テキストエディタでいう、「フォルダ内を検索」のようなもの

logFilesSearch.ps1

$searchKey = "keyword which you want to search"
$outFileName = "result.txt"

$Items = get-childitem
foreach($aItem in $Items) {
   $aLog = get-content $aItem
    $results = @($aLog | select-string $searchKey)
    $results | out-file $outFileName -append
}