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#---------------------
# Pipelined functions
#---------------------
#Good reference: https://www.simple-talk.com/dotnet/.net-tools/down-the-rabbit-hole--a-study-in-powershell-pipelines,-functions,-and-parameters/
#___________________________________________________________________________________________
#
# Safety net to protect against accidentally running the whole script with F5
#___________________________________________________________________________________________
write-host -foregroundcolor yellow 'You hit F5. This will run this whole script.'
$response = Read-Host -Prompt 'Did you really mean to run the entire script (y/n)?'
throw [System.ArgumentException] "You CANNOT run using F5. Period. Please highlight and run selection with F8."
#Simple function - Stating the obvious
#---------------
FUNCTION Get-Sum($a, $b)
{
$a + $b
}
## Usage variations
Get-Sum 9 10
Get-Sum -a 9 -b 10
#Simple function - With parameter default values
#---------------
FUNCTION Get-Sum($a = 0, $b = 0)
{
$a + $b
}
## Usage variations
Get-Sum
Get-Sum 9 10
Get-Sum -a 9 -b 10
#Simple Function - That accepts pipeline inputs
#---------------
FUNCTION Out-Screen(
[Parameter(ValueFromPipeline=$True)] $a = 0
)
{
$a
}
#What is the expected result here?
1..10 | Out-Screen
#Not what was expected.. What happened?
#Some things to note:
# 1. The input is an array
# 2. Only the last vaule got output
# 3. Why only the last value?
# ...because function is treated only having an "End" block, we will see next!
#Pipelined Function - What goes wrong? Try Verbose
#---------------
FUNCTION Out-Screen(
[Parameter(ValueFromPipeline=$True)] $a = 0
)
{
Write-Verbose ('$a = {0}' -f $a)
$a
}
1..10 | Out-Screen -verbose
#Only the last value from the input is printed...
#Pipelined Functions - NEED "Begin", "Process", "End"
#---------------
FUNCTION Out-Screen(
[Parameter(ValueFromPipeline=$True)] $a = 0
)
{
Begin
{
Write-Verbose ('Begin $a = {0}' -f $a)
}
Process
{
Write-Verbose ('Process $a = {0}' -f $a)
}
End
{
Write-Verbose ('End $a = {0}' -f $a)
write-host $a
}
}
#What is the expected result here?
1..10 | Out-Screen -Verbose
#Things to note:
#
# Begin - Got called exactly once at the beginning
# Parameter $a is not in the scope of Begin as seen by its default value of 0
#
# Process - Got called once for every pipeline input value
#
# End - Got called exactly once at the end
# Holds the last value of the parameter $a when control flow gets to it
#Pipelined Functions - Total of pipeline input using "Begin", "Process", "End"
#---------------
FUNCTION Get-SumOfPipelineInput(
[Parameter(ValueFromPipeline=$True)] $a = 0
)
{
Begin
{
[int] $sum = 0
Write-Verbose ('Begin $a = {0}, $sum = {1}' -f $a, $sum)
}
Process
{
$sum += $a
Write-Verbose ('Process $a = {0}, $sum = {1}' -f $a, $sum)
}
End
{
Write-Verbose ('End $a = {0}, $sum = {1}' -f $a, $sum)
write-host $sum
}
}
#Try it out
1..10 | Get-SumOfPipelineInput -Verbose
#Pipelined Functions - foreach-object shortcut way of doing the same as above
#---------------
@(1..10) | ForEach-Object -Begin{$sum = 0} -Process{$sum += $_} -End{$sum}
#Note the use of $_ to refer to the values coming from the pipe
#Everyone knows this but now we know what goes behind pipelining (somewhat!)
#Start NotePad
Invoke-Expression "Notepad.exe"
#Kill the innocent "Notepad"
Get-Process -Name notepad | Stop-Process
#Above, Stop-Process accepts a pipeline input (input being of type "System.Diagnostics.Process")
(Get-Process)[0].GetType()
(Get-Process)[0] | Get-Member
#In fact, we could have written Stop-Process something like this
FUNCTION Stop-Process-Jana(
#Note that we are taking an process array as input - [System.Diagnostics.Process[]]
[Parameter(ValueFromPipeline=$True)]
[System.Diagnostics.Process[]]
$process = $null
)
{
Begin
{
[int] $counter = 0
#We cannot access input parameter here!
Write-Verbose ('Begin')
}
Process
{
Write-Verbose ('Process $process = {0}, ID = {1}' -f $process.Name, $process.Id)
$process.Kill()
$counter ++
}
End
{
Write-Verbose ('End')
Write-Verbose "Killed [$counter] processes!"
}
}
#Invoke 3 NotePad's so that we can see all three getting killed!
Invoke-Expression "Notepad.exe"
Invoke-Expression "Notepad.exe"
Invoke-Expression "Notepad.exe"
#Kill all notepads using our new pipelined function
Get-Process -Name notepad | Stop-Process-Jana -Verbose
#Up to now, we saw multiple inputs ....but a single output
#
# What if we wanted a separate output for each of the inputs?
#
#Pipelined Functions - More than 1 pipelined input parameter and separate output for each input
#---------------
FUNCTION Get-Sum-EachPair(
[Parameter(ValueFromPipeline=$True, ValueFromPipelineByPropertyName=$true)]
[int]$a = 0,
[Parameter(ValueFromPipeline=$True, ValueFromPipelineByPropertyName=$true)]
[int]$b = 0
)
{
Begin
{
[int[]] $sums = @()
Write-Verbose ('Begin $a = {0}, $b = {1}' -f $a, $b)
}
Process
{
$sum = $a + $b
Write-Verbose ('Process $a = {0}, $b = {1}' -f $a, $b)
$sums += $sum
}
End
{
Write-Verbose ('End $a = {0}, $b = {1}' -f $a, $b)
$sums
}
}
#What is expected here?
1,2 | Get-Sum-EachPair -Verbose
#This is wrong on so many fronts
# 1. This is an array - we are trying to pass it to two input parameters
# (1,2).GetType()
# 2. Each input value from the pipeline gets sent to every pipelined parameter
# 3. Results will be unpredictable
# Combine this with pipeline + non-pipeline input + defaults + parameter sets!
#So, how is it done?
#Custom objects!
#Input Pair1 - Sum values in this pair
#
$pair1 = New-Object -TypeName PSObject
$pair1 | Add-Member -MemberType NoteProperty -Name a -Value 1
$pair1 | Add-Member -MemberType NoteProperty -Name b -Value 2
#How does $pair1 look?
$pair1 | ft -auto
#Get the sum of values
$pair1 | Get-Sum-EachPair -Verbose
#Input Pair2
#
$pair2 = New-Object -TypeName PSObject
$pair2 | Add-Member -MemberType NoteProperty -Name a -Value 3
$pair2 | Add-Member -MemberType NoteProperty -Name b -Value 4
#Make an array with Pair1 and Pair2
"`$pair1 = $pair1"
"`$pair2 = $pair2"
$pairs = @($pair1, $pair2)
#Did you notice above? A mix of pipelined input and non-pipelined input in the same line?
#$pair2 | Add-Member -MemberType NoteProperty -Name a -Value 3
#Above, what is implied in the call to Add-Member is "-InputObject $pair2". Same as above is below:
#Add-Member -InputObject $pair2 -MemberType NoteProperty -Name a -Value 3
#Get the sum of values of two pairs that are in $pairs
$pairs | Get-Sum-EachPair -Verbose
#Should return 4 results
@($pair1, $pair2, $pair1, $pair2) | Get-Sum-EachPair -Verbose
#Scroll up to the function and notice
# 1. The additional parameter decoration "ValueFromPipelineByPropertyName"
# 2. We are collecting results into an array in "Process"
# 3. We just output what was already done, in "End"
#Pipelined Functions - Greater than 1 pipelined parameter, separate output for each input + non-pipelined input parameter
#---------------
FUNCTION Get-Sum-EachPair(
[Parameter(ValueFromPipeline=$True, ValueFromPipelineByPropertyName=$true)]
[int]$a = 0,
[Parameter(ValueFromPipeline=$True, ValueFromPipelineByPropertyName=$true)]
[int]$b = 0,
#This is a non-piplelined input parameter
[Parameter(ValueFromPipeline=$False, Mandatory=$false)]
[bool]$WriteOutputAsCSV = $True
)
{
Begin
{
[int[]] $sums = @()
Write-Verbose ('Begin $a = {0}, $b = {1}' -f $a, $b)
}
Process
{
$sum = $a + $b
Write-Verbose ('Process $a = {0}, $b = {1}' -f $a, $b)
$sums += $sum
}
End
{
Write-Verbose ('End $a = {0}, $b = {1}' -f $a, $b)
$sums
if ($WriteOutputAsCSV -eq $True)
{
"Sums (from non-pipelined parameter input!) : {0}" -f ($sums -join ',')
}
}
}
#Usage examples
Get-Sum-EachPair
@($pair1, $pair2, $pair1, $pair2) | Get-Sum-EachPair -WriteOutputAsCSV: $true
#PowerShell automatic variable - $input
#---------------------------------------
# Example is a variation of what I found online
FUNCTION Make-HTML
(
[string] $FileName
)
{
# process the input from pipeline using automatic variable $input
$input | ConvertTo-HTML > $FileName
}
$file = 'c:\Temp\Test.html'
Get-Process | Make-HTML -FileName $file
Invoke-Item $file
#Remove-Item $file
#Notice the the input came from the pipeline into the function via automatic variable $input!
#Above is "hiding" the usage of $input automatic variable...good idea to be explicit
FUNCTION Make-HTML
(
[string] $FileName,
[object[]] $InputObject
)
{
#If input is coming from the pipeline, use that, else set $input to the value passed in as parameter explicitly
if ($InputObject) { $input = $InputObject }
# process the input from pipeline using automatic variable $input
$input | ConvertTo-HTML > $FileName
}
#Usage variation 1 - Using pipelined method
$file = 'c:\Temp\Test.html'
Get-Process | Make-HTML -FileName $file
Invoke-Expression $file
#Remove-Item $file
#Usage variation 2 - Using non-pipelined method
$file = 'c:\Temp\Test.html'
Make-HTML -FileName $file -InputObject (Get-Process)
Invoke-Expression $file
#Remove-Item $file
#WARNING: $input is not an array but rather an enumerator
# ..which only moves forward on each access and does not reset itself
# ...so, one can get weird results if iterating through multiple times.
#https://dmitrysotnikov.wordpress.com/2008/11/26/input-gotchas/