Windows PowerShell - Overview

Overview

There are four kinds of commands Windows PowerShell can execute:

• cmdlets, which are .NET programs designed to interact with PowerShell
• PowerShell scripts (files suffixed by .ps1)
• PowerShell functions
• standalone executable programs

If a command is a standalone executable program, PowerShell.exe launches it in a separate process; if it is a cmdlet, it is executed in the PowerShell process. PowerShell also provides an interactive command line interface, wherein the commands can be entered and their output displayed. The user interface, based on the Win32 console, offers customizable tab completion but lacks syntax highlighting. PowerShell also enables the creation of aliases for cmdlets, which are textually translated by PowerShell into invocations of the original commands. PowerShell also supports both named and positional parameters for commands. In executing a cmdlet, the job of binding the argument value to the parameter is done by PowerShell itself, but, for external executables, arguments are passed via the argv (or equivalent) variable array to be parsed by the executable.

Another concept used by PowerShell is that of a pipeline. Like Unix pipelines, PowerShell pipelines are used to compose complex commands, allowing the output of one command to be passed as input to another, using the | operator. But unlike its Unix counterpart, the PowerShell pipeline is an object pipeline; that is, the data passed between cmdlets are fully typed objects, rather than character streams. When data is piped as objects, the elements they encapsulate retain their structure and types across cmdlets, without the need for any serialization or explicit parsing of the stream, as would be the need if only character streams were shared. An object can also encapsulate certain functions that work on the contained data. These also become available to the recipient command for use. For the last cmdlet in a pipeline, PowerShell automatically pipes its output object to the Out-Default cmdlet, which transforms the objects into a stream of format objects and then renders those to the screen.

Because all PowerShell objects are .NET objects, they share a .ToString method, which retrieves the text representation of the data in an object. Windows PowerShell uses this method to convert an object to text. In addition, it also allows formatting definitions to be specified, so the text representation of objects may be customized by choosing which data elements to display, and how. However, in order to maintain backwards compatibility, if an external executable is used in a pipeline, it receives a text stream representing the object, and does not integrate with the PowerShell type system.

The PowerShell Extended Type System (ETS) is based on the .NET type system, but with extended semantics (for example, propertySets and third-party extensibility). For example, it enables the creation of different views of objects by exposing only a subset of the data fields, properties, and methods, as well as specifying custom formatting and sorting behavior. These views are mapped to the original object using XML-based configuration files.