Powershell pitfalls

Question

What Powershell pitfalls you have fall into? :-)

Mine are:

# -----------------------------------
function foo()
{
    @(\"text\")
}

# Expected 1, a         


        

Answer 1

# $x is not defined
[70]: $x -lt 0
True
[71]: [int]$x -eq 0
True

So, what's $x..?

Answer 2

This one has tripped me up before, using $o.SomeProperty where it should be $($o.SomeProperty).

Answer 3

On Functions...

• The subtleties of processing pipeline input in a function with respect to using $_ or $input and with respect to the begin, process, and end blocks.
• How to handle the six principal equivalence classes of input delivered to a function (no input, null, empty string, scalar, list, list with null and/or empty) -- for both direct input and pipeline input -- and get what you expect.
• The correct calling syntax for sending multiple arguments to a function.

I discuss these points and more at length in my Simple-Talk.com article Down the Rabbit Hole- A Study in PowerShell Pipelines, Functions, and Parameters and also provide an accompanying wallchart--here is a glimpse showing the various calling syntax pitfalls for a function taking 3 arguments:

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On Modules...

These points are expounded upon in my Simple-Talk.com article Further Down the Rabbit Hole: PowerShell Modules and Encapsulation.

• Dot-sourcing a file inside a script using a relative path is relative to your current directory -- not the directory where the script resides! To be relative to the script use this function to locate your script directory: [Update for PowerShell V3+: Just use the builtin $PSScriptRoot variable!]

  function Get-ScriptDirectory
  { Split-Path $script:MyInvocation.MyCommand.Path }
  

• Modules must be stored as ...Modules\name\name.psm1 or ...\Modules\any_subpath\name\name.psm1. That is, you cannot just use ...Modules\name.psm1 -- the name of the immediate parent of the module must match the base name of the module. This chart shows the various failure modes when this rule is violated:

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2015.06.25 A Pitfall Reference Chart

Simple-Talk.com just published the last of my triumvirate of in-depth articles on PowerShell pitfalls. The first two parts are in the form of a quiz that helps you appreciate a select group of pitfalls; the last part is a wallchart (albeit it would need a rather high-ceilinged room) containing 36 of the most common pitfalls (some adapted from answers on this page), giving concrete examples and workarounds for most. Read more here.