Do not continue until a file exists in powershell

Question

Hi,
I was wondering if there is a method in powershell to get it to 
continually check whether a file exists and do not progress until it finds it.
i.e
DO WHILE  
c:\flag.txt NOT EXISTS
Wait 10 seconds
END
<Next command >
So that the <next Command> section is only executed once it finds the file c:\flag.txt
Is this possible?
Thanks,
Zoe

Answer 1

I doubt that it would have been very helpful anyway.  The requirement was
"check whether a file exists and
do not progress until it finds it"
so blocking is exactly what's called for.  Putting it into a background job is probably just going to trade checking of the file status for checking on the job status.
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My post was very helpful, indeed.
Blocking any instance of an application (PowerShell in this case) is almost always a very bad decision (*) because it blocks the UI.
Do you understand that a UI, when blocked cannot process any other command from user, just because the UI is blocked?
( Alright, Ctrl+Break can stop the process, and the terminate the PowerShell session too! )
Your argumentation is proper of a non-programmer.
Imagine the situation: you, the user, has a powerful computer at your finger tips, with last generation of MS server, which is running your smart PS script that's blocking (how many hours for?) your PS instance. In this case, you don't need a powerful computer,
what you need is a better PowerShell scripter.
Haven't heard of Star-Job?
Spend sometime reading about it, it's worth the reading.
( Alright, your mvp friend seems never heard of it, when I look at the crap code he did using Star-Job in another thread. )
The solution is very simple: Start a job to wait for whatever have to be waited; then, keep using the PS instance
normally, and check once or another if the job is completed (or use another sync mechanism).
This is smart!
Blocking the UI is since ever, not smart.
(*) except very specific situations; fortunately, current computers are preemptively multitask.
I understand it blocks the UI, but we don't have any indication in the question that this is going to be used in an interactive session or that the OP doesn't know about background jobs.  If it's going to have to monitor/wait for an extended period
of time it could well be being implemented as a scheduled task or job, or even as a persistent job.  No matter what the session environment is, some solution for implementing that blocking behavior will be needed to satisfy the stated requirements, and
the posted solution will do that.
The assumption is that the OP isn't smart enough to figure out or experienced enough to know that what they're asking for will block their interactive session while it waits for the file.  I don't see anything in the question to warrant that assumption. 
If it's going to have to wait an arbitrarily long time, a permanent registered event might be a better solution than the time loop.
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