PowerShell: Can you hook or intercept PowerShell execution of external applications?

Question

I want to do some housekeeping before executing any external console applications (setting some environment vars).

In my web research, it looks like overriding NotifyBeginApplication() in $host might do the trick. Unfortunately, I can't figure out how to do that.

Here's essentially what I want to do...

$host = $host | `
   Add-Member -force -pass -mem scriptmethod NotifyBeginApplication `
   { $env:_startTime = [datetime]::now; $env:_line = $myInvocation.Line }

This doesn't work as $host is constant and it may be the wrong approach anyway.

The documentation that I've been able to find states that this function is called before any "legacy" console application is executed, but another blog entry says that it's only called for console applications that have no I/O redirection.

So, is this the right way to do this? If so, how would I override the function?

If not, how could this be done?

The only alternative I've seen that might work is to fully implement a custom PSHost. That seems possible with existing available source code, but beyond what I want to attempt.

Answer 1

I also agree with your (unfortunate) conclusion that you will need to create your own custom host to handle this.

You can create additional runspaces easily enough via scripting, but this method isn't accessible in your currently running host (the default console).

Answer 2

If this is code that you can modify, then create this function:

Function call {
    ## Set environment here
    $env:FOO = "BAR"
    Invoke-Expression "$args"
}

Now pass your native commands to the call function. Examples:

call cmd /c set
call cmd /c dir
call somefunkyexe /blah -ooo aaah -some:`"Funky Argument`"

If this is code that you can't modify, then things will be complicated.