PowerShell

Windows PowerShell

File:Windows PowerShell icon.png
File:PowerShell.PNG Screenshot of a sample PowerShell session
Developer(s)	Microsoft Corporation
Stable release	1.0 / November 14, 2006
Preview release	2.0 CTP / November 6, 2007
Repository	github.com/PowerShell/PowerShell
Operating system	Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008
Platform	x86, x86-64 and Itanium
Available in	multilanguage
Type	Operating system shell
License	MS-EULA
Website	Windows PowerShell

Windows PowerShell (formerly codenamed Monad as a reference to the monadism philosophy of Leibnitz), is an extensible command line interface shell and associated scripting language from Microsoft. It was released in 2006 and is currently available for Windows XP SP2, Windows Server 2003 and Windows Vista. Furthermore, it is built into Windows Server 2008 as an optional feature.

PowerShell integrates with .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. These work by accessing data in different data stores, like filesystem or registry, which are made available to the PowerShell runtime via PowerShell providers.

PowerShell also provides a hosting mechanism with which the PowerShell runtime can be embedded inside other applications, which can then leverage PowerShell functionality to implement certain operations, including those exposed via the graphical interface. This capability has been utilized by Microsoft Exchange Server 2007 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 also expose their management interface via PowerShell cmdlets. Going forward, graphical interface-based management applications on Windows will be layered on top of PowerShell.

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 shells which include only a handful of basic commands. For other purposes, a separate console-based 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. While with Windows Server 2003, the situation was improved, 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 with Windows Server 2003. 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 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 17 2005, Beta 2 on September 11 2005 and Beta 3 on January 10 2006. They announced on April 25 2006 that Monad 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. Release Candidate 2 of PowerShell was released September 26 2006 and released to web (RTW) on November 14 2006. PowerShell for Vista was released on January 30 2007. A CTP release of Windows PowerShell v2.0 has been made available since November 6 2007.

Overview

PowerShell 1.0

Like all command shells, Windows PowerShell enables the execution of commands. The commands it executes may be in the form of cmdlets, which are specialized .NET classes designed expressly to expose a functionality via PowerShell, 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 command-line interface, based on cmd.exe, offers basic 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 typed objects, rather than byte streams. Byte streams require the receiving command to parse them to extract the data it needs, using either the capabilities of the shell or special-purpose tools, such as Grep and Awk. But when data is piped as objects, the elements they encapsulate retain their types, making them distinguishable through separate properties. The recipient cmdlet can then access the data elements by their properties without implementing a parser. 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 Write-Host cmdlet, which creates a formatted text representation of its data, writing it 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 external executables are 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.

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. 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 collection 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.

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. 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. 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 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.

The implementation of these cmdlet classes can refer to any .NET API 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, and registry. Windows PowerShell also includes various cmdlets for managing various Windows systems, including the file system, or using WMI 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 an untyped scripting language somewhat like C# which can implement complex operations using cmdlets imperatively. The scripting language supports variables, functions, branching (if-then-else), loops (while, do, for, and foreach), and error handling, as well as integration with .NET. Variables in PowerShell scripts start with $ and 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. 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:

1. <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.
2. <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.

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. 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.

For error handling, PowerShell provides .NET style Try…Catch…Finally blocks. In case of errors, objects containing errors about the error (Exception object) are thrown, which are caught using the trap. However, the action-or-error is configurable; in case of an error PowerShell can be configured to silently resume execution, without trapping the exception.

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. PowerShell scripts can be signed to verify their integrity, and are subject to .NET CAS.

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. 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. 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. Once the pipeline is setup, 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.

PowerShell 2.0

Template:Future software Microsoft is working on the next version of PowerShell and has made a CTP release of the same publicly available. It includes changes to the scripting language and hosting API, in addition to including a number of cmdlets. A non-exhaustive list of the new features is:

1. 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.
2. 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.
3. ScriptCmdlets: These are cmdlets written using the PowerShell scripting language.
4. SteppablePipelines: This allows the user to control when the BeginProcessing(), ProcessRecord() and EndProcessing() functions of a cmdlet are called.
5. 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.
6. 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.
7. New Cmdletes: Including Out-GridView, which displays tabular data in the WPF GridView object.
8. New Operators: -Split -Join and @ operators.
9. 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 (Runspace Pools) as well as the ability to create restricted Runspaces which only allow a configured subset of PowerShell to be invoked.
10. Graphical PowerShell: PowerShell 2.0 includes a GUI-based PowerShell host that provides integrated debugger, syntax highlighting and up to 8 PowerShell consoles (Runspaces) in a tabbed UI, as well as to run only the selected parts in a script.. However, it is a very early release which suffers from performance problems with large scripts and does not include tab completion.

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.

Windows PowerShell (Cmdlet)	Windows PowerShell (Alias)	cmd.exe / COMMAND.COM (MS-DOS, Windows, OS/2, etc.)	Bash (Unix, BSD, Linux, etc.)	Description
Set-Location	sl, cd, chdir	cd, chdir	cd	Change the current directory
Clear-Host	cls, clear	cls	clear	Clear the screen
Copy-Item	cpi, copy, cp	copy	cp	Copy one or several files / a whole directory tree
Get-Help	help, man	help	man	Help on commands
Remove-Item	ri, del, rmdir, rd, rm	del, rmdir, rd	rm, rmdir	Delete a file / a directory
Rename-Item	rni, ren	ren	mv	Rename a file / a directory
Get-ChildItem	gci, dir, ls	dir	ls	List all files / directories in the (current) directory
Write-Output	echo, write	echo	echo	Print strings, variables etc. to screen
Pop-Location	popd	popd	popd	Change the current directory to the directory most recently pushed onto the stack
Push-Location	pushd	pushd	pushd	Push the current directory onto the stack
Set-Variable	sv, set	set	set	Set the value of a variable / create a variable
Get-Content	gc, type, cat	type	cat	Get the content of a file
Get-Process	gps, ps	tlist, tasklist	ps	List all currently running processes
Stop-Process	spps, kill	kill, taskkill	kill	Stop a running process
Tee-Object	tee	?	tee	Pipe input to a file or variable, then pass the input along the pipeline

Examples

• Stop all processes that begin with the letter "p":

 PS> get-process p* | stop-process

• Find the processes that use more than 1000 MB of memory and kill them:

 PS> get-process | where { $_.WS -gt 1000MB } | stop-process

• Calculate the number of bytes in the files in a directory:

 PS> get-childitem | measure-object -property length -sum

• Determine whether a specific process is no longer running:

 PS> $processToWatch = get-process notepad
 PS> $processToWatch.WaitForExit()

• Change the case of a string from lower to upper:

 PS> "hello, world!".ToUpper()

• Insert the string "ABC" after the first character in the word "string" to have the result "sABCtring":

 PS> "string".Insert(1, "ABC")

• Download a specific RSS feed and show the titles of the 8 most recent entries:

 PS> $rssUrl = "http://blogs.msdn.com/powershell/rss.aspx"
 PS> $blog = [xml](new-object System.Net.WebClient).DownloadString($rssUrl)
 PS> $blog.rss.channel.item | select title -first 8

• Sets $UserProfile to the value of the UserProfile environment variable

 PS> $UserProfile = $env:UserProfile

File extensions

• PS1 – Windows PowerShell shell script
• PS1XML – Windows PowerShell format and type definitions
• PSC1 – Windows PowerShell console file

Application support

Application	Version	Cmdlets	Provider	Management GUI
Exchange Server	2007	Yes	Yes	Yes
Windows Server	2008	Yes	Yes	No
Microsoft SQL Server	2008	Yes	Yes	No
System Center Operations Manager	2007	Yes	No	No
System Center Virtual Machine Manager	2007	Yes	Yes	No
System Center Data Protection Manager	2007	Yes	No	No
Windows Compute Cluster Server	2007	Yes	Yes	No
IBM WebSphere MQ	6.0.2.2	Yes	No	No
Quest Management Shell for Active Directory	1.0.5	Yes	No	No

See also

• Comparison of computer shells
• Comparison of programming languages

References

Books

• Payette, Bruce. Windows PowerShell in Action. ISBN 1-932394-90-7.
• Holmes, Lee. Windows PowerShell Quick Reference. ISBN 0-596-52813-2.
• Holmes, Lee. Windows PowerShell Cookbook. ISBN 0-596-52849-3.
• Watt, Andrew. Professional Windows PowerShell. ISBN 0-471-94693-1.
• Kopczynski, Tyson. Microsoft Powershell Unleashed. ISBN 0672329530.
• Wilson, Ed. Microsoft® Windows PowerShell™ Step By Step. ISBN 0-7356-2395-3.
• Ford, Jerry Lee Jr. Microsoft Windows Powershell Programming for the Absolute Beginner. ISBN 1-59863-354-6.
• Oakley, Andy. Monad (AKA PowerShell). ISBN 0-596-10009-4.
• Jones, Don. Windows PowerShell: TFM. ISBN 0-9776597-2-0.

External links

• PowerShell homepage
• Windows PowerShell Team Blog
• Scripting with Windows PowerShell
• Monad Manifesto - The Origin of Windows PowerShell
• Channel9 Interviews/Demos with Jeffrey Snover - the Architect of Windows PowerShell
• 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

• Powershell.com - Powershell Analyzer and Powershell Plus - Powershell Hosting, Editor and Debugging Products.
• PowerShell Community Extensions
• sudo for Powershell - run specific commands as an administrator on Windows Vista

PowerShell providers

• S3Nas PowerShell Provider - S3Nas Amazon S3 PowerShell Provider
• PowerShell SharePoint Provider - PowerShell SharePoint Provider
• OneNote PowerShell Provider - OneNote PowerShell Provider
• Windows Mobile PowerShell Provider - Windows Mobile PowerShell Provider