Windows PowerShell integrates with the Microsoft .NET Framework and provides an environment to perform administrative tasks by execution of cmdlets (pronounced commandlets) which are specialized .NET classes implementing a particular operation, scripts which are composition of cmdlets along with imperative logic, executables which are standalone applications, or by instantiating regular .NET classes.[4][5] These work by accessing data in different data stores, like the filesystem or registry, which are made available to the PowerShell runtime via Windows PowerShell providers.
Windows PowerShell also provides a hosting mechanism with which the Windows PowerShell runtime can be embedded inside other applications, which can then leverage Windows PowerShell functionality to implement certain operations, including those exposed via the graphical interface. This capability has been utilized by Microsoft Exchange Server 2007[4][6] to expose its management functionality as PowerShell cmdlets and providers and implement the graphical management tools as PowerShell hosts which invoke the necessary cmdlets. Other Microsoft applications including Microsoft SQL Server 2008[7] also expose their management interface via PowerShell cmdlets. In the future, graphical interface-based management applications on Windows will be layered on top of Windows PowerShell.
Windows PowerShell includes its own extensive, console-based help, reminiscent of man pages in Unix shells. The help topics include help for cmdlets, providers, and concepts in PowerShell. To see the help, use the Get-Help cmdlet. Online help for Windows PowerShell is updated weekly in the TechNet Library.
Background
Every released version of Microsoft DOS and Microsoft Windows for personal computers has featured a command-line interface tool. These are COMMAND.COM (in installations relying on MS-DOS, including Windows 9x) and cmd.exe (in Windows NT-based installations). These are regular command line interpreters which include only a handful of basic commands. For other purposes, a separate console application needs to be provided, to be invoked from these shells. They also include a scripting language (batch files) which can be used to automate various tasks. However, they cannot be used to automate all facets of GUI functionality, in part because command-line equivalents of operations exposed via the graphical interface are limited, and the scripting language is elementary, preventing the creation of complex scripts by composing available functionality. In Windows Server 2003, the situation was improved,[8] but scripting support was still considered unsatisfactory.
Microsoft attempted to address some of these shortcomings by introducing the Windows Script Host in 1998 with Windows 98, and its command-line based host: cscript.exe . It integrates with the Active Script engine and allows scripts to be written in compatible languages, such as JScript and VBScript, leveraging the APIs exposed by applications via COM. However, it has its own deficiencies, as well. It is not integrated with the shell, its documentation not very accessible, and it quickly gained a reputation as a system vulnerability vector after several high-profile computer viruses exploited weaknesses in its security provisions. Different versions of Windows provided various special-purpose command line interpreters (such as netsh and WMIC) with their own command sets. None of them were integrated with the command shell, nor were they interoperable.
By 2003 Microsoft had started to develop a new shell called Monad (aka Microsoft Shell or MSH). Monad was to be a new extensible command shell with a fresh design which would be capable of automating a full range of core administrative tasks. Microsoft published the first Monad public beta release on June 172005, Beta 2 on September 112005 and Beta 3 on January 102006. They announced on April 252006 that Monad was renamed to Windows PowerShell, positioning it as a significant part of their management technology offerings.[9] Release Candidate 1 of PowerShell was released at the same time.
Release Candidate 2 of PowerShell was released September 262006 and released to web (RTW) on November 142006. PowerShell for Vista was released on January 302007.[10] A CTP release of Windows PowerShell v2.0 has been made available since November 62007.[11]
Overview
PowerShell 1.0
The commands Windows PowerShell executes may be in the form of 'cmdlets', which are specialized .NETclasses designed expressly to expose a functionality via PowerShell, PowerShell scripts (*.ps1) or regular executables. If a command is an executable file, PowerShell 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 parameter value to the parameter is done by PowerShell itself, but for external executables, parameters 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. A pipeline is set up by piping the output of one command (or pipeline) to another command, using the | operator. But unlike its Unix counterpart, the PowerShell pipeline is an object pipeline; that is, the data passed between cmdlets are fully typedobjects, rather than byte 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 byte 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.[12][13] For the last cmdlet in a pipeline, PowerShell automatically pipes its output object to the Write-Host cmdlet, which creates a formatted text representation of its data, writing it to the screen.[14][15]
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 certain extensions. 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 an XML-based language.[16]
Cmdlets
Cmdlets are specialized commands in the PowerShell environment that implement specific functions. These are the native commands in the PowerShell stack. Cmdlets follow a <verb>-<noun> naming pattern, such as Get-ChildItem, helping to make them (according to Microsoft) self-descriptive.[17] Cmdlets output their results as objects, or collections thereof (including arrays), and can optionally receive input in that form, making them suitable for use as recipients in a pipeline. But while PowerShell allows arrays and other collections of objects to be written to the pipeline, cmdlets always process objects individually. For collections of objects, PowerShell invokes the cmdlet on each object in the collection, in sequence.[17]
Cmdlets are specialized .NET classes, which the PowerShell runtime instantiates and invokes when they are run. Cmdlets derive either from Cmdlet or from PSCmdlet, the latter being used when the cmdlet needs to interact with the PowerShell runtime.[17] These base classes specify certain methods - BeginProcessing(), ProcessRecord() and EndProcessing() - one of which the cmdlet's implementation overrides to provide the functionality. Whenever a cmdlet is run, these methods are invoked by PowerShell in sequence, with ProcessRecord() being called if it receives pipeline input.[18] If a collection of objects are piped, the method is invoked for each object in the collection. The class implementing the Cmdlet must have one .NET attribute - CmdletAttribute - which specifies the verb and the noun that make up the name of the cmdlet. Common verbs are provided as an enum.
If a cmdlet receives either pipeline input or command-line parameter input, there must be a corresponding property in the class, with a mutator implementation. PowerShell invokes the mutator with the parameter value or pipeline input, which is saved by the mutator implementation in class variables. These values are then referred to by the methods which implement the functionality. Properties that map to command-line parameters are marked by ParameterAttribute[19] and are set before the call to BeginProcessing(). Those which map to pipeline input are also flanked by ParameterAttribute, but with the ValueFromPipeline attribute parameter set.[20]
The implementation of these cmdlet classes can refer to any .NETAPI and may be in any .NET language. In addition, PowerShell makes certain APIs available, such as WriteObject(), which is used to access PowerShell-specific functionality, such as writing resultant objects to the pipeline. Cmdlets can use .NET data access APIs directly or use the PowerShell infrastructure of PowerShell Providers, which make data stores addressable using unique paths. Data stores are exposed using drive letters, and hierarchies within them, addressed as directories. Windows PowerShell ships with providers for the file system, registry, the certificate store, as well as the namespaces for command aliases, variables, and functions.[21] Windows PowerShell also includes various cmdlets for managing various Windows systems, including the file system, or using Windows Management Instrumentation to control Windows components. Other applications can register cmdlets with PowerShell, thus allowing it to manage them, and if they enclose any datastore (such as databases), they can add specific providers as well.
Scripting
Windows PowerShell includes a dynamically typedscripting language which can implement complex operations using cmdlets imperatively. The scripting language supports variables, functions, branching (if-then-else), loops (while, do, for, and foreach), structured error/exception handling and closures/lambda expressions, as well as integration with .NET. Variables in PowerShell scripts have names that start with $; they can be assigned any value, including the output of cmdlets. While the language is untyped, internally the variables are stored with their types, which can be either primitive types or objects. Strings can be enclosed either in single quotes or in double quotes; in the former case, variables will be expanded even if they are inside the quotation marks. According to the variable syntax, if the path to a file is enclosed in braces preceded by a dollar sign (as in ${C:\foo.txt}), it refers to the contents of the file. If it is used as an L-value, anything assigned to it will be written to the file. When used as an R-value, it will be read from the file. If an object is assigned, it is serialized before storing it.
Object members can be accessed using . notation, as in C# syntax. PowerShell provides special variables, such as $args, which is an array of all the command line arguments passed to a function from the command line, and $_, which refers to the current object in the pipeline.[22] PowerShell also provides arrays and associative arrays. The PowerShell scripting language also evaluates arithmetic expressions entered on the command line immediately, and it parses common abbreviations, such as GB, MB, and KB.
Using the function keyword, PowerShell provides for the creation of functions, which can take parameters. It provides two syntaxes for invoking a function:
<function> <param1> <param2> …': In this case, the function need not define the parameters it accepts; the parameters will be passed as an array accessible from inside the function via the $args array.[23]
<function>(<param1>, <param2>): In this case, the function declaration must specify the parameters it accepts, and their types. The parameters passed will be bound to the parameter variables when the function is called.[24]
PowerShell allows any .NET methods to be called by providing their namespaces enclosed in brackets ([]), and then using a pair of colons (::) to indicate the static method.[25] For example,
[System.Console]::WriteLine("PowerShell")
Objects are created using the New-Object cmdlet. Calling methods of .NET objects is accomplished by using the regular . notation.[25]
For error handling, PowerShell provides a .NET-based exception handling mechanism. In case of errors, objects containing information about the error (Exception object) are thrown, which are caught using the trap keyword. However, the action-or-error is configurable; in case of an error PowerShell can be configured to silently resume execution, without trapping the exception.[26]
Scripts written using PowerShell can be made to persist across sessions in a .ps1 file. Later, either the entire script or individual functions in the script can be used. Scripts and functions are used analogously with cmdlets, in that they can be used as commands in pipelines, and parameters can be bound to them. Pipeline objects can be passed between functions, scripts, and cmdlets seamlessly. However, script execution is disabled by default and must be enabled explicitly.[27] PowerShell scripts can be signed to verify their integrity, and are subject to .NET Code Access Security.
The PowerShell scripting language supports binary prefix notation similar to the scientific notation supported by many programming languages in the C-family.
Hosting
Another use of PowerShell is being embedded in a management application, which then uses the PowerShell runtime to implement the management functionality. For this, PowerShell provides a managed hosting API. Via the APIs, the application can instantiate a runspace (one instantiation of the PowerShell runtime), which runs in the application's process and is exposed as a Runspace object.[4] The state of the runspace is encased in a SessionState object. When the runspace is created, the Windows PowerShell runtime initializes the instantiation, including initializing the providers and enumerating the cmdlets, and updates the SessionState object accordingly. The Runspace then must be opened for either synchronous processing or asynchronous processing. After that it can be used to execute commands.
To execute a command, a pipeline (represented by a Pipeline object) must be created and associated with the runspace. The pipeline object is then populated with the cmdlets that make up the pipeline. For sequential operations (as in a PowerShell script), a Pipeline object is created for each statement and nested inside another Pipeline object.[4] When a pipeline is created, Windows PowerShell invokes the pipeline processor, which resolves the cmdlets into their respective assemblies (the command processor) and adds a reference to them to the pipeline, and associates them with an InputPipe, Outputpipe and ErrorOutputPipe objects, to represent the connection with the pipeline. The types are verified and parameters bound using reflection.[4] Once the pipeline is set up, the host calls the Invoke() method to run the commands, or its asynchronous equivalent - InvokeAsync(). If the pipeline has the Write-Host cmdlet at the end of the pipeline, it writes the result onto the console screen. If not, the results are handed over to the host, which might either apply further processing or display it itself.
The hosting APIs are used by Microsoft Exchange Server 2007 to provide its management GUI. Each operation exposed in the GUI is mapped to a sequence of PowerShell commands (or pipelines). The host creates the pipeline and executes them. In fact, the interactive PowerShell console itself is a PowerShell host, which interprets the scripts entered at command line and creates the necessary Pipeline objects and invokes them.
Graphical Windows PowerShell, with multiple open RunSpaces in the Windows PowerShell 2.0 CTP2
Microsoft is working on the next version of PowerShell and has made several CTP releases of the same publicly available. The most recent CTP, CTP3, was issued in late 2008[28]. Windows PowerShell 2.0 is installed by default on Windows Server 2008 R2 and Windows 7[29].
PowerShell V2 includes changes to the scripting language and hosting API, in addition to including more than 240 new cmdlets.[30][31]
A non-exhaustive list of the new features included in PowerShell V2 is:[32][33]
PowerShell Remoting: Using WS-Management, PowerShell 2.0 allows scripts and cmdlets to be invoked on a remote machine or a large set of remote machines.
Background Jobs: Also called a PSJob, it allows a command sequence (script) or pipeline to be invoked asynchronously. Jobs can be run on the local machine or on multiple remote machines. A PSJob cannot include interactive cmdlets.
Transactions: Enable cmdlet and provider developers to perform transacted operations. PowerShell 2.0 includes transaction cmdlets for starting, committing, and rolling back a PSTransaction as well as features to manage and direct the transaction to the participating cmdlet and provider operations. The PowerShell Registry provider supports transactions.
ScriptCmdlets: These are cmdlets written using the PowerShell scripting language.
SteppablePipelines: This allows the user to control when the BeginProcessing(), ProcessRecord() and EndProcessing() functions of a cmdlet are called.
Modules: This allows script developers and administrators to organize and partition PowerShell scripts in self-contained, reusable units. Code from a module executes in its own self-contained context and does not affect the state outside of the module. Modules can define a restricted runspace environment by using a script. They have a persistent state as well as public and private members.
Data Language: A domain-specific subset of the PowerShell scripting language, that allows data definitions to be decoupled from the scripts and allow localized string resources to be imported into the script at runtime (Script Internationalization).
Script Debugging: It allows breakpoints to be set in a PowerShell script or function. Breakpoints can be set on lines, line & columns, commands and read or write access of variables. It includes a set of cmdlets to control the breakpoints via script.
Eventing: This feature allows listening, forwarding, and acting on management and system events. Eventing allows PowerShell Hosts to be notified about state changes to their managed entities. It also enables PowerShell scripts to subscribe to ObjectEvents, PSEvents, and WmiEvents and process them synchronously and asynchronously.
Windows PowerShell Integrated Scripting Environment: PowerShell 2.0 includes a GUI-based PowerShell host (formerly known as Graphical Windows PowerShell) that provides integrated debugger, syntax highlighting, tab completion and up to 8 PowerShell unicode-enabled consoles (Runspaces) in a tabbed UI, as well as to run only the selected parts in a script.
Network File Transfer: Native support for prioritized, throttled, and asynchronous transfer of files between machines using the Background Intelligent Transfer Service (BITS).[34]
New Cmdlets: Including Out-GridView, which displays tabular data in the WPFGridView object.
New Operators: -Split, -Join, and Splatting (@) operators.
Exception Handling with Try-Catch-Finally: Unlike other .NET languages, this allows multiple exception types for a single catch block.
Nestable Here-Strings: PowerShell Here-Strings have been improved and can now nest.[35]
New APIs: The new APIs range from handing more control over the PowerShell parser and runtime to the host, to creating and managing collection of Runspaces (RunspacePools) as well as the ability to create Restricted Runspaces which only allow a configured subset of PowerShell to be invoked. The new APIs also support participation in a Windows PowerShell managed transaction.
Cmdlets
The following table contains a selection of the more than 129 Cmdlets that ship with PowerShell as well as the equivalent commands in other command line interpreters.
For administrators, PowerShell provides efficient, powerful access to key indicators of the operating system. […] For developers, PowerShell is helpful in SQL scripting or developing DLLs. The concepts and cmdlets of PowerShell offer new ways to work with old information. […] PowerShell is flexible, extensible, and easily integrated.[40]
Windows has never enjoyed the powerful shell scripting environments that its Unix rivals have long included. That's changed now with the inclusion of PowerShell […] in Windows Server 2008 and a passel of other Microsoft server products as well. Finally, Windows has a classy, robust and powerful solution that can access just about every part of the operating system.[41]
Unlike the CMD shell's meager batch language, PowerShell's scripting language supports a large variety of flow-control elements, type casting, functions, full-featured arrays, object reference variables, default variables in flow-control blocks, regular expressions, and other constructs often associated only with UNIX shells like bash and ksh. At first glance, the PowerShell language looks somewhat similar to Perl or C#, but it's not even half as difficult to learn.[42]
In his Windows Server 2008 review technology reporter and editor of the popular SuperSite for WindowsPaul Thurrott states:
PowerShell is a complex but technically impressive environment, with support for discoverable .NET-based objects, properties, and methods. It provides all of the power of UNIX command line environments with none of the inconsistencies.[43]
According to Microsoft, PowerShell has had almost a million downloads in its first six months.[44]
There is also a growing list of products that are shipping with PowerShell support.
Examples
Examples are provided first using the long-form canonical syntax and then using more terse UNIX-like and DOS-like aliases that are set up in the default configuration. Examples that could harm a system include the -whatif parameter to prevent them from actually executing
Stop all processes that begin with the letter "p":
PS> get-process p* | stop-process -whatif
PS> ps p* | kill -whatif
Find the processes that use more than 1000 MB of memory and kill them:
Sense/Net 6.0 A currently beta open source implementation of a .NET based Enterprise Content Management System accessible from PowerShell, information availbale from the Sense/Net development blog
Tyson Kopczynski, Pete Handley, Marco Shaw. Windows PowerShell Unleashed. ISBN 978-0672329883. {{cite book}}: Cite has empty unknown parameter: |subtitle= (help)CS1 maint: multiple names: authors list (link)
Windows PowerShell and the “PowerShell Worm” - an analysis, by Leonard Chung of the Windows PowerShell Team, of the "PowerShell Worm" and a walkthrough of Windows PowerShell's security features and how they strongly reduce the risk of scripting malware.
3rd-party tools
sudo for Powershell - run specific commands as an administrator on Windows Vista
