How to continuously try something in powershell until it succeeds? - windows

Question

Using the basic construct:
try
{
Do-Something
}
catch
{
Write-Output "Something threw an exception"
}
Is it possible to keep trying the Do-Something until it succeeds? Perhaps using a while loopp like this:
$Timeout = 60
$timer = [Diagnostics.Stopwatch]::StartNew()
$t = 0
while (($timer.Elapsed.TotalSeconds -lt $Timeout) -and ($t -ne 1))) {
Start-Sleep -Seconds 1
try
{
Do-Something
$t = 1
}
catch
{
Write-Output "Something threw an exception"
}
}
$timer.Stop()
Here I use a timer to make sure PowerShell doesn't run indefinitely.
It should keep trying until the try succeeds and $t = 1 is executed. However, it fails in about 2 seconds. Please help.
More specifically the Do-Something is:
(Get-Process -Name FineReader).MainWindowHandle
I'd like the code to keep trying until FineReader exists and it can get the MainWindowHandle.

Answer 1

You can use break keyword.
# Set the erroracton preference to stop when an error occurs,
$ErrorActionPreferenceBak = $ErrorActionPreference
$ErrorActionPreference = 'Stop'
While($True){
try{
Do-Something
break
}
catch{
Write-Output "Something failed"
Start-Sleep -Seconds 1 # wait for a seconds before next attempt.
}
finally{
#Reset the erroracton preference
$ErrorActionPreference = $ErrorActionPreferenceBak
}
}

Answer 2

Your Do-Something should be called with the switch -ErrorAction Stop in order to issue a terminating exception that can be caught by try
For that, you also need to bind your function as a CmdLet. For example:
function DoSomething {
[CmdLetBinding()]
Param(
)
# Your code
}
And then call your function with the -ErrorAction Stop switch:
try {
Do-Something -ErrorAction Stop
}
If your DoSomething is not a function but rather an existing powershell CmdLet, then ... you guessed it, just call it with -ErrorAction Stop
You can learn more about the try/catch/finally in powershell here

Answer 3

Assume that you have a function failing in 9 out of 10 calls:
function Get-RandomFail{
$value = [Random]::new().Next(10);
if ($value -ne 5) { throw }
return $value
}
And you want to limit time window, you can use following:
function Try-Invoke{
[CmdletBinding()]
param(
$Action,
$MaxSeconds,
$Delay=1
)
$timeout = [timespan]::FromSeconds($MaxSeconds)
$start = [DateTime]::Now
do {
try {
return &$Action
} catch {
Write-Verbose "Error"
}
Start-Sleep -Seconds $Delay
} until (([DateTime]::Now - $start) -gt $timeout)
throw
}
$result = Try-Invoke {Get-RandomFail} -MaxSeconds 5 -Verbose
Get-RandomFail will be invoked until no error occurs or until time is over. You can also use Delay argument to modify sleep time after every unsuccesfull Get-RandomFail invocation.