How do I – Validate user input is numeric from Powershell?

Written by Cornelius J. van Dyk on . Posted in Blog

As the level of complexity and power of our Powershell scripts increase, it is inevitable that we reach a point where we need to interact with the user and prompt them for some information.  In this case we were building a script to automate the creation and scaling of Search so we needed to know how many servers would be configured as Crawl servers.  We all know the Read-Host cmdlet can provide us with the input, but how do we know that the user actually entered numeric only data?  The easiest way is to simply use a try/catch block and attempt to convert the input to an int value.  Here’s how we do that:

$numberOfCrawl = 0
$inputOK = $false
    [int]$numberOfCrawl = Read-Host -ForegroundColor red "Please enter the number of servers you wish to configure as Crawl servers."
    $inputOK = $true
    Write-Host -ForegroundColor red "INVALID INPUT!  Please enter a numeric value."
until ($inputOK)
Write-Host –ForegroundColor green "You chose to configure [$numberOfCrawl] Crawl servers."

We start by defining the $inputOK variable as false and then wrap the entire piece of code in a do/until loop that repeats until the user enters a valid numeric only value.  Inside of this loop, we have a try/catch block.  Inside the try, we attempt to assign the entered value from Read-Host to the $numberofCrawl variable which we cast as an int.  If the user enters a non-numeric value, the cast will fail and will be caught by the catch.  In the catch we output an error message in red text and simply allow the loop to restart from the top.  If however, the user entered a valid numeric value, the cast will succeed afterwich we set the $inputOK variable to true thus allowing the loop to end gracefully.  As a final step, we display the entered value to the user.



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Cornelius J. van Dyk

Born and raised in South Africa during the 70's I got my start in computers when a game on my Sinclair ZX Spectrum crashed, revealing it's BASIC source code. The ZX had a whopping 48K of memory which was considered to be a lot in the Commodore Vic20 era, but more importantly, it had BASIC built into the soft touch keyboard. Teaching myself to program, I coded my first commercial program at age 15.

After graduating high school at 17, I joined the South African Air Force, graduating the Academy and becoming a Pilot with the rank of First Lieutenant by age 20. After serving my country for six years, I made my way back into computer software.

Continuing my education, I graduated Suma Cum Laude from the Computer Training Institute before joining First National Bank where my work won the Smithsonian Award for Technological Innovation in the field of Banking and Insurance. Soon I met Will Coleman from Amdahl SA, who introduced me to a little known programming language named Huron/ObjectStar. As fate would have it, this unknown language and Y2K brought me to the USA in 1998.

I got involved with SharePoint after playing around with the Beta for SharePoint Portal Server 2003. Leaving my career at Rexnord to become a consultant in 2004, I was first awarded the Microsoft Most Valuable Professional Award for SharePoint in 2005, becoming only the 9th MVP for WSS at the time. I fulfilled a life long dream by pledging allegiance to the Flag as a US citizen in 2006. I met the love of my life and became a private consultant in 2008. I was honored to receive my ninth MVP award for SharePoint Server in 2013.

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