Mastering PowerShell PSDefaultParameterValues for Streamlined Automation

$PSDefaulParameterValues.Add("Command:Parameter","Value")

As you can see we simply add a new key to the hashtable specifying the cmdlet and parameter to set a default for, separated by a :. The value obviously needs to be of a type accepted by the parameter or you will get errors when using the cmdlet.
A concrete example would be to specify a custom separator for Write-Host.

# Default
PS> Write-Host "Hello","World"
Hello World
# Adding the custom default
PS> $PSDefaultParameterValues.Add("Write-Host:Separator", "`n")
# Changed output
PS> Write-Host "Hello","World"
Hello
World

PS> $PSDefaultParameterValues.Add("*:Verbose", $true)

Staying with our verbosity example from before we could make its behavior dependent on a variable $verbose.

PS> $PSDefaultParameterValues.Add("*:Verbose", {$verbose -eq $true})

Now we can control the default verbosity of our session by setting and removing a variable. Keep in mind this script block is run every time you use the cmdlet, so keep it fast. You can measure it like this:

PS> Measure-Command {$verbose -eq $true} | ft
Days	Hours	Minutes Seconds Milliseconds
---- 	----- 	------- ------- ------------
0       0       0       0       0

Get-History | Where-Object CommandLine -like "`$PSDEfaultParameterValues*"

Let’s say you always export data to CSV files for someone to open in Excel. If they just double-click your CSV they will not get a great result.

Fig. 1: Imported data in Excel

Most of us would then simply open up a new Excel file and import the data with Data > From Text, clicking through the plethora of screens and configuring Excel to actually detect the delimiter.

Fig. 2: Manually edited data import

But there is a better way! Instead of just exporting CSV.

Get-Process -Name Microsoft* | Export-Csv msproc.csv

you could specify a custom delimiter and, while your at it, remove type information and clobber.

Get-Process -Name Microsoft* | Export-Csv bettermsproc.csv -Delimiter ";"
-NoTypeInformation -NoClobber

Now importing, exporting, converting from and converting to CSV will use the delimiter ; and not include any type information or clobber.

If you open the resulting CSV file directly with excel you can see a result similar to the data import dance we did before.

Fig. 3: Data import after configuration with PowerShell

But always typing out those parameters is cumbersome and has an inherently high margin of error.
This is where we will set some defaults

$PSDefaultParams.Add("Export-Csv:Delimiter", ";")
$PSDefaultParams.Add("Export-Csv:NoTypeInformation", $true)
$PSDefaultParams.Add("Export-Csv:NoClobber", $true)

With this we can go back to simply using Export-Csv and the resulting file will be opened in Excel just as expected.
We could even go one step further and change those parameters for all cmdlets in the CSV family using wildcards.

$PSDefaultParams.Add("*-Csv:Delimiter", ";")
$PSDefaultParams.Add("*-Csv:NoTypeInformation", $true)
$PSDefaultParams.Add("*-Csv:NoClobber", $true)

in your scripts.

There are some good reasons for this:

• Other users may not have your default values and therefore the script will break for them
• You may change your defaults down the line and the script will break for you

To mitigate this I would suggest just disabling PSDefaultParameterValues first thing in all your scripts:

# Disable PSDefaultParameterValues
$PSDefaultParameterValues.Add("Disabled", $true)

And re-enabling them at the end:

# Re-Enable PSDefaultParameterValues
$PSDefaultParameterValues.Remove("Disabled")