From the TAIL(1) man page:
The -f option causes tail to not stop when end of file is reached, but rather to wait for additional data to be appended to the input.My approach to replicating this functionality in PowerShell is to once again take advantage of System.IO.FileStream Class and read from the end of file as I did in my previous tail emulations. Originally, I thought this was going to be a more difficult task to accomplish but since I learned so much from my prior attempts, it turned out to be fairly simple and a lot less code to implement. My solution focuses on the fact that the file size grows as more data is appended to the text file. If I know how large the file was in a prior reading than it is currently, I know how many bytes have been added to the file and from there, I know the number of bytes I need to read from the end of the file and return to the console. All I need is a looping routine to constantly check the file for changes. In my code sample below, I emulate "tail -f" to the console by first reading the last 10 lines of a hypothetical BlackBerry Enterprise Server Management Agent log file (on an active server this file grows constantly) on a remote server as Unix "tail -f" would then start to monitor the log file for changes by comparing the file size waiting 100 milliseconds between each comparison. This process will continue until you send a break or the file is deleted.
With this bit of starter code, you should be able to implement some unique tools to monitor and react to data in log files in real-time.
UPDATE: I have made some improvements to this code here.
Function Read-EndOfFileByByteChunk($fileName,$totalNumberOfLines,$byteChunk) {
if($totalNumberOfLines -lt 1) { $totalNumberOfLines = 1 }
if($byteChunk -le 0) { $byteChunk = 10240 }
$linesOfText = New-Object System.Collections.ArrayList
if([System.IO.File]::Exists($fileName)) {
$fileStream = New-Object System.IO.FileStream($fileName,[System.IO.FileMode]::Open,[System.IO.FileAccess]::Read,[System.IO.FileShare]::ReadWrite)
$asciiEncoding = New-Object System.Text.ASCIIEncoding
$fileSize = $fileStream.Length
$byteOffset = $byteChunk
[byte[]] $bytesRead = New-Object byte[] $byteChunk
$totalBytesProcessed = 0
$lastReadAttempt = $false
do {
if($byteOffset -ge $fileSize) {
$byteChunk = $fileSize - $totalBytesProcessed
[byte[]] $bytesRead = New-Object byte[] $byteChunk
$byteOffset = $fileSize
$lastReadAttempt = $true
}
$fileStream.Seek((-$byteOffset), [System.IO.SeekOrigin]::End) | Out-Null
$fileStream.Read($bytesRead, 0, $byteChunk) | Out-Null
$chunkOfText = New-Object System.Collections.ArrayList
$chunkOfText.AddRange(([System.Text.RegularExpressions.Regex]::Split($asciiEncoding.GetString($bytesRead),"\r\n")))
$firstLineLength = ($chunkOfText[0].Length)
$byteOffset = ($byteOffset + $byteChunk) - ($firstLineLength)
if($lastReadAttempt -eq $false -and $chunkOfText.count -lt $totalNumberOfLines) {
$chunkOfText.RemoveAt(0)
}
$totalBytesProcessed += ($byteChunk - $firstLineLength)
$linesOfText.InsertRange(0, $chunkOfText)
} while($totalNumberOfLines -ge $linesOfText.count -and $lastReadAttempt -eq $false -and $totalBytesProcessed -lt $fileSize)
$fileStream.Close()
if($linesOfText.count -gt 1) {
$linesOfText.RemoveAt($linesOfText.count-1)
}
$deltaLines = ($linesOfText.count - $totalNumberOfLines)
if($deltaLines -gt 0) {
$linesOfText.RemoveRange(0, $deltaLines)
}
} else {
$linesOfText.Add("[ERROR] $fileName not found") | Out-Null
}
return $linesOfText
}
#--------------------------------------------------------------------------------------------------#
Function Read-FileUpdates($fileName,$startSize) {
$asciiEncoding = New-Object System.Text.ASCIIEncoding
while([System.IO.File]::Exists($fileName)) {
$fileStream = New-Object System.IO.FileStream($fileName,[System.IO.FileMode]::Open,[System.IO.FileAccess]::Read,[System.IO.FileShare]::ReadWrite)
$currentFileSize = $fileStream.Length
if($currentFileSize -gt $startSize) {
$byteChunk = $currentFileSize - $startSize
[byte[]] $bytesRead = New-Object byte[] $byteChunk
$fileStream.Seek((-$byteChunk), [System.IO.SeekOrigin]::End) | Out-Null
$fileStream.Read($bytesRead, 0, $byteChunk) | Out-Null
Write-Host ($asciiEncoding.GetString($bytesRead)) -noNewLine
$startSize = $currentFileSize
}
$fileStream.Close()
Start-Sleep -milliseconds 100
}
}
#--------------------------------------------------------------------------------------------------#
Set-Variable -name inputFile -option Constant -value "\\japan-bes.ad.mydomain.local\E$\Program Files\Research In Motion\BlackBerry Enterprise Server\Logs\20110418\JAPAN-BES_MAGT_02_20110418_0001.txt"
#--------------------------------------------------------------------------------------------------#
if([System.IO.File]::Exists($inputFile)) {
Write-Host (@(Read-EndOfFileByByteChunk $inputFile 10 1280) | Out-String) -noNewLine
$fileStream = New-Object System.IO.FileStream($inputFile,[System.IO.FileMode]::Open,[System.IO.FileAccess]::Read,[System.IO.FileShare]::ReadWrite)
$fileSize = $fileStream.Length
$fileStream.Close()
Read-FileUpdates $inputFile $fileSize
} else {
Write-Host "Could not find $inputFile..." -foregroundColor Red
}
