Trying to implement a custom tail-like function, after checking several examples i've come to the bellow code, which works quite well (doesn't load whole file to read the X ending lines, works for network paths ...)
the problem I have is I don't how to move the stream pointer says 10 lines before its current position ?
as a workaround i move the pointer 1024 bytes before current position , but i don't know how much lines this is really concerning.
$sr=New-Object System.IO.StreamReader($fs)
$lastPosition=$sr.BaseStream.Length # final position of the file
$currentPosition=$lastPosition - 1024
Can anyone point me to right direction please ?
Here is the complete code :
function tail{
[cmdletBinding()]
Param(
[Parameter(Position=0, Mandatory=$true)]
[ValidateNotNullOrEmpty()]
[System.String]
$filename, # path
[int]$n=10, # number of lines to output
[switch]$continous, # continue to monitor for changes ?
[switch]$hilight # hilight lines containing keywords ?
)
# Initialising stuff
$hilightError=@("erreur","error","fatal","critic")
$hilightWarning=@("attention","warning","notice")
$errorColor="red"
$warningColor="darkyellow"
if ( (test-Path $filename) -eq $false){
write-Error "Cant read this file !"
exit
}
function tailFile($ptr){
# return each line from the pointer position to the end of file
$sr.BaseStream.Seek($ptr,"Begin")
$line = $sr.ReadLine()
while ( $line -ne $null){
$e=$w=$false
if( $hilight){
$hilightError | %{ $e = $e -or ($line -match $_) } # find error keywords ?
if( $e) {wh $line -ForegroundColor $errorColor }
else{
$hilightWarning | %{ $w = $w -or ($line -match $_ ) } # find warning keywords ?
if($w){ wh $line -ForegroundColor $warningColor }
else{ wh $line}
}
}
else{ #no hilight
wh $line
}
$line = $sr.ReadLine()
}
}
# Main
$fs=New-Object System.IO.FileStream ($filename,"OpenOrCreate", "Read", "ReadWrite",8,"None") # use ReadWrite sharing permission to not lock the file
$sr=New-Object System.IO.StreamReader($fs)
$lastPosition=$sr.BaseStream.Length # final position of the file
$currentPosition=$lastPosition - 1024 # take some more bytes (to get the last lines)
tailfile $currentPosition
if($continous){
while(1){
start-Sleep -s 1
# have the file changed ?
if ($sr.BaseStream.Length -eq $lastPosition){
write-verbose "no change..."
continue
}
else {
tailfile $lastPosition
$lastPosition = $sr.BaseStream.Position
write-Verbose "new position $lastPosition"
}
}
}
$sr.close()
}
thanks to Christian and Keith for your tips. Finally I've decided just to get backward in the streamreader until it returns enough readline(). I let the possibility to specify the codepage and seems to be ok with unicode. Here is the code if anyone is interested in
function tail{
[cmdletBinding()]
Param(
[Parameter(Position=0, Mandatory=$true)]
[ValidateNotNullOrEmpty()]
[System.String]
$filename, # path
[int]$n=10, # number of lines to output,
[Alias("cp")]
$codepage=65001,#utf8
[Alias("f")]
[switch]$continous, # continue to monitor for changes ?
[switch]$hilight # hilight lines containing keywords ?
)
# Initialising stuff
$hilightError=@("erreur","error","fatal","critic")
$hilightWarning=@("attention","warning","notice")
$errorColor="red"
$warningColor="yellow"
[System.Text.Encoding]$enc = [System.Text.Encoding]::GetEncoding($codepage)
function render($render){
$e=$w=$false
if( $hilight){
foreach ($line in $render){
$hilightError | %{ $e = $e -or ($line -match $_) } # find error keywords ?
if( $e) {wh $line -ForegroundColor $errorColor }
else{
$hilightWarning | %{ $w = $w -or ($line -match $_ ) } # find warning keywords ?
if($w){ wh $line -ForegroundColor $warningColor }
else{ wh $line}
}
$e=$w=$false
}
}
else{ #no hilight
wh $render
}
}
function TailFileBeforeEnd{
#try to find $n lines before eof
$buffer=1024
$ptr=$lastPosition #start at the end of the file
$found=0
while($ptr -gt 0 -and $found -lt $n){
$ptr-=$buffer
if ($ptr -le 0){$ptr=0}
$sr.BaseStream.Seek($ptr,"Begin")|out-null #step backward
$line = $sr.ReadLine()
$found=0
$output=@()
$render=@()
while ( $line -ne $null){ #read to the end
$output+=$line
$found++
$line=$sr.ReadLine()
}
if($found -ge $n){ #found enough lines
Write-Verbose "OK found $found / $n"
foreach($i in ($output.length - $n)..($output.length)){ #take only lines needed
$render+=$output[$i]
}
continue
}
else{ #move backward and retry to find lines
Write-Verbose "not enough line ($found displayed)"
$ptr-=$buffer
if ($ptr -le 0) { #eof without finding suffisant lines
$ptr=0
Write-host "not enough line ($found displayed)"
$render=$output
}
}
}
render $render
}
function tailFile($ptr){
# return each line from the pointer position to the end of file
$render=@()
$sr.BaseStream.Seek($ptr,"Begin")|out-null
$line = $sr.ReadLine()
while ( $line -ne $null){
$render+=$line
$line = $sr.ReadLine()
}
render $render
}
# Main loop
# use ReadWrite sharing permission to not lock the file
$fs=New-Object System.IO.FileStream ($filename,"OpenOrCreate", "Read", "ReadWrite",8,"None")
$sr=New-Object System.IO.StreamReader($fs, $enc)
$lastPosition=$sr.BaseStream.Length
tailFileBeforeEnd
if($continous){
while(1){
start-Sleep -s 2
# has the file changed ?
if ($sr.BaseStream.Length -eq $lastPosition){
write-verbose "no change..."
continue
}
else {
tailfile $lastPosition
$lastPosition = $sr.BaseStream.Position
write-Verbose "new position $lastPosition"
}
}
}
$sr.close()
}