Scripting Games 2012 comments: #14 local computer name

There are a number of ways to pass the names of the local machine into a script or function:

• use the actual name
• use the IP address (if the processing in the script can work with IP addresses)
• use 127.0.0.1 – the loop back address (if the processing in the script can work with IP addresses)
• use a dot  “.”  to signify the local machine
• use “localhost”
• use the environmental variable holding the machine name

Normally we don’t use any of the first three because you either have to type out the name – chance of error or the functionality within the script won’t accept IP addresses. Also the PSsession cmdlets only use computer names by default

Traditionally we have used . or localhost especially when working with WMI

Get-WmiObject -Class Win32_OperatingSystem -ComputerName .
Get-WmiObject -Class Win32_OperatingSystem -ComputerName localhost

There are a number of cmdlets that will accept a computer name as a parameter

PS> Get-Help * -Parameter ComputerName | select name

Name
—-
Get-WinEvent
Get-Counter
Test-WSMan
Invoke-WSManAction
Connect-WSMan
Disconnect-WSMan
Get-WSManInstance
Set-WSManInstance
Remove-WSManInstance
New-WSManInstance
Invoke-Command
New-PSSession
Get-PSSession
Remove-PSSession
Receive-Job
Enter-PSSession
Get-EventLog
Clear-EventLog
Write-EventLog
Limit-EventLog
Show-EventLog
New-EventLog
Remove-EventLog
Get-WmiObject
Invoke-WmiMethod
Get-Process
Remove-WmiObject
Register-WmiEvent
Get-Service
Set-Service
Set-WmiInstance
Get-HotFix
Test-Connection
Restart-Computer
Stop-Computer

This works:

Get-Process -ComputerName .

but this throws an error

PS> Get-Process -ComputerName localhost
Get-Process : Couldn’t connect to remote machine.
At line:1 char:12
+ Get-Process <<<<  -ComputerName localhost
    + CategoryInfo          : NotSpecified: (:) [Get-Process], InvalidOperationException
    + FullyQualifiedErrorId : System.InvalidOperationException,Microsoft.PowerShell.Commands.GetProcessCommand

I’ve seen similar errors with Get-Eventlog

PS> Get-EventLog -List -ComputerName .

  Max(K) Retain OverflowAction        Entries Log
  —— —— ————–        ——- —
  20,480      0 OverwriteAsNeeded      21,814 Application
     512      7 OverwriteOlder              0 DemoMate
  20,480      0 OverwriteAsNeeded           0 HardwareEvents
     512      7 OverwriteOlder              0 Internet Explorer
  20,480      0 OverwriteAsNeeded           0 Key Management Service
   8,192      0 OverwriteAsNeeded           3 Media Center
     512      7 OverwriteOlder              0 MyNewLog
     128      0 OverwriteAsNeeded         357 OAlerts
  20,480      0 OverwriteAsNeeded           1 Scripts
                                              Security
  20,480      0 OverwriteAsNeeded      56,225 System
  15,360      0 OverwriteAsNeeded      10,646 Windows PowerShell

PS> Get-EventLog -List -ComputerName localhost
Get-EventLog : The network path was not found.
At line:1 char:13
+ Get-EventLog <<<<  -List -ComputerName localhost
    + CategoryInfo          : NotSpecified: (:) [Get-EventLog], IOException
    + FullyQualifiedErrorId : System.IO.IOException,Microsoft.PowerShell.Commands.GetEventLogCommand

My solution and recommendation is to use the environmental variable for the computer name.

PS> Get-ChildItem -Path env: | where {$_.name -like "*computer*"}

Name                           Value
—-                           —–
COMPUTERNAME                   RSLAPTOP01

The environment provider doesn’t allow the use of filters so we have to use where

How do we use this:

Get-WmiObject -Class Win32_OperatingSystem -ComputerName $env:COMPUTERNAME
Get-Process -ComputerName $env:COMPUTERNAME
Get-EventLog -List -ComputerName $env:COMPUTERNAME

It can also be used as the default value on a function parameter that asks for a computername – that way you get the local machine if you don’t specify a value to the parameter.

I have never seen this fail – doesn’t mean it can’t just that I’ve never seen it – and it has the advantage of being easier to read than using a dot