Passing a cmdlet as a parameter - function

Question

I need to restart a service in a powershell script. The problem is that this service is a bit buggy and frequently needs to be shut down several times before it gets into the "stopped" state. Because of that I can't seem to use the Restart-Service cmdlet, instead I need to retry the Stop-Service cmdlet a few times. The same applies to starting the service.
So I figure this is a good place to write a function that will take an action (start or stop) and retry it a few times until it works. The problem is I'm not sure how to pass the action in as a parameter. I could just have the action be a String and then say if action == "start" do starcAction, but that won't be very clean. Is there any way I could pass a cmdlet like Stop-Service in as a parameter?

Answer 1

For the scenario you described you'd normally do something like this:
$maxTries = 5
switch ($args(0)) {
'start' {
Start-Service 'MySvc'
}
'stop' {
$i = 0
do {
Stop-Service 'MySvc'
$i++
} until ((Get-Service 'MySvc').Status -eq 'Stopped' -or $i -ge $maxTries)
if ((Get-Service 'MySvc').Status -ne 'Stopped') {
Write-Error "Cannot stop service."
exit 1
}
}
default {
Write-Error "Unknown action: $_"
exit 1
}
}
If you really want to avoid string arguments, you could use parameter sets like this:
[CmdletBinding(DefaultParameterSetName='none')]
Param(
[Parameter(Mandatory=$true,ParameterSetName='start')]
[Switch][bool]$Start = $false,
[Parameter(Mandatory=$true,ParameterSetName='stop')]
[Switch][bool]$Stop = $false
)
$maxTries = 5
switch ($PSCmdlet.ParameterSetName) {
'start' {
Start-Service 'MySvc'
}
'stop' {
$i = 0
do {
Stop-Service 'MySvc'
$i++
} until ((Get-Service 'MySvc').Status -eq 'Stopped' -or $i -ge $maxTries)
if ((Get-Service 'MySvc').Status -ne 'Stopped') {
Write-Error "Cannot stop service."
exit 1
}
}
'none' {
Write-Error "Usage: $($MyInvocation.MyCommand.Name) {-Start|-Stop}"
exit 1
}
}

Answer 2

Param([Parameter(Mandatory)] [ValidateSet('Start','Stop')] [string] $Action)
This allows the user to press Tab to select the possible values and will automatically reject all invalid input.
Passing in a well defined parameter (doesn't matter if it's a string or not) is actually cleaner than "passing in a commandlet" would be if there was such a thing.