#include <File.au3>
$sFile = @DesktopDir & '\Test.txt'
FileClose(FileOpen($sFile, 2))
For $i = 1 To 10
FileWriteLine($sFile, 'Line #' & $i)
Next
MsgBox(64, 'Before', FileRead($sFile))
_FileWriteToLine($sFile, 5, ' ' & FileReadLine($sFile, 5), 1)
MsgBox(64, 'After', FileRead($sFile))
If StringInStr($sFile, 'test.txt') Then
FileDelete($sFile)
EndIf