Tail a log file (text file)

[crunnluadh]

My question is how would I go about monitoring the last line of a log file and displaying it in a txtfield? Here is the scenario:

I have an application running that sends some events to a log file it keeps. Keep in mind that this file is opened by the application. I would like to read the last line of the log file and print that in my app, and then when ever it is updated print the new update.

Additionally a more advanced way would be the last 5 lines of the log file to get a semi-history. An even more advanced way would be to have a scrolling function to track all of it if the user needed to see if they'd missed something.

thank you

[DevGrp]

Why dont you just load the log file into the textbox? Easiest solution. If you are going to do that, make sure the textbox is set to multiline.

[crunnluadh]

That partially works, but how would I get it to update as the log updated. I'd rather not keep passing the entire file every time. The log gets rather large, it's for one of our security devices.

So my question is partially answered but now I need to know how to keep updating the entries as the log is updated.

Thanks again.

[Slow_Learner]

Try something like this:

Imports System.IO

Public Class Form1
    Inherits System.Windows.Forms.Form

#Region " Windows Form Designer generated code "

    Public Sub New()
        MyBase.New()

        'This call is required by the Windows Form Designer.
        InitializeComponent()

        'Add any initialization after the InitializeComponent() call

    End Sub

    'Form overrides dispose to clean up the component list.
    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing Then
            If Not (components Is Nothing) Then
                components.Dispose()
            End If
        End If
        MyBase.Dispose(disposing)
    End Sub

    'Required by the Windows Form Designer
    Private components As System.ComponentModel.IContainer

    'NOTE: The following procedure is required by the Windows Form Designer
    'It can be modified using the Windows Form Designer.  
    'Do not modify it using the code editor.
    Friend WithEvents btnTail As System.Windows.Forms.Button
    Friend WithEvents TextBox1 As System.Windows.Forms.TextBox
    Friend WithEvents Label1 As System.Windows.Forms.Label
    Friend WithEvents TextBox2 As System.Windows.Forms.TextBox
    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
        Me.btnTail = New System.Windows.Forms.Button()
        Me.TextBox1 = New System.Windows.Forms.TextBox()
        Me.Label1 = New System.Windows.Forms.Label()
        Me.TextBox2 = New System.Windows.Forms.TextBox()
        Me.SuspendLayout()
        '
        'btnTail
        '
        Me.btnTail.Location = New System.Drawing.Point(272, 464)
        Me.btnTail.Name = "btnTail"
        Me.btnTail.TabIndex = 0
        Me.btnTail.Text = "Tail"
        '
        'TextBox1
        '
        Me.TextBox1.Location = New System.Drawing.Point(64, 24)
        Me.TextBox1.Name = "TextBox1"
        Me.TextBox1.Size = New System.Drawing.Size(552, 20)
        Me.TextBox1.TabIndex = 1
        Me.TextBox1.Text = ""
        '
        'Label1
        '
        Me.Label1.Location = New System.Drawing.Point(0, 24)
        Me.Label1.Name = "Label1"
        Me.Label1.Size = New System.Drawing.Size(64, 32)
        Me.Label1.TabIndex = 2
        Me.Label1.Text = "Filename and path:"
        '
        'TextBox2
        '
        Me.TextBox2.Location = New System.Drawing.Point(8, 72)
        Me.TextBox2.Multiline = True
        Me.TextBox2.Name = "TextBox2"
        Me.TextBox2.ReadOnly = True
        Me.TextBox2.ScrollBars = System.Windows.Forms.ScrollBars.Vertical
        Me.TextBox2.Size = New System.Drawing.Size(608, 376)
        Me.TextBox2.TabIndex = 3
        Me.TextBox2.Text = ""
        '
        'Form1
        '
        Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)
        Me.ClientSize = New System.Drawing.Size(624, 501)
        Me.Controls.AddRange(New System.Windows.Forms.Control() {Me.TextBox2, Me.Label1, Me.TextBox1, Me.btnTail})
        Me.Name = "Form1"
        Me.Text = "Form1"
        Me.ResumeLayout(False)

    End Sub

#End Region

    Private Sub btnTail_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnTail.Click
        Dim SRead As Stream
        Try
            SRead = File.OpenRead(TextBox1.Text)
            Dim SrRead As StreamReader = New StreamReader(SRead, System.Text.Encoding.ASCII)
            ' set the file pointer to the beginning
            SrRead.BaseStream.Seek(0, SeekOrigin.Begin)
            SrRead.BaseStream.Position = SrRead.BaseStream.Length - 500
            Dim buffer(500) As Char
            SrRead.Read(buffer, 0, (SrRead.BaseStream.Length - SrRead.BaseStream.Position))
            TextBox2.Text = buffer
            SrRead.DiscardBufferedData()
            SrRead.Close()
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Sub
End Class

although you may want to add some code to trap for files less than 500 bytes, or of course you may want to read more than 500 bytes from the end. Hope this helps.

[Slow_Learner]

Originally posted by crunnluadh
That partially works, but how would I get it to update as the log updated.

Take a look at the FileSystemWatcher:

ms-help://MS.VSCC/MS.MSDNVS/cpref/html/frlrfSystemIOFileSystemWatcherClassTopic.htm

prolly you can tack one onto the tail code above pretty easy

[Edneeis]

The FileSystemWatcher is definately the way to go for this, here is an example. Sorry I forgot to comment it.

[DevGrp]

The example that Edneeis provided should get you started.

Good work Edneeis.

[crunnluadh]

Thank you all for the great advice and timely . It was a rather important side project a few of us are tackling. Thanks again.

[Slow_Learner]

Went ahead and added the tail code instead of reading the whole file

[Edneeis]

I forgot to scroll the textbox to the bottom, so here is that part:

'scroll to the bottom
            txtLog.SelectionStart = txtLog.Text.Length
            txtLog.ScrollToCaret()

And if you want to just grab a certain number of 'lines' from the log then you can use this. I used a Queue to just grab the bottom lines other than that it is a lot like Slow_Leaners that grabs the end in bytes.

Private Sub UpdateText(ByVal sender As System.Object, ByVal e As FileSystemEventArgs)
        Try
            Dim SRead As Stream
            'open file
            SRead = File.OpenRead("Log.txt")
            Dim SrRead As StreamReader = New StreamReader(SRead, System.Text.Encoding.ASCII)
            'setup queue
            Dim q As New Queue()
            'read lines into que and only keep the last lines upto the value of txtLinesToTail 
            While (SrRead.Peek() > -1)
                Dim Buffer As String = SrRead.ReadLine()
                'add line to queue
                q.Enqueue(Buffer)
                'ditch any lines more than needed
                If q.Count > CInt(txtLinesToTail.Text) Then q.Dequeue()
            End While
            'transfer the queue contents to the textbox
            Dim line As String
            txtLog.Text = String.Empty
            For Each line In q
                txtLog.Text &= String.Concat(line, ControlChars.NewLine)
            Next
            'scroll to the bottom
            txtLog.SelectionStart = txtLog.Text.Length
            txtLog.ScrollToCaret()
            'close the file
            SrRead.Close()
        Catch ex As Exception
            'show error
            MsgBox(ex.Message)
        End Try
    End Sub
 
    Private Sub txtLinesToTail_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtLinesToTail.TextChanged
        'update with new line count
        UpdateText(Me, Nothing)
    End Sub

[Slow_Learner]

I may be reading this wrong but:

            While (SrRead.Peek() > -1)
                Dim Buffer As String = SrRead.ReadLine()
                'add line to queue
                q.Enqueue(Buffer)
                'ditch any lines more than needed
                If q.Count > CInt(txtLinesToTail.Text) Then q.Dequeue()
            End While

Doesn't that actually read the entire file? What if the file's 10mb or more?

[Edneeis]

That is true but there is no way to tell how long a line is (that I know of). So you have to read it all in and then only keep the last x amount of lines. Although I thought yours was an excellent example of doing it conversatively with bytes.

[Slow_Learner]

ms-help://MS.VSCC/MS.MSDNVS/cpref/html/frlrfSystemIOFileInfoClassLengthTopic.htm

So if you wanted to be extra careful, before you move the .BaseStream.Position to a certain position you'd check to see how long the file was (I believe FileInfo.Length returns bytes) and then trap against that to keep an exception from happening. Right?
...
duh woops how long a LINE is eh? As in how many lines in a given file? Yeah I see what you mean. :/

[Edneeis]

Especially since it appears that the ReadLine reads the text until it encounters a CrLf character so each line is not the same size.

[Slow_Learner]

Yeah I've been working with reading delimited text files rather a lot lately so I figured ReadLine was right out for this problem, for just the reason you say.

[crunnluadh]

Thank you to everyone that responded and contributed code. We got it working and for the moment correctly . Thanks again.