powershell - Where-Object, Select-Object and ForEach-object - Differences and Usage

Question

Where-Object, Select-Object and ForEach-Object

I am a PowerShell beginner. I don't understand too much. Can someone give examples to illustrate the differences and usage scenarios between them?

Answer 1

If you are at all familiar with either LINQ or SQL then it should be much easier to understand because it uses the same concepts for the same words with a slight tweak.

Where-Object

is used for filtering out objects from the pipeline and is similar to how SQL filters rows. Here, objects are compared against a condition, or optionally a ScriptBlock, to determine whether it should be passed on to the next cmdlet in the pipeline. To demonstrate:

# Approved Verbs
Get-Verb | Measure-Object # count of 98
Get-Verb | Where-Object Verb -Like G* | Measure-Object # 3

# Integers 1 to 100
1..100 | Measure-Object # count of 100
1..100 | Where-Object {$_ -LT 50} | Measure-Object # count of 49

This syntax is usually the most readable when not using a ScriptBlock, but is necessary if you want to refer to the object itself (not a property) or for more complicated boolean results. Note: many resources will recommend (as @Iftimie Tudor mentions) trying to filter sooner (more left) in the pipeline for performance benefits.

Select-Object

is used for filtering properties of an object and is similar to how SQL filters columns. Importantly, it transforms the pipeline object into a new PSCustomObject that only has the requested properties with the object's values copied. To demonstrate:

Get-Process
Get-Process | Select-Object Name,CPU

Note, though, that this is only the standard usage. Explore its parameter sets using Get-Help Select-Object where it has similar row-like filtering capabilities like only getting the first n objects from the pipeline (aka, Get-Process | Select-Object -First 3) that continue onto the next cmdlet.

ForEach-Object

is like your foreach loops in other languages, with its own important flavour. In fact, PowerShell also has a foreach loop of its own! These may be easily confused but are operationally quite different. The main visual difference is that the foreach loop cannot be used in a pipeline, but ForEach-Object can. The latter, ForEach-Object, is a cmdlet (foreach is not) and can be used for transforming the current pipeline or for running a segment of code against the pipeline. It is really the most flexible cmdlet there is.

The best way to think about it is that it is the body of a loop, where the current element, $_, is coming from the pipeline and any output is passed onto the next cmdlet. To demonstrate:

# Transform
Get-Verb | ForEach-Object {"$($_.Verb) comes from the group $($_.Group)"
# Retrieve Property
Get-Verb | ForEach-Object Verb
# Call Method
Get-Verb | ForEach-Object GetType
# Run Code
1..100 | ForEach-Object {
    $increment = $_ + 1
    $multiplied = $increment * 3
    Write-Output $multiplied
}