Archives for the Month of December, 2007

PowerShell Cookbook Sample Chapter: Environmental Awareness

When O’Reilly put up the overview page for the Windows PowerShell Cookbook, the sample chapter was originally “Looping and Flow Control.” While a useful chapter in its own right, it didn’t really highlight the solution-oriented focus the book takes.

They’ve now updated the sample chapter to one much more representative: Environmental Awareness.

The goal of this chapter is to help you work with the global environment (environment variables and common system paths,) and script’s environment (its location, name, and invocation info.)

It covers:

14. Environmental Awareness
       14.0 Introduction  
       14.1 View and Modify Environment Variables  
       14.2 Access Information About Your Command's Invocation  
       14.3 Program: Investigate the InvocationInfo Variable  
       14.4 Find Your Script's Name  
       14.5 Find Your Script's Location  
       14.6 Find the Location of Common System Paths  
       14.7 Program: Search the Windows Start Menu  
       14.8 Get the Current Location  
       14.9 Safely Build File Paths Out of Their Components  
       14.10 Interact with PowerShell's Global Environment 

You can download it by following the sample chapter link at the book’s overview page.

PowerShell's -EQ Operator: Reference Equality vs Value Equality

A question recently came up asking why the -eq operator doesn't work for two different (but equal) SecureStrings.

 

By default, almost all environments (the .NET Framework included) test if two things are exactly the same – that they are stored in the same place in memory. This is called reference equality.

 

[D:\documents\WindowsPowerShell]

PS:14 > $test = Read-Host -AsSecureString

****

 

[D:\documents\WindowsPowerShell]

PS:15 > [Object]::ReferenceEquals($test, $test)

True

 

[D:\documents\WindowsPowerShell]

PS:16 > $test -eq $test

True

 

That’s not usually what people want, so each individual type of object is (optionally) responsible for supporting a value-based equality test. They do this by implementing interfaces (software contracts) called IComparable and / or IEquatable:

 

[D:\documents\WindowsPowerShell]

PS:17 > $string1 = "Test"

 

[D:\documents\WindowsPowerShell]

PS:18 > $string2 = "Test"

 

[D:\documents\WindowsPowerShell]

PS:19 > [Object]::ReferenceEquals($string1, $string2)

False

 

[D:\documents\WindowsPowerShell]

PS:20 > [String].GetInterfaces()

 

IsPublic IsSerial Name

-------- -------- ----

True     False    IComparable

True     False    ICloneable

True     False    IConvertible

True     False    IComparable`1

True     False    IEnumerable`1

True     False    IEnumerable

True     False    IEquatable`1

 

[D:\documents\WindowsPowerShell]

PS:21 > $string1 -eq $string2

True

 

 

The implementors of the SecureString class have chosen to not implement these contracts, so PowerShell supports only reference equality on these types:

 

[D:\documents\WindowsPowerShell]

PS:30 > [System.Security.SecureString].GetInterfaces()

 

IsPublic IsSerial Name

-------- -------- ----

True     False    IDisposable

 

[D:\documents\WindowsPowerShell]

PS:31 > $test1 = Read-Host -AsSecureString

****

 

[D:\documents\WindowsPowerShell]

PS:32 > $test2 = Read-Host -AsSecureString

****

 

[D:\documents\WindowsPowerShell]

PS:33 > $test1 -eq $test2

False