Run ForEach loop in parallel in PowerShell Core

If you are like me, writing all sorts of scripts in PowerShell Core to automate processes, this is a post for you.

There might be times when you have a time-consuming process, and running them in sequence might be too slow. If you can put the process in the ForEach loop, you are lucky because there is a parallel flavor of ForEach loop.

PowerShell Core allows you to run the ForEach loop in parallel like this:

$services = "Service1","Service1","Service3","Service4","Service5","Service6","Service7"
$tag="latest"

$services | ForEach-Object -Parallel {
    pwsh -Command "docker build -t $($_):$($using:tag) -f $($_).Dockerfile ."
} -ThrottleLimit 3

The script above will loop through the service list in the batch of 3 and process them in parallel. While we look at the script, we can also address two things.

Current variable in the loop

The $_ is used to access the current variable in the loop, a current value from the array $services.

Subexpression operator $()

For example, $($_) means that the expressions inside the subexpression operator will execute first and return it as a variable.

Access variables with $using:

We’ve defined a variable $tag outside of the ForEach loop. In C#, for example, we do not have issues accessing that variable in the loop. But in PowerShell, you need to retrieve it by $using.

So $($using:tag) will result in value latest.