content for the string and produce a count
$counter = 0
#$reader = [System.IO.File]::OpenText($filePath)
#$reader.DiscardBufferedData()
$pos = $reader.BaseStream.Seek(0, [System.IO.SeekOrigin]::Begin)

while($null -ne ($line = $reader.ReadLine())) {
  $counter++
  if ($counter -gt $seenRows){
    $matchCount += ($line | Select-String "$matchString" | Measure-Object).Count
  }
}

$reader.Close()

Write-Output "Statistic: $matchCount"
Exit 0