PowerShell: Run Cmdlet If Another Was Successful (And Keep Trying Until It Is)

Today I’m sharing a useful bit of PowerShell I gracelessly punt from script to script whenever I need to make sure a prerequisite it met before running something and to keep checking until it’s met, then run what I need: “do X when Y is ready and keep checking Y until it’s ready”.

The original use for this was my script to create a new Microsoft 365 user, but hold off on some parts of it – such as time zone settings – until the Exchange Online mailbox is provisioned.  That takes some time, so I wanted to keep checking and as soon as I could, continue the script.

Another scenario this could potentially be used is if you have to execute something with a dependency to connectivity to a remote server, but you need to wait on a connection being established, e.g. VPN connection separate to the script.

Here’s what it looks like using my “don’t proceed until the mailbox exists” example:

I can hear some screaming in the distance at my use of MessageBox, but for everyone else: what’s happening here?

First, Get-Mailbox runs against the mailbox we are interested in setting.  It does not find the mailbox, so passes it to else, which says “I’ll wait a minute, then try again”, when until confirms the mailbox exists.  And repeat.  In your own use of checking the mailbox existed, all you’d need to do is change the variable of $email and the messages displayed.

I am also using Start-Countdown, an awesome function by Martin Pugh which is like Start-Sleep but with a visual countdown. Available here.

In another example, I only run the $install block if Test-Connection confirms I can connect to $server.  This one uses Start-Sleep instead of Start-Countdown and consequently, if it’s running interactively, the user will see error messages until the server’s available.  I use this when installing apps using PowerShell running as SYSTEM.  For example, run at startup but only when the VPN’s connected.

The template minus any example information is below, where Check-This presenents your cmdlet for a prerequisite, and Do-This is the cmdlet for your action.