How to prompt for $args when missing

Question

My script:

$computername=$args[0]
if ($args -eq $null) { $computername = Read-Host "enter computer name" }
Get-ADComputer -Id $computername -Properties * | select name,description

If I pass the argument with the script i.e.:

get-ComputerName.ps1 computer01

it works fine. However if I skip the computer I want it to prompt me but instead I'm getting this error:

Get-ADComputer : Cannot validate argument on parameter 'Identity'. The argument
is null. Provide a valid value for the argument, and then try running the
command again.
At U:\get-ADComputer-assigned-user.ps1:9 char:20
+ Get-ADComputer -Id $computername -Properties * | select name,description
+                    ~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Get-ADComputer], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationError,Microsoft.ActiveDirectory.Management.Commands.GetADComputer

I cannot figure out how to make it work.

Answer 1

Don't use the automatic variable $args, but define a specific mandatory parameter for the computername:

[CmdletBinding()]
Param(
  [Parameter(Mandatory=$true, Position=0)]
  [string]$ComputerName
)

Get-ADComputer -Id $ComputerName -Properties * | Select-Object Name, Description

That will allow you to run your script like this:

./Get-ComputerName.ps1 -ComputerName computer01

or like this:

./Get-ComputerName.ps1 computer01

If the parameter is missing you'll be prompted for it:

PS C:\> ./Get-ComputerName.ps1

cmdlet test.ps1 at command pipeline position 1
Supply values for the following parameters:
ComputerName: _

If you want the script to throw an error instead of prompting for a missing parameter you could do it like this:

[CmdletBinding()]
Param(
  [Parameter(Mandatory=$false, Position=0)]
  [string]$ComputerName = $(throw 'Parameter missing!')
)

Get-ADComputer -Id $ComputerName -Properties * | Select-Object Name, Description

Check the documentation for further information about parameter handling in PowerShell.