Lessons in PowerShell - Logging From A Script Running Via Windows Task Scheduler

Logging and tracing information can normally be written to the host console, standard output or standard error streams. However, when a PowerShell script is run from the Windows Task Scheduler, those mechanisms are typically not available.

To work around this situation, I decided to create a log file for my PowerShell Script which is executed from Windows Task Scheduler.

For a start, we can use the .NET StreamWriter class to create a file stream.

1$logFile = New-Object System.IO.StreamWriter("C:\temp\Script.log")

Please note: you must ensure that the user account the script runs under needs to have file system security access to that specific folder and file.

We can then create a function that will write a message to the log file and also flush the stream (I wanted to closely monitor the progress of the script while it may be running, so I am explicitly flushing the stream).

1Function LogMessage($message)
2{
3    $logFile.WriteLine($message)
4
5    $logFile.Flush()
6}

And using the mimicked PowerShell Version 1 Try / Catch / Finally script block, we can ensure that even if an error occurs, the stream will be closed. See the bottom line below.

Here is the script in its entirety.

 1# Trap settings
 2$ReportErrorShowExceptionClass=$true
 3$ReportErrorShowInnerException=$true
 4$ReportErrorShowSource=$true
 5$ReportErrorShowStackTrace=$true
 6
 7# Setup log file functionality
 8$now=Get-Date
 9$logFile = New-Object System.IO.StreamWriter("C:\temp\Script.log")
10
11Function LogMessage($message)
12{
13    $logFile.WriteLine($message)
14    $logFile.Flush()
15}
16
17&{
18    &{ #Try 
19          LogMessage("Started at " + $now)
20          ...
21          LogMessage("Successfully finished")
22    } 
23    trap #Catch
24    {
25        LogMessage("TRAPPED: " + $_.Exception.GetType().FullName) 
26        LogMessage("TRAPPED: " + $_.Exception.Message)
27        continue # So that the "Finally" gets executed
28    }
29} #Finally
30$logFile.Close()

There we have it… a PowerShell script that runs under Windows Task Scheduler and writes out to a log file.

Of course, using the PowerShell Version 2 Try/Catch statement is much nicer than this PowerShell Version 1 equivalent.