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Tail utility for windows

, 18 Jan 2004
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Tail utility for windows

Introduction

This simple program imitates the unix "tail -f" utility on windows.

Source code and Demo

The submitted source code contains one main file: Tail.cs. This is the core of the tail program. This file is used in two wrappers - a GUI wrapper and a CUI wrapper. The two projects in the solution illustrate using Tail.cs in a windows form application and a console application.

Code Walkthrough

.NET introduced the FileSystemWatcher, which is the main component of this program. By configuring this component one can listen to file system changes.

I first check whether the file exists. If it does exist then I monitor any changes to the file thus:

fileSystemWatcher.Changed += new FileSystemEventHandler(TargetFile_Changed);

If it does not exist yet, then I wait until the file is created before watching for file change notifications:

fileSystemWatcher.Created += new FileSystemEventHandler(TargetFile_Created);

When a change is made to the file, the method below gets the notification

  public void TargetFile_Changed(object source, FileSystemEventArgs e)
  {
   lock (this)
   {
    byte[] bytesRead = new byte[maxBytes];
    FileStream fs = new FileStream(this.filename, FileMode.Open);
    if (fs.Length > maxBytes)
    {
     this.previousSeekPosition = fs.Length - maxBytes;
    }
    this.previousSeekPosition = (int)fs.Seek(
        this.previousSeekPosition, SeekOrigin.Begin);
    int numBytes = fs.Read(bytesRead, 0, maxBytes);
    fs.Close();
    this.previousSeekPosition += numBytes;  

    StringBuilder sb = new StringBuilder();
    for (int i=0; i<numBytes; i++)
    {
     sb.Append((char)bytesRead[i]);   
    }

    this.MoreData(this, sb.ToString());
   }
  }

Within this, I ensure that the last characters are read in a large file which is longer than the number of characters that can be read. I then read the last set of characters starting from where we left off the previous time. The string of new characters is then sent to all listeners.

The sample programs listen to this notfication and then updates its user interface.

Issues

I have noticed an occasional exception saying that the target file is being accessed by another program. In the environment that I am working in, the target file is not always opened for writing, so I am able to get away with the above code for now. If it works, don't touch it uh. Smile | :) But you feel free to correct it and post changes.

Bug Fixes

  • Added fix for file access exception. Thanks to Gary a.k.a garythom_work for this fix. (see thread below) - 17 Jan 2004

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

SathishVJ

France France
~/sathishvj

Comments and Discussions

 
QuestionCross-thread operation not valid PinmemberTinyTony_16-Oct-13 0:05 
BugWell, this definitely does not work PinmemberZacky Pickholz20-Nov-12 8:05 
GeneralSlower than native code... Pinmembertherearefartoomanybens10-Mar-09 11:56 
GeneralRe: Slower than native code... PinmemberDEGT24-Sep-09 1:14 
GeneralNice little utility Pinmemberrc_cohn10-Nov-05 12:11 
GeneralSome text editors does not trigger fileSystemWatcher.Changed event! PinmemberTomer Shalev3-Aug-05 7:32 
GeneralRe: Some text editors does not trigger fileSystemWatcher.Changed event! Pinmembermeraydin2-Dec-05 6:51 
GeneralUse in threads Pinmemberbollwerj24-May-04 8:11 
GeneralRe: Use in threads PinmemberDEGT24-Sep-09 1:36 
GeneralCall to TargetFile_Changed Pinmemberbollwerj28-Apr-04 9:41 
Hi
 
First I'd like to say thank you very much for this article.   I'm fairly new to .Net and had not come across either the FileSystemWatcher class or the StringBuilder class so this article was very beneficial in that respect.
 
I don't speak C#, I work in VC++ so I converted your code and to VC++ and customized it a bit to fit into a VC++ GUI that I'm building.   (Made for a good learning experience.) I'm happy to say it works like a champ for what I need it to do.
 
I think I understand most of what is going on with the exception of the call to TargetFile_Changed(NULL< NULL) that is made during initialization.   I guessing that the purpose of that call is to simply pick up anything that may currently exist in the file.   Is that correct?
 
One last thing, when converting the Byte array data to StreamBuilder data, in VC++, a cast to char doesn't work. It produced decimal numbers instead of the character equivilants. I had to use the Convert::ToChar() to get characters instead of the decimal codes.
 
Thanks again for the project, it makes for a good tutorial as well as a useful tool.
 
Cheers
 
John Bollwerk
Smile | :)
 

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