What is the recommended coding style for PowerShell?

Question

Is there any recommended coding style how to write PowerShell scripts? It's not about how to structure the code (how many functions, if to use module, ...). It's about 'how to write the code so that it is readable'.

In programming languages there are some recommended coding styles (what to indent, how to indent - spaces/tabs, where to make new line, where to put braces,...), but I haven't seen any suggestion for PowerShell.

What I'm interested particularly in:

---

How to write parameters

function New-XYZItem
  ( [string] $ItemName
  , [scriptblock] $definition
  ) { ...

(I see that it's more like 'V1' syntax) or

function New-PSClass  {
  param([string] $ClassName
       ,[scriptblock] $definition
  )...

or (why to add empty attribute?)

function New-PSClass  {
  param([Parameter()][string] $ClassName
       ,[Parameter()][scriptblock] $definition
  )...

or (other formatting I saw maybe in Jaykul's code)

function New-PSClass {
  param(
        [Parameter()]
        [string]
        $ClassName
        ,
        [Parameter()]
        [scriptblock]
        $definition
  )...

or ..?

---

How to write complex pipeline

Get-SomeData -param1 abc -param2 xyz | % {
    $temp1 = $_
    1..100 | % { 
      Process-somehow $temp1 $_
    }
  } | % { 
    Process-Again $_
  } | 
  Sort-Object -desc

or (name of cmdlet on new line)

Get-SomeData -param1 abc -param2 xyz | 
  % {
    $temp1 = $_
    1..100 | 
      % { 
        Process-somehow $temp1 $_
      }
  } | 
  % { 
    Process-Again $_
  } | 
  Sort-Object -desc |

and what if there are -begin -process -end params? how to make it the most readable?

Get-SomeData -param1 abc -param2 xyz | 
  % -begin {
     init
  } -process {
     Process-somehow2 ...
  } -end {
     Process-somehow3 ...
  } |
  % -begin {
  } ....

or

Get-SomeData -param1 abc -param2 xyz | 
  %  `
    -begin {
      init
    } `
    -process {
      Process-somehow2 ...
    } `
    -end {
      Process-somehow3 ...
    } |
  % -begin {
  } ....

the indentitation is important here and what element is put on new line as well.

---

I have covered only questions that come on my mind very frequently. There are some others, but I'd like to keep this SO question 'short'.

Any other suggestions are welcome.

Answer 1

After spending a couple years diving pretty deep into Powershell v2.0, here's what I've settled on:

<#
.SYNOPSIS
Cmdlet help is awesome.  Autogenerate via template so I never forget.

.DESCRIPTION
.PARAMETER
.PARAMETER
.INPUTS
.OUTPUTS
.EXAMPLE
.EXAMPLE
.LINK
#>
function Get-Widget
{
    [CmdletBinding()]
    param (
        # Think about which params users might loop over.  If there is a clear
        # favorite (80/20 rule), make it ValueFromPipeline and name it InputObject.
        [parameter(ValueFromPipeline=$True)]
        [alias("Server")]
        [string]$InputObject,

        # All other loop candidates are marked pipeline-able by property name.  Use Aliases to ensure the most 
        # common objects users want to feed in will "just work".
        [parameter(Mandatory=$true, Position=0, ValueFromPipelineByPropertyName=$True)]
        [alias("FullName")]
        [alias("Path")]
        [string[]]$Name,

        # Provide & document defaults for optional params whenever possible.
        [parameter(Position=1)]
        [int]$Minimum = 0,

        [parameter(Position=2)]
        [int]$ComputerName = "localhost",

        # Stick to standardized parameter names when possible.  *Especially* with switches.  Use Aliases to support 
        # domain-specific terminology and/or when you want to expose the parameter name of the .Net API you're wrapping.
        [parameter()]
        [Alias("IncludeFlibbles")]
        [switch]$All,
    )

    # The three main function blocks use this format if & only if they are short one-liners    
    begin { $buf = new-list string }

    # Otherwise they use spacing comparable to a C# method
    process    
    {
        # Likewise, control flow statements have a special style for one-liners
        try
        {
            # Side Note: internal variables (which may be inherited from a parent scope)  
            # are lowerCamelCase.  Direct parameters are UpperCamelCase.
            if ($All)
                { $flibbles = $Name | Get-Flibble }   
            elseif ($Minimum -eq 0)          
                { $flibbles = @() }
            else
                { return }                       

            $path = $Name |
                ? { $_.Length -gt $Minimum } |
                % { $InputObject.InvokeGetAPI($_, $flibbles) } |
                ConvertTo-FullPath
        }
        finally { Cleanup }

        # In general, though, control flow statements also stick to the C# style guidelines
        while($true)
        {
            Do-Something
            if ($true)
            {
                try
                {
                    Do-Something
                    Do-Something
                    $buf.Add("abc")
                }
                catch
                {
                    Do-Something
                    Do-Something
                }
            }            
        }    
    }    
}

<# 
Pipelines are a form of control flow, of course, and in my opinion the most important.  Let's go 
into more detail.

I find my code looks more consistent when I use the pipeline to nudge all of Powershell's supported 
language constructs (within reason) toward an "infix" style, regardless of their legacy origin.  At the 
same time, I get really strict about avoiding complexity within each line.  My style encourages a long,
consistent "flow" of command-to-command-to-command, so we can ensure ample whitespace while remaining
quite compact for a .Net language. 

Note - from here on out I use aliases for the most common pipeline-aware cmdlets in my stable of 
tools.  Quick extract from my "meta-script" module definition:
sal ?? Invoke-Coalescing
sal ?: Invoke-Ternary
sal im Invoke-Method
sal gpv Get-PropertyValue
sal spv Set-PropertyValue
sal tp Test-Path2
sal so Select-Object2        
sal eo Expand-Object        

% and ? are your familiar friends.
Anything else that begins with a ? is a pseudo-infix operator autogenerated from the Posh syntax reference.
#>        
function PipelineExamples
{
    # Only the very simplest pipes get to be one-liners:
    $profileInfo = dir $profile | so @{Path="fullname"; KBs={$_.length/1kb}}
    $notNull = $someString | ?? ""        
    $type = $InputObject -is [Type] | ?: $InputObject $InputObject.GetType()        
    $ComObject | spv Enabled $true
    $foo | im PrivateAPI($param1, $param2)
    if ($path | tp -Unc)
        { Do-Something }

    # Any time the LHS is a collection (i.e. we're going to loop), the pipe character ends the line, even 
    # when the expression looks simple.
    $verySlowConcat = ""            
    $buf |
        % { $verySlowConcat += $_ }
    # Always put a comment on pipelines that have uncaptured output [destined for the caller's pipeline]
    $buf |
        ? { $_ -like "*a*" }


    # Multi-line blocks inside a pipeline:
    $orders |
        ? { 
            $_.SaleDate -gt $thisQuarter -and
            ($_ | Get-Customer | Test-Profitable) -and
            $_.TastesGreat -and
            $_.LessFilling
        } |
        so Widgets |        
        % {                
            if ($ReviewCompetition)
            {
                $otherFirms |
                    Get-Factory |
                    Get-ManufactureHistory -Filter $_ |
                    so HistoryEntry.Items.Widgets                     
            }
            else
            {
                $_
            }
        } |            
        Publish-WidgetReport -Format HTML


    # Mix COM, reflection, native commands, etc seamlessly
    $flibble = Get-WmiObject SomethingReallyOpaque |
        spv AuthFlags 0xf -PassThru |
        im Put() -PassThru |
        gpv Flibbles |
        select -first 1

    # The coalescing operator is particularly well suited to this sort of thing
    $initializeMe = $OptionalParam |
        ?? $MandatoryParam.PropertyThatMightBeNullOrEmpty |
        ?? { pwd | Get-Something -Mode Expensive } |
        ?? { throw "Unable to determine your blahblah" }           
    $uncFolderPath = $someInput |
        Convert-Path -ea 0 |
        ?? $fallback { tp -Unc -Folder }

    # String manipulation        
    $myName = "First{0}   Last{1}   " |
        ?+ "Suffix{2}" |
        ?replace "{", ": {" |
        ?f {eo richard berg jr | im ToUpper}            

    # Math algorithms written in this style start to approach the elegance of functional languages
    $weightedAvg = $values |
        Linq-Zip $weights {$args[0] * $args[1]} |
        Linq-Sum |
        ?/ ($weights | Linq-Sum)
}

# Don't be afraid to define helper functions.  Thanks to the script:Name syntax, you don't have to cram them into 
# the begin{} block or anything like that.  Name, params, etc don't always need to follow the cmdlet guidelines.
# Note that variables from outer scopes are automatically available.  (even if we're in another file!)
function script:Cleanup { $buf.Clear() }

# In these small helpers where the logic is straightforward and the correct behavior well known, I occasionally 
# condense the indentation to something in between the "one liner" and "Microsoft C# guideline" styles
filter script:FixComputerName
{
    if ($ComputerName -and $_) {            
        # handle UNC paths 
        if ($_[1] -eq "\") {   
            $uncHost = ($_ -split "\\")[2]
            $_.Replace($uncHost, $ComputerName)
        } else {
            $drive = $_[0]
            $pathUnderDrive = $_.Remove(0,3)            
            "\\$ComputerName\$drive`$\$pathUnderDrive"
        }
    } else {
        $_
    }
}

Oops, that got longer than I expected. Hope the answers you wanted are in there somewhere :)

edit - StackOverflow's syntax highlighter is giving up on me completely. Paste it into the ISE.