Can Powershell Run Commands in Parallel?
You can execute parallel jobs in Powershell 2 using Background Jobs. Check out Start-Job and the other job cmdlets.
# Loop through the server list
Get-Content "ServerList.txt" | %{
# Define what each job does
$ScriptBlock = {
param($pipelinePassIn)
Test-Path "\\$pipelinePassIn\c`$\Something"
Start-Sleep 60
}
# Execute the jobs in parallel
Start-Job $ScriptBlock -ArgumentList $_
}
Get-Job
# Wait for it all to complete
While (Get-Job -State "Running")
{
Start-Sleep 10
}
# Getting the information back from the jobs
Get-Job | Receive-Job
The answer from Steve Townsend is correct in theory but not in practice as @likwid pointed out. My revised code takes into account the job-context barrier--nothing crosses that barrier by default! The automatic $_
variable can thus be used in the loop but cannot be used directly within the script block because it is inside a separate context created by the job.
To pass variables from the parent context to the child context, use the -ArgumentList
parameter on Start-Job
to send it and use param
inside the script block to receive it.
cls
# Send in two root directory names, one that exists and one that does not.
# Should then get a "True" and a "False" result out the end.
"temp", "foo" | %{
$ScriptBlock = {
# accept the loop variable across the job-context barrier
param($name)
# Show the loop variable has made it through!
Write-Host "[processing '$name' inside the job]"
# Execute a command
Test-Path "\$name"
# Just wait for a bit...
Start-Sleep 5
}
# Show the loop variable here is correct
Write-Host "processing $_..."
# pass the loop variable across the job-context barrier
Start-Job $ScriptBlock -ArgumentList $_
}
# Wait for all to complete
While (Get-Job -State "Running") { Start-Sleep 2 }
# Display output from all jobs
Get-Job | Receive-Job
# Cleanup
Remove-Job *
(I generally like to provide a reference to the PowerShell documentation as supporting evidence but, alas, my search has been fruitless. If you happen to know where context separation is documented, post a comment here to let me know!)
There's so many answers to this these days:
- jobs (or threadjobs in PS 6/7 or the module for PS 5)
- start-process
- workflows (PS 5 only)
- powershell api with another runspace
- invoke-command with multiple computers, which can all be localhost (have to be admin)
- multiple session (runspace) tabs in the ISE, or remote powershell ISE tabs
- Powershell 7 has a
foreach-object -parallel
as an alternative for #4
Using start-threadjob in powershell 5.1. I wish this worked like I expect, but it doesn't:
# test-netconnection has a miserably long timeout
echo yahoo.com facebook.com |
start-threadjob { test-netconnection $input } | receive-job -wait -auto
WARNING: Name resolution of yahoo.com microsoft.com facebook.com failed
It works this way. Not quite as nice and foreach-object -parallel in powershell 7 but it'll do.
echo yahoo.com facebook.com |
% { $_ | start-threadjob { test-netconnection $input } } |
receive-job -wait -auto | ft -a
ComputerName RemotePort RemoteAddress PingSucceeded PingReplyDetails (RTT) TcpTestS
ucceeded
------------ ---------- ------------- ------------- ---------------------- --------
facebook.com 0 31.13.71.36 True 17 ms False
yahoo.com 0 98.137.11.163 True 97 ms False
Here's workflows with literally a foreach -parallel:
workflow work {
foreach -parallel ($i in 1..3) {
sleep 5
"$i done"
}
}
work
3 done
1 done
2 done
Or a workflow with a parallel block:
function sleepfor($time) { sleep $time; "sleepfor $time done"}
workflow work {
parallel {
sleepfor 3
sleepfor 2
sleepfor 1
}
'hi'
}
work
sleepfor 1 done
sleepfor 2 done
sleepfor 3 done
hi
Here's an api with runspaces example:
$a = [PowerShell]::Create().AddScript{sleep 5;'a done'}
$b = [PowerShell]::Create().AddScript{sleep 5;'b done'}
$c = [PowerShell]::Create().AddScript{sleep 5;'c done'}
$r1,$r2,$r3 = ($a,$b,$c).begininvoke() # run in background
$a.EndInvoke($r1); $b.EndInvoke($r2); $c.EndInvoke($r3) # wait
($a,$b,$c).streams.error # check for errors
($a,$b,$c).dispose() # clean
a done
b done
c done