Huddled Masses

[new] Update, I created a better version of a pipeline function for powershell …

Every once in a while the question of how to best use the process block of a function to process pipeline objects comes up on IRC, and although I’m sure others have already written this up on the web in the past, we’ve been polishing up this example sending it back and forth to each other for a couple of weeks, and it seems to me it’s about time to publish it a little more prominently.

The basic idea was to write a function that could be used to process a set of inputs from either the pipeline or an argument, while still allowing other arguments to be passed and processed. This is something that’s very easy for a cmdlet, because you can specify that a certain parameter will receive pipeline input (and even control which attribute of the objects on the pipeline will be used), but in a script it’s not really supported. In addition, we require that the function process items as they come in, rather than waiting until it’s received all input before processing them.

Some explanation

This is just an example method, it doesn’t really do much, except that it outputs exactly the same thing whether you call it like Test-Pipeline -inputObject "Foo","Bar","Baz" "Kill Each Process" or like: ""Foo","Bar","Baz" | Test-Pipeline "Kill Each Process".

It works by calling itself with the inputObject argument passed on the pipeline when you specify it as an argument, so that code is processed exactly right. This means that the first line you see in the process block must remain the first line, even if you add additional code to the block.

We use a trick to make sure that additional unnamed parameters work normally by using a switch parameter -inputObject followed by a second parameter $io which we treat as inputObject if inputObject is set. This works because switch parameters don’t expect values, so we’re able to sort-of hijack it to emulate the cmdlet behavior. Note that if it is not set, we move the value of the $in parameter into the $args array which contains any additional parameters. This arrangement allows it to behave rather like a cmdlet would, but it’s a little delicate: if you’re going to pass input objects, you must specify the switch as the first argument, and pass the $io (inputObjects) immediately following (as though they were the value of the inputObjects parameter). All of that is necessary so that we can optionally pass additional parameters.

You can also add additional named parameters — but you would need to either always specify their names when you called them, or carefully adjust the code so that they get shifted if -inputObject isn’t specified.

Here’s some sample output, in case you’re wondering:

Credit where it’s due

I wanted to officially acknowledge that /\/\o\/\/ and Gaurhoth and Oisin and Brandon have all contributed to the polishing and testing of this script in various forms … thanks to all

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