PowerShell Pipeline and object filtering

Pipeline is a functionality in PowerShell where it allows the output of a cmdlet to be used as input to the next cmdlet in the pipeline and work with them. By default at the end of a pipeline is a special cmdlet called Out-Default. Let's have a look at the example below.

Get-Service | Where-Object {$_.Status -eq "Running"}

The above example will provide you a list of services that are currently running on your machine. Here whats actually happening is objects that are produced by the first cmdlet (Get-Service) is passed to the next cmdlet in the pipeline. The second cmdlet will filter the services that are having "Running" status and displays the final result. "$_" holds the current object in the pipeline. 

Let's go through another example to get Windows event logs for last 24 hours.

Get-eventlog -LogName System -EntryType Error -After (Get-Date).AddDays(-1) | select EventID, TimeGenerated, Message |  convertto-html | Out-File C:\errorlist.htm

As you can see this example has multiple stages where output produced by one cmdlet is passed to the next cmdlet in the pipeline. Stage 1 filters System event logs with type error in the last 24 hours. The output of stage 1 is passed to next cmdlet in the pipeline which filters EventID, TimeGenerated, and Message. Results from stage 2 are passed to the next cmdlet where it converts the result to HTML and finally passed to the last stage where the final result is written to a file.

Reference video: