Windows PowerShell

Windows PowerShell, previously Microsoft Shell or MSH (codenamed Monad) is a command line interface (CLI) shell and scripting language product being developed by Microsoft. The interface is similar to Unix shells, but the product is based on object-oriented programming and the Microsoft .NET framework, and is highly extensible.

Microsoft originally intended to launch PowerShell along with Windows Vista, but later defined a separate release schedule for PowerShell. Microsoft has stated that it will be included as part of a general availability release which is scheduled for release in the second half of 2006. Windows PowerShell is the foundation of the administrative experience for Microsoft Exchange Server 2007 where everything is done via command line interfaces and the administrative GUI is layered on top of those commands.

PowerShell requires version 2.0 of the .NET Framework, and is supported on Windows XP, Windows Server 2003, Windows Vista, and Windows Server "Longhorn", and is available in x86, x64 and Itanium editions.

History

Every released version of Microsoft DOS and Microsoft Windows for personal computers has featured a command line interface tool. These have been command.com (in installations relying on MS-DOS) and cmd.exe (in Windows NT-based installations). These were not always capable of reproducing or automating all the administrative functions available in the graphical user interface (GUI), although under Windows Server 2003, this situation is improved. This has been due to a combination of limitations within the command-line equivalents or because Microsoft did not provide full fidelity command-line equivalents.

Additionally, for Unix admins, neither command.com nor cmd.exe are POSIX.2-conformant. Microsoft's Services for UNIX does include a POSIX.2-conformant shell pdksh but this and other Unix shells are still incapable of performing many routine tasks relegated to the Windows GUI. While the Services for UNIX suite is now built into Windows Server 2003 R2, and will be included with some editions of Windows Vista, it was previously a for-sale add-on, which provided a barrier to adoption.

Users have also been hindered by a lack of adequate documentation of command line functionality. By contrast, many GUI applications in Unix are only wrappers for documented command-line tools.

Microsoft attempted to address some of these shortcomings by introducing the Windows Script Host in 1998 with Windows 98. The Windows Script Host was a new scripting layer which could implement a variety of scripting languages to control applications. However, the Script Host proved unpopular because it was not integrated with the shell, its documentation was not very accessible, and it quickly gained a reputation as a system vulnerability after several high-profile computer viruses exploited substantial weaknesses in its security provisions.

Windows Server 2003 and certain versions of Windows XP included a command-line-based script host called Cscript.exe (essentially just a portion of the Windows Script Host introduced earlier), but it was not integrated into the existing shell (cmd.exe). These operating systems provided other ad hoc CLIs as well (netsh, for example), but they were not fully integrated, either.

Ultimately, the problem with Windows' automation was the fact that its native administrative interfaces were open to Microsoft's proprietary GUI-based tools, and any scriptable interfaces Microsoft chose to provide. The GUI was the primary interface, and it proved difficult to provide scriptable "wrappers;" this is exactly opposite most Unix implementations where the scriptable command-line provided the base functionality with any GUI simply wrapping around it.

Microsoft designed PowerShell to mitigate these problems — and to alleviate their onus of developing discrete GUI and CLI applications — by integrating a more secure scripting language into a new, extensible command shell capable of performing and automating the same core administrative tasks as graphical user interfaces typically have in Windows. Exchange Server 2007's management user interface, for example, is built on top of PowerShell. In most everyday tasks, PowerShell can be used in place of CMD.EXE and Windows Script Host (WSH). However, for backward-compatibility purposes both CMD.EXE and WSH will continue to be supported by Windows. Microsoft has committed to building future GUI tools on top of PowerShell, ensuring that the core administrative functionality remains scriptable, along the lines of a Unix operating system.

Microsoft published the second "Monad" public beta release on September 11 2005 and released Beta 3 on January 10 2006.

Microsoft announced on April 25 2006 that MSH was renamed to Windows PowerShell, positioning it as a significant part of their management technology offerings. Release Candidate 1 of PowerShell was released at the same time.

Central concepts

PowerShell's codename, Monad, comes from Gottfried Leibniz's monadology, a philosophy in which the universe is a composite of fundamental elements called Monads that are integrated in a "pre-established harmony". Similarly, PowerShell is a composite of the complex tasks of a series of components. The components are special programs called cmdlets (pronounced command lets), which are .NET classes designed to use the features of the environment. The key difference between the usual Unix approach and the PowerShell one is that rather than creating a "pipeline" based on textual input and output, MSH passes data between the various cmdlets as objects (structured data). Unix does not preclude the use of arbitrary streams of data being passed between command line utilities, but textual input and output is the primary and most popular model.

If accessed individually from the command line, a commandlet's output will automatically be converted into text, but if its output is to be used by another commandlet, it will be converted into whatever form of object is most appropriate for that commandlet's input. This has the advantage of eliminating the need for the many text-processing utilities which are common in Unix pipelines, such as grep and awk, as well as allowing things to be combined interactively, or in a scripting environment, which would otherwise require a more complex programming language. For instance, a listing of processes will consist not of text describing them, but objects representing them, so that methods can be called on those objects without explicit reference to any outside structure or library.

Features

Although still in development, a number of key features of PowerShell have already been revealed, including:

A C#-like scripting language with many power user features such as direct support for hashtables, switch statements that can case on regular expressions, array slicing and anonymous methods (script blocks) that can be stored as data and then later executed. That is in addition to the normal scripting language features such as looping (for/foreach/while), conditionals (if/switch), variable scoping (global/script/local/private) and the abililty to define functions.
Cmdlets all inherit certain options, allowing the user to choose things such as the level of interaction and how to deal with errors — including a "suspend" feature, which allows the user to enter a new command shell, investigate a problem, and then continue with the original command. A simple mechanism is built-in for the programmer to define the prompts to be shown in such circumstances. Cmdlets that produce side-effects support the options: -WHATIF and -CONFIRM. -WHATIF informs the user what would have happened but no action takes place. -CONFIRM informs the user what is about to happen and allows the user to control whether it takes place nor not.
An extensible provider model provides not only access to and manipulation of the file system but also other hierarchical stores. For instance, PowerShell comes with a registry provider that allows access to the registry via the "HKLM" and "HKCU" hives. With this, the registry can be browsed by executing commands like "dir HKLM:\SOFTWARE\Microsoft" at the shell prompt. PowerShell comes with providers for the certificate store as well as environment variables, shell variables, local certification store, functions and aliases. Like cmdlets, the provider model is extensible, allowing third parties to create their own provider model and snap it into PowerShell.
The command-line options are generally whole words, but can be specified as the minimum number of letters necessary (e.g. the option -show-detailed-information could be entered as -s if no other option began with 's')
Comprehensive, user-extendable, tab completion features. The cmd.exe shell in current versions of Windows can only complete file or directory names, in contrast to the advanced completion in shells such as bash and zsh.
The ability to assign the output of a command to a variable, which will then be an object or array of objects inspectable in any way desired.

References

Books

Monad - Introducing the MSH Command Shell and Language (the Toad book) by Andy Oakley, ISBN 0-596-10009-4
Windows PowerShell in Action (to be published on November 2006) by Bruce Payette, ISBN 1932394-90-7
Windows PowerShell™: TFM®, by Don Jones and Jeffery Hicks, ISBN 0-9776597-1-2

External links

Channel9 Monad Interviews/Demos with Jeffrey Snover, the Architect of Windows PowerShell
Monad: The Future of Windows Scripting - an article in TechNet Magazine by Thomas Lee.
Future Windows component could spur old-school script viruses - Monad's role in future security concerns, by Paul Roberts, IDG News Service
''Monad and the "First Vista Virus", a clarification by Lee Holmes, a developer on the Windows PowerShell Team
Scripting with Windows PowerShell
Windows PowerShell Team Blog

Windows PowerShell | Microsoft Command Shell