Wait for all child processes to finish - powershell

Question

Currently I am trying to run a script multiple times and let the parent wait for all the child processes to finish.
The creation of the children is as follows:
# Spawn balance load processes
$command = "-i $ScriptPath\balanceLoaders\%BALANCE_DIR% -o $outputDirectory -c %BALANCE_DIR%"
#$command = "$ScriptPath\convertFiles.ps1 -i $ScriptPath\balanceLoaders\%BALANCE_DIR% -o $outputDirectory -c %BALANCE_DIR%"
for ([int]$i = 0; $i -lt $b; $i++) {
#$proc = Start-Process powershell.exe -WindowStyle Hidden -Argument $command.Replace('%BALANCE_DIR%', $i) -PassThru
$proc = Start-Process $ScriptPath\convertfiles.exe -WindowStyle Hidden -Verb Runas -ArgumentList $command.Replace('%BALANCE_DIR%', $i) -PassThru
}
The children are spawned dynamically. Currently I can only track the last spawned child with $proc. However, I want to track them all.
Currently using $proc.WaitForExit() to wait for the child.

Answer 1

Collect all processes in an array and run an idle loop until all of them have exited.
$proc = for ($i = 0; $i -lt $b; $i++) {
Start-Process ... -PassThru
}
while (($proc | Select-Object -Expand HasExited) -contains $false) {
Start-Sleep -Milliseconds 100
}
If you have PowerShell v3 or newer you can simplify the loop to something like this:
while ($proc.HasExited -contains $false) {
Start-Sleep -Milliseconds 100
}
because PowerShell v3 introduced a new feature called member enumeration that allows accessing properties or methods of array elements via the array object.

Answer 2

Ansgar Wiecher's helpful answer contains good pointers and a working solution, but if you simply want to wait for all spawned processes to terminate, use of Wait-Process is simpler.
That said, if you want to check exit codes as they become available, before all processes have exited, a loop that sleeps periodically (Start-Sleep), as in Ansgar's answer, is necessary.
The following solution demonstrates the synchronously-wait-for-all technique with Wait-Process with a simplified example that creates 3 instances of Notepad and waits for all of them to terminate:
# Create all processes and store objects representing them in array $procs.
$b = 3 # number of processes to create
$procs = foreach ($i in 0..($b-1)) { Start-Process -PassThru Notepad }
# Wait for all processes to exit.
# (Close the Notepad windows manually or, if no Notepad windows were previously
# open, use `Stop-Process -Name Notepad` from another session to forcefully close them.)
# Use Wait-Process -Timeout <secs> to limit the max. period of time to wait.
$procs | Wait-Process
# Get the exit codes.
$exitCodes = $procs.ExitCode # PSv3+; v2: $procs | Select-Object -ExpandProperty ExitCode
Or, as a single pipeline:
# Use $_ inside { ... } to refer to the current iteration value.
# Use $procs.ExitCode to get the exit codes.
0..($b-1) | ForEach-Object -ov procs { Start-Process -PassThru Notepad } | Wait-Process
Note the use of -ov - short for common parameter -OutVariable, which collects the process objects output by the ForEach-Object cmdlet's script block invocations.