# encoding: ascii
# api: powershell
# title: Get-FileTail
# description: An update to the orignal Get-FileTail that replaces a specific int with a long to accommodate particularly large files. Original text: A PowerShell 2.0 function to do the equivalent of the ‘nix tail, including the continuous feature (tail -f). Notice this does not try to emulate the ‘nix command parameters, which never made sense to me. I’ll leave that to someone else.
# version: 1.01
# type: script
# author: Joel Bennett
# license: CC0
# function: Get-FileTail
# x-poshcode-id: 6101
# x-archived: 2016-03-18T21:23:33
# x-published: 2016-11-18T20:35:00
#
#
#.Synopsis
# Show the last n lines of a text file
#.Description
# This is just a tail script for PowerShell, using seekable streams to avoid reading the whole file and using v2 eventing to detect changes and provide a -Continuous mode.
#.Parameter Name
# The file name to tail
#.Parameter Lines
# The number of lines to display (or start with, in -Continuous mode)
#.Parameter Continuous
# Whether or not to continue watching for new content to be added to the tail of the function
#.Parameter Linesep
# Allows you to override the line separator character used for counting lines.
# By default we use `n, which works for Windows `r`n and Linux `n but not old-school Mac `r files
#.Parameter Encoding
# Allows you to manually override the text encoding.
# By default we can detect various unicode formats, and we default to UTF-8 which will handle ASCII
#.Example
# Get-FileTail ${Env:windir}\Windowsupdate.log 25
#
# get the last 25 lines from the specified log
#.Example
# Get-FileTail ${Env:windir}\Windowsupdate.log -Continuous
#
# Start reading from WindowsUpdate.log as it is written to.
#function Get-FileTail {
PARAM( $Name, [int]$lines=10, [switch]$continuous, $linesep = "`n", [string]$encoding )
BEGIN {
if(Test-Path $Name) {
$Name = (Convert-Path (Resolve-Path $Name))
}
[byte[]]$buffer = new-object byte[] 1024
if($encoding) {
[System.Text.Encoding]$encoding = [System.Text.Encoding]::GetEncoding($encoding)
Write-Debug "Specified Encoding: $encoding"
}
# else {
# $detector = New-Object System.IO.StreamReader $Name, $true
# [Text.Encoding]$encoding = $detector.CurrentEncoding
# Write-Debug "Detected Encoding: $encoding"
# $detector.Dispose()
# }
function tailf {
PARAM($StartOfTail=0)
[string[]]$content = @()
#trap { return }
## You must use ReadWrite sharing so you can open files which other apps are writing to...
$reader = New-Object System.IO.FileStream $Name, "OpenOrCreate", "Read", "ReadWrite", 8, "None"
if(!$encoding) {
$b1 = $reader.ReadByte()
$b2 = $reader.ReadByte()
$b3 = $reader.ReadByte()
$b4 = $reader.ReadByte()
if (($b1 -eq 0xEF) -and ($b2 -eq 0xBB) -and ($b3 -eq 0xBF)) {
Write-Debug "Detected Encoding: UTF-8"
[System.Text.Encoding]$encoding = [System.Text.Encoding]::UTF8
} elseif (($b1 -eq 0) -and ($b2 -eq 0) -and ($b3 -eq 0xFE) -and ($b4 -eq 0xFF)) {
Write-Debug "Detected Encoding: 12001 UTF-32 Big-Endian"
[System.Text.Encoding]$encoding = [System.Text.Encoding]::GetEncoding(12001)
} elseif (($b1 -eq 0xFF) -and ($b2 -eq 0xFE) -and ($b3 -eq 0) -and ($b4 -eq 0)) {
Write-Debug "Detected Encoding: 12000 UTF-32 Little-Endian"
[System.Text.Encoding]$encoding = [System.Text.Encoding]::UTF32
} elseif (($b1 -eq 0xFE) -and ($b2 -eq 0xFF)) {
Write-Debug "Detected Encoding: 1201 UTF-16 Big-Endian"
[System.Text.Encoding]$encoding = [System.Text.Encoding]::BigEndianUnicode
} elseif (($b1 -eq 0xFF) -and ($b2 -eq 0xFE)) {
Write-Debug "Detected Encoding: 1200 UTF-16 Little-Endian"
[System.Text.Encoding]$encoding = [System.Text.Encoding]::Unicode
} elseif (($b1 -eq 0x2B) -and ($b2 -eq 0x2F) -and ($b3 -eq 0x76) -and (
($b4 -eq 0x38) -or ($b4 -eq 0x39) -or ($b4 -eq 0x2b) -or ($b4 -eq 0x2f))) {
Write-Debug "Detected Encoding: UTF-7"
[System.Text.Encoding]$encoding = [System.Text.Encoding]::UTF7
} else {
Write-Debug "Unknown Encoding: [$('0x{0:X}' -f $b1) $('0x{0:X}' -f $b2) $('0x{0:X}' -f $b3) $('0x{0:X}' -f $b4)] using UTF-8"
[System.Text.Encoding]$encoding = [System.Text.Encoding]::UTF8
}
}
#trap { Write-Warning $_; $reader.Close(); throw }
if($StartOfTail -eq 0) {
$StartOfTail = $reader.Length - $buffer.Length
} else {
$OnlyShowNew = $true
}
#Write-Verbose "Starting Tail: $StartOfTail of $($reader.Length)"
do {
$pos = $reader.Seek($StartOfTail, "Begin")
#Write-Verbose "Seek: $Pos"
do {
$count = $reader.Read($buffer, 0, $buffer.Length);
#Write-Verbose "Read: $Count"
$content += $encoding.GetString($buffer,0,$count).Split($linesep)
} while( $count -gt 0 )
$StartOfTail -= $buffer.Length
# keep going if we don't have enough lines,
} while(!$OnlyShowNew -and ($content.Length -lt $lines) -and $StartOfTail -gt 0)
## ADJUST OUR OUTPUT ...
$end = $reader.Length
#Write-Verbose "Ended Tail: $end of $($reader.Length)"
if($content) {
$output = [string]::Join( "`n", @($content[-$lines..-1]) )
$len = $output.Length
$output = $output.TrimEnd("`n")
$end -= ($len - $output.Length) - 1
}
Write-Output $end
if($output.Length -ge 1) {
Write-Host $output -NoNewLine
}
#trap { continue }
$reader.Close();
}
}
PROCESS {
[long]$StartOfTail = tailf 0
if($continuous) {
$Null = unregister-event "FileChanged" -ErrorAction 0
$fsw = new-object system.io.filesystemwatcher
$fsw.Path = split-path $Name
$fsw.Filter = Split-Path $Name -Leaf
$fsw.EnableRaisingEvents = $true
$null = Register-ObjectEvent $fsw Changed "FileChanged" -MessageData $Name
while($true) {
wait-event FileChanged | % {
[int]$StartOfTail = tailf $StartOfTail -newonly
$null = Remove-Event $_.EventIdentifier
}
}
unregister-event "FileChanged"
}
Write-Host
}
#}