Change the Default Parameter Values in PowerShell v 3.0
It is possible to modify the default action of a PowerShell cmdlet by modifying its parameters. For example, it annoys me that Get-EventLog does not focus on the Application log. Thanks to $PSDefaultParameterValues we can launch a Cmdlet in the way that we want.
$PSDefaultParameterValues Topics
- Change the Behaviour of Get-Eventlog
- How to Reset $PSDefaultParameterValues
- Identify Cmdlets with Suitable Parameters
- PowerShell Parameters Home
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Change the Behaviour of Get-Eventlog with $PSDefaultParameterValues
Normally, if you run the cmdlet Get-Eventlog it prompts you with:
Logname:
However, in PowerShell version 3.0 you can change what happens when you execute a cmdlet by using an interesting built-in function called: $PSDefaultParameterValues.
# Change Default Values of a Cmdlet.
# N.B. Requires PowerShell 3.0
$PSDefaultParameterValues=@{
"Get-Eventlog:logname"="Application";
"Get-Eventlog:newest"=50;
}
Get-Eventlog
Note 1: In the present PowerShell session, whenever you type Get-Eventlog on its own, it returns the 50 lastest entries in the application log.
How to Reset $PSDefaultParameterValues
The quickest way to nullify $PSDefaultParameterValues is to close, then reopen PowerShell. A better way to over-ride the command temporarily is thus:
$PSDefaultParameterValues.Add("Disabled", $true)
Alternatively, you could remove values from the hash table.
$PSDefaultParameterValues.Add("Disabled", $False) $PSDefaultParameterValues.Remove("Get-Eventlog:newest")
Get-Eventlog
Note 2: If you now try Get-Eventlog the results should reveal the Application events, but it won’t halt after the 50th because we removed the :newest parameter.
Tip: Once you have identified suitable jobs for $PSDefaultParameterValues, place them in your profile.ps1 file.
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Research Methods for $PSDefaultParameterValues
We have already used the .add method, now let us see what other methods and properties are available:
# Research $PSDefaultParameterValues
Clear-Host
$PSDefaultParameterValues | Get Member
Useful methods include ‘Clear’, and property ‘Values’.
Identify Cmdlets with Suitable Parameters
These are the parameter characteristics that I look for when I want to change their default actions:
Required? True
Position? 1
Default value [blank, nothing set]
Here is a work-In-progress script to help with our research.
# PowerShell Parameters to change default action
Clear-Host
$Source = Get-Command Get*
Foreach ($cmdlet in $Source) {
$Cmdlet.ParameterSets | Select-Object -ExpandProperty parameters | `
Where {$_.Position -gt "-1" -And $_.isMandatory -eq $true } `
| FT $Cmdlet.Name, Name, Position, IsMandatory -auto
}
Note 3: This relies on Select-Object -ExpandProperty parameters.
Note 4: I have been experiment with different values for the ‘Where’ clause.
See more on setting default PowerShell parameters »
Summary of $PSDefaultParameterValues
You can improve productivity by changing the default behaviour of a PowerShell cmdlet. For example, you can set the action of the plain command: Get-EventLog so that it starts outputting the contents of the Application log. The secret is to master PowerShell’s $PSDefaultParameterValues.
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See more example of PowerShell’s parameters
• PowerShell Tutorials • PowerShell Parameter Introduction • Parameter Examples of Technique
• Mandatory Parameters • PowerShell $PSDefaultParameterValues • Top 10 PowerShell Parameters
• PowerShell Parameters Index • Parameter Hashtable Splatting • PowerShell 3.0 Default Parameters
Please email me if you have a better example script. Also please report any factual mistakes, grammatical errors or broken links, I will be happy to correct the fault.
