Watch file for changes and run command with powershell

Question

Is there any simple way(i.e., script) to watch file in Powershell and run commands if file changes. I have been googling but can\'t find simple solution. Basically I run scr

Answer 1

Here is the solution I ended up with based on several of the previous answers here. I specifically wanted:

1. My code to be code, not a string
2. My code to be run on the I/O thread so I can see the console output
3. My code to be called every time there was a change, not once

Side note: I've left in the details of what I wanted to run due to the irony of using a global variable to communicate between threads so I can compile Erlang code.

Function RunMyStuff {
    # this is the bit we want to happen when the file changes
    Clear-Host # remove previous console output
    & 'C:\Program Files\erl7.3\bin\erlc.exe' 'program.erl' # compile some erlang
    erl -noshell -s program start -s init stop # run the compiled erlang program:start()
}

Function Watch {    
    $global:FileChanged = $false # dirty... any better suggestions?
    $folder = "M:\dev\Erlang"
    $filter = "*.erl"
    $watcher = New-Object IO.FileSystemWatcher $folder, $filter -Property @{ 
        IncludeSubdirectories = $false 
        EnableRaisingEvents = $true
    }

    Register-ObjectEvent $Watcher "Changed" -Action {$global:FileChanged = $true} > $null

    while ($true){
        while ($global:FileChanged -eq $false){
            # We need this to block the IO thread until there is something to run 
            # so the script doesn't finish. If we call the action directly from 
            # the event it won't be able to write to the console
            Start-Sleep -Milliseconds 100
        }

        # a file has changed, run our stuff on the I/O thread so we can see the output
        RunMyStuff

        # reset and go again
        $global:FileChanged = $false
    }
}

RunMyStuff # run the action at the start so I can see the current output
Watch

You could pass in folder/filter/action into watch if you want something more generic. Hopefully this is a helpful starting point for someone else.