PowerShell has simple logging built in:

Write-Debug "This will be output to the console if `$DebugPreference = `"Continue`""

Echo "This will be output to the console and can be piped to Out-File (remember to use -append)"

Sometimes more complex logging is needed - for instance, NLog, a proven and well-documented .NET library that can log to file or the Windows event log/e-mail/database as needed, controlled by a flexible XML config file.

I got NLog working in a PowerShell script. Here’s how:

• download NLog e.g. from NuGet https://www.nuget.org/packages/NLog
• copy an appropriate NLog.dll to the same directory as your PowerShell script (I used version 4.3.3 .NET 4 DLL on Windows Server 2012)
• create an NLog.config file in the same directory as your PowerShell script
• copy the following text into the newly-created NLog.config file, changing <YOUR DIRECTORY NAME HERE> to a valid directory. This is the directory NLog will write the log file called “logfile.txt” to - it can be the same as the the directory your PowerShell script, NLog.dll and NLog.config files are:

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      autoReload="true">
  <targets async="true">
    <!-- text file - absolute directory e.g. not using "basedir" variable -->
    <target name="logfile"
            xsi:type="File"
            fileName="<YOUR DIRECTORY NAME HERE>\logfile.txt"
            layout="${longdate}|${message} ${exception:format=tostring}"/>
  </targets>
  <rules>
    <!-- log only info and above -->
    <logger name="*" minLevel="Info" writeTo="logfile"/>
  </rules>
</nlog>

• in your PowerShell script, copy the following text to somewhere near the top:

# get current running script path
$scriptPath = $MyInvocation.MyCommand.Path
$scriptPath = Split-Path $scriptPath
# set up logging with NLog
# requires NLog.dll and NLog.config to be in the same directory as this script
# adapted from https://github.com/NLog/NLog/issues/233
# needs absolute path to NLog.dll and NLog.config (use variable "scriptPath")
# ignore version and location output from LoadFile
[Reflection.Assembly]::LoadFile("$scriptPath\NLog.dll") | Out-Null

# load NLog config file
# note all logging targets in NLog config file need to be absolute (not relative) paths
$ne = New-Object NLog.Config.XmlLoggingConfiguration("$scriptPath\NLog.config")
# assign config file
([NLog.LogManager]::Configuration) = $ne
# get the default logger
$logger = [NLog.LogManager]::GetCurrentClassLogger()

• you can now use NLog in your PowerShell script by calling methods on the $logger object, like:

$logger.Debug("Debug...") # note this won't be written using my sample NLog config file
$logger.Info("Info...")
$logger.Error("Error...")

If all goes well, after running the PowerShell script with output to the $logger object, the PowerShell script directory will have 4 files:

...
├── <YOUR AWESOME POWERSHELL SCRIPT>.ps1
├── logfile.txt
├── NLog.config
└── NLog.dll

Logging to a text file is only scratching the surface of what NLog is capable of. For more examples of NLog targets and layouts, see the NLog wiki at https://github.com/nlog/nlog/wiki.

One gotcha mentioned above is relative paths - NLog did not seem to write the log file without an absolute path. To troubleshoot, I added attributes throwExceptions and internalLogToConsole to the NLog.config file - more help can be found at at https://github.com/nlog/nlog/wiki/Logging-troubleshooting.