Wednesday, January 26, 2011


As 2020 fLogViewer is available at with more monitoring possibilities
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fLogViewer is a free Windows program which allows one to view large log text files with color coding and filtering. is the replacement of

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  • Dynamically updates the display when the file changes. (Support IIS log file)
  • Support large files up to 4GB while not using too much memory. Load a 1 Gb text file with 15.6 millions lines using about 172 Mb of memory.
  • Support color coding based on keywords or regular expressions.
  • Support filtering based on keywords, regular expressions or IronPython plug-ins.
  • Support searching based on keywords or regular expressions.
  • Export in HTML format using the user defined colors.
  • Automatically archive and zip file.
  • Drag and drop text and zip files (automatically unzip the zip file content).
  • Special plug-in to visualize CSV, TAB separated and W3C Extended Log File (IIS log file).
  • Write your own plug-in to process the selected text in C#, VB.NET or IronPython.
  • Write your own plug-in to view the selected line in C#, VB.NET or IronPython.
  • View and monitor the local event viewer logs.
  • View remote file via http.
  • Support of Unicode and Windows code pages.
  • Require .NET 2.0 and no install. Download and run


Data Processing Plug-In

Plug in written in IronPython, can be defined to process the selected rows of the file. Save the .py file in the folder %APPDATA%\fLogViewer.Net\PlugIns and start
import  sys
 import  clr
 import  System

 class  PlugIn:

      def  __init__( self):
         self. PlugInType     =  "OnSelection"
         self. Text           =  "&Export As HTML (IronPython)"
         self. UserMessage    =  "Exporting As HTML"
         self. Extensions     =  ""

      def  Open( self,  fileName,  backGroundColor,  textGroundColor,  font):
         self. FileName =  "%s\\ExportedAsHTML.html"  %  ( System. Environment. GetEnvironmentVariable("TEMP"),  )
         self. handler  =  open( self. FileName,  "w")
         self. handler. write("<HTML><TABLE''''>\\n"  %  ( backGroundColor,  ))
         return  True

      def  Close( self):
         self. handler. write("</TABLE></HTML>\\n")
         self. handler. close()
         return  self. FileName

      def  Execute( self,  line,  textGroundColor):
         line =  line. replace("&mp;",  "&")
         line =  line. replace("&gt;",  ">")
         line =  line. replace("&lt;",  "<")

         self. handler. write("<TR><TD nowrap><FONT'Courier''>"  %  ( textGroundColor,  ))
         self. handler. write("\\n")
         self. handler. write( line)
         self. handler. write("\\n")
         self. handler. write("</FONT></TD></TR>")
         self. handler. write("\\n")
         return  True

Filter Plug-In

Filter can be written as an IronPython Plug in.

import  sys
 import  clr
 import  System
 class  PlugIn: 
      def  __init__( self): 
         self. PlugInType     =  "Filter" 
         self. Text           =  "Error Filter Plug In" 
         self. UserMessage    =  "" 
         self. Extensions     =  "" 
      def  Open( self): 
         return  True
      def  Close( self): 
         return  True
      def  Execute( self,  line): 
         return  line. find("[ERROR]")  !=  - 1

What people said about fLogViewer

fLogViewer was part of the Scott Hanselman's 2005 and 2006 Ultimate Developer and Power Users Tool List. Scott's comment:Great freeware highlighting log viewer for large log files. 2006 2005

...But those of us stuck on Windows would usually prefer some sort of GUI tool.
fLogViewew fits into this niche nicely. It's a MDI program wirtten in VB6, but seems to be plenty fast.

Features include color-coding (done by looking for keywords in the lines of the log), the smarts to monitor log files and refresh them on the fly (even with IIS's caching scheme), and a variety of filtering and export options...

Mike Gunderloy's NewsLettter.

Thanks for developing flogviewer, it is a great help for me. If it wouldn´t be freeware I even would pay for it. ;-)
Jens. M.

LogViewer - Great freeware highlighting log viewer for large log files.

Scott Galloway's Possibly the ultimate developer tools list.

Je m'étais débrouillé avec fLogViewer (freeware: Bon, il faut définir soi-même les filtres mais c'est un outil très puissant... Unknown

Hej! Jeg kan stærkt anbefale dette freeware program til at læse diverse logfiler med.

Det kan bla. opdatere logfilerne dynamisk mens de er åbne, hvilket er super til f.eks. troubleshooting osv. :)
... Hent programmet fLogViewer her:

Peter Schmidt

Since we do not speak Danish at, we hope this comment is good.

I've posted about baretail previously as a tail program for Windows, but now I see there is a similar tool with some more functionality to it. fLogViewer picks up and runs with the "Windows way" by taking a simple tool and putting more and more features onto it (note: Yes, I am fairly sarcastic there, but the features are appreciated nonetheless!). I kinda like this tool, although the necessity of an install and the way it uses some older system files than what I have on my XP system anyway are detractors to replacing baretail with fLogViewer.

Michael L. Dickey

I had been searching for a free logfile viewer supporting filtering and coloring for a while.
I finally found flogviewer: free, support regexp based filtering, and regexp line coloring. All I need to dive into those 80mb logfiles.
Although not perfect (I find it a bit slow for logfile of 80mb), it does the job really well.

Thibault Barrere

We are developing a consumer device with an embedded ARM microcontroller.
Our prototype system has a very simple ASCII serial trace output. In order to confirm reliability of the product, it must be run for long periods and trace logs taken for subsequent analysis. I have found that flogviewer is ideal not only for this, but also for real time analysis of the trace output. From experience, most terminal emulators have problems with large log files, in particular if one attempts to look at the trace history. However, by viewing the trace log file with flogviewer, the problems are significantly reduced.
Furthermore, filters can be used to look at particular traces in real time.
All in all flogviewer is an excellent tool for our purposes.
Hope that´s OK and many thanks for a great tool.

Lisa Kingscott.

A Tail with more power in its tip than the original has in 'full length'. Powerful filter options, together with extensive highlighting options and scripting support will make sure you will spot just those lines in your logs that matter to you. Even if the structure of your logfiles varies.

Ruud, NL

I stumbled over flogviewer a few years ago on a post by Scott Hanselman.
I was looking for a freeware logviewer which would give me the possibility to
adapt it's color-highlighting to my differing needs.
Flogviewer is an awesome piece of software which helped me a lot in my daily routines and
in tracking down probs on my servers.

Great job Fred. Thanks a lot for your great work.

Ralf R, Germany.


You must read the following license agreement before installing fLogViewer. By doing so, you indicate your acceptance of what is written here. (the SOFTWARE) is provided "as is" without warranty of any kind, including, but not limited to, implied warranties of merchantability or fitness for a particular purpose. 

In no event shall the author be liable for any damages to hardware or loss of data whatsoever arising out the use of (or inability to use) the SOFTWARE even if advised of the possibility. is free.

Installing and using fLogViewer signifies acceptance of these terms and conditions of the license. If you do not agree with the terms of this license you must remove fLogViewer files from your storage devices and cease to use the product.

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