windows - PowerShell Function to check whether certain port is in LISTENING state on localhost

Introduction

Aim: to check whether a port is in LISTENING state on localhost by using a PowerShell function

The following command:

New-Object Net.Sockets.TcpClient.Connect("127.0.0.1",10389)

results in:

PS C:\Windows\system32> New-Object Net.Sockets.TcpClient.Connect("127.0.0.1",10389)
At line:1 char:33
+ New-Object Net.Sockets.TcpClient.Connect("127.0.0.1",10389)
+                                 ~
Unexpected token ')' in expression or statement.
    + CategoryInfo          : ParserError: (:) [], ParentContainsErrorRecordException
    + FullyQualifiedErrorId : UnexpectedToken

PS C:\Windows\system32>

Question

Which PowerShell Function could be used to check whether a certain port is in LISTENING state on localhost?

If you're using PowerShell v3.0+, then instead of trying to connect to the port to determine the state, you can simply use Get-NetTCPConnection:

Get-NetTCPConnection -State Listen

To me this is more accurate as it's reading the status of the port on the computer. Using a connection to test can make it seem like it's not "LISTENING" when it is, if a firewall or alike gets in the way or something.

First create and store the connection:

$connection = (New-Object Net.Sockets.TcpClient)
$connection.Connect("127.0.0.1",10389)

Then check if it's connected

if ($connection.Connected) {
    "We're connected"
    }

Or as suggested by Colyn1337

Try {
    $connection = (New-Object Net.Sockets.TcpClient)
    $connection.Connect("127.0.0.1",10389)
    "Connected"
    }
Catch {
    "Can't Connect"
    }

I had tried to add this to techie's answer, so this is an expansion of his. You can fine tune the output like so:

Get-NetTCPConnection -State Listen | Where-Object {$_.LocalAddress -eq "192.168.56.1" -and $_.LocalPort -eq "139"}

That would return an array of data if it were listening on that port. If there is no listener, it returns null and therefore no need for error handling.