WINDOWS POWERSHELL
In PowerShell, administrative tasks are generally
performed by cmdlets (pronounced command-lets), which are specialized
.NET classes implementing a particular operation. Sets of
cmdlets may be combined into scripts, executables (which are standalone
applications), or by instantiating regular .NET classes (or WMI/COM
Objects). These work by accessing data in different data
stores, like the file system or registry, which are made available to
the PowerShell runtime via PowerShell providers.
PowerShell includes its own extensive, console-based help, similar to man pages in Unix shells, via the Get-Help cmdlet. Local help contents can be retrieved from the Internet via Update-Help cmdlet. Alternatively, help from the web can be acquired on a case-by-case basis via the -online switch to Get-Help.
Every released version of Microsoft DOS and Microsoft Windows for personal computers has included a command-line interface tool (shell). These are COMMAND.COM (in installations relying on MS-DOS, including Windows 9x) and cmd.exe (in Windows NT family operating systems). The shell is a command line interpreter that supports a few basic commands. For other purposes, a separate console application must be invoked from the shell. The shell also includes a scripting language (batch files), which can be used to automate various tasks. However, the shell 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 and does not allow the creation of complex scripts. In Windows Server 2003, the situation was improved, but scripting support was still considered unsatisfactory.
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. Not much later, on April 25, 2006 Microsoft formally announced that Monad had been renamed Windows PowerShell, positioning it as a significant part of their management technology offerings. Release Candidate 1 of PowerShell was released at the same time. A significant aspect of both the name change and the RC was that this was now a component of Windows, and not an add-on product.
If a command is a standalone executable program, PowerShell.exe launches it in a separate process; if it is a cmdlet, it executes in the PowerShell process. PowerShell 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. PowerShell enables the creation of aliases for cmdlets, which PowerShell textually translates into invocations of the original commands. PowerShell 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 parsed by the external executable independently of PowerShell interpretation.
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.
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 self-descriptive. Cmdlets output their results as objects or as collections thereof (including arrays), and can optionally receive input in that form, making them suitable for use as recipients in a pipeline. But whereas 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.
Cmdlets are specialized .NET classes, which the PowerShell runtime instantiates and invokes at run-time. 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() – which the cmdlet's implementation overrides to provide the functionality. Whenever a cmdlet runs, PowerShell invokes these methods in sequence, with ProcessRecord() being called if it receives pipeline input. If a collection of objects is 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, registry, the certificate store, as well as the namespaces for command aliases, variables, and functions. 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.
PowerShell implements the concept of a pipeline, which enables piping the output of one cmdlet to another cmdlet as input. For example, the output of the Get-Process cmdlet could be piped to the Where-Object to filter any process that has less than 1 MB of paged memory, and then to the Sort-Object cmdlet (e.g., to sort the objects by handle count), and then finally to the Select-Object cmdlet to select just the first 10 (i.e., the 10 processes based on handle count).
Because all PowerShell objects are .NET objects, they share a .ToString() method, which retrieves the text representation of the data in an object. In addition, PowerShell allows formatting definitions to be specified, so the text representation of objects can be customized by choosing which data elements to display, and in what manner. However, in order to maintain backwards compatibility, if an external executable is used in a pipeline, it receives a text stream representing the object, instead of directly integrating with the PowerShell type system.
Scripts written using PowerShell can be made to persist across sessions in either a .ps1 file or a .psm1 file (the latter is used to implement a module). Later, either the entire script or individual functions in the script can be used. Scripts and functions operate 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. To prevent unintentional running of scripts, script execution is disabled by default and must be enabled explicitly. Enabling of scripts can be performed either at system, user or session level. PowerShell scripts can be signed to verify their integrity, and are subject to Code Access Security.
One can also use PowerShell embedded in a management application, which 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 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 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 the output itself.
Initially using the code name "Monad", PowerShell was first shown publicly at the Professional Developers Conference in September 2003. All major releases are still supported, and each major release has featured backwards compatibility with preceding versions.
The following table contains a selection of the cmdlets that ship with PowerShell, noting similar commands in other well-known command-line interpreters. Many of these similar commands come out-of-the-box defined as aliases within PowerShell, making it easy for people familiar with other common shells to start working.
A project named Pash (the name is a pun on the well-known "bash" Unix shell) has been an open source and cross-platform re-implementation of PowerShell via the Mono framework. Pash was created by Igor Moochnick, written in C# and was released under the GNU General Public License. Pash development stalled in 2008, but was restarted in 2012.
Cmdlets are the heart-and-soul of Windows PowerShell, Microsoft's new command shell/scripting language. This series provides a task-based introduction to Windows PowerShell cmdlets: rather than focusing on the individual cmdlets themselves, the emphasis is on the tasks you can carry out using those cmdlets. These tasks include everything from reading and writing text files to managing event logs to sorting and filtering data.
PowerShell includes its own extensive, console-based help, similar to man pages in Unix shells, via the Get-Help cmdlet. Local help contents can be retrieved from the Internet via Update-Help cmdlet. Alternatively, help from the web can be acquired on a case-by-case basis via the -online switch to Get-Help.
Every released version of Microsoft DOS and Microsoft Windows for personal computers has included a command-line interface tool (shell). These are COMMAND.COM (in installations relying on MS-DOS, including Windows 9x) and cmd.exe (in Windows NT family operating systems). The shell is a command line interpreter that supports a few basic commands. For other purposes, a separate console application must be invoked from the shell. The shell also includes a scripting language (batch files), which can be used to automate various tasks. However, the shell 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 and does not allow the creation of complex scripts. In Windows Server 2003, the situation was improved, but scripting support was still considered unsatisfactory.
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. Not much later, on April 25, 2006 Microsoft formally announced that Monad had been renamed Windows PowerShell, positioning it as a significant part of their management technology offerings. Release Candidate 1 of PowerShell was released at the same time. A significant aspect of both the name change and the RC was that this was now a component of Windows, and not an add-on product.
If a command is a standalone executable program, PowerShell.exe launches it in a separate process; if it is a cmdlet, it executes in the PowerShell process. PowerShell 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. PowerShell enables the creation of aliases for cmdlets, which PowerShell textually translates into invocations of the original commands. PowerShell 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 parsed by the external executable independently of PowerShell interpretation.
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.
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 self-descriptive. Cmdlets output their results as objects or as collections thereof (including arrays), and can optionally receive input in that form, making them suitable for use as recipients in a pipeline. But whereas 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.
Cmdlets are specialized .NET classes, which the PowerShell runtime instantiates and invokes at run-time. 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() – which the cmdlet's implementation overrides to provide the functionality. Whenever a cmdlet runs, PowerShell invokes these methods in sequence, with ProcessRecord() being called if it receives pipeline input. If a collection of objects is 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, registry, the certificate store, as well as the namespaces for command aliases, variables, and functions. 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.
PowerShell implements the concept of a pipeline, which enables piping the output of one cmdlet to another cmdlet as input. For example, the output of the Get-Process cmdlet could be piped to the Where-Object to filter any process that has less than 1 MB of paged memory, and then to the Sort-Object cmdlet (e.g., to sort the objects by handle count), and then finally to the Select-Object cmdlet to select just the first 10 (i.e., the 10 processes based on handle count).
Because all PowerShell objects are .NET objects, they share a .ToString() method, which retrieves the text representation of the data in an object. In addition, PowerShell allows formatting definitions to be specified, so the text representation of objects can be customized by choosing which data elements to display, and in what manner. However, in order to maintain backwards compatibility, if an external executable is used in a pipeline, it receives a text stream representing the object, instead of directly integrating with the PowerShell type system.
Scripts written using PowerShell can be made to persist across sessions in either a .ps1 file or a .psm1 file (the latter is used to implement a module). Later, either the entire script or individual functions in the script can be used. Scripts and functions operate 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. To prevent unintentional running of scripts, script execution is disabled by default and must be enabled explicitly. Enabling of scripts can be performed either at system, user or session level. PowerShell scripts can be signed to verify their integrity, and are subject to Code Access Security.
One can also use PowerShell embedded in a management application, which 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 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 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 the output itself.
Initially using the code name "Monad", PowerShell was first shown publicly at the Professional Developers Conference in September 2003. All major releases are still supported, and each major release has featured backwards compatibility with preceding versions.
The following table contains a selection of the cmdlets that ship with PowerShell, noting similar commands in other well-known command-line interpreters. Many of these similar commands come out-of-the-box defined as aliases within PowerShell, making it easy for people familiar with other common shells to start working.
A project named Pash (the name is a pun on the well-known "bash" Unix shell) has been an open source and cross-platform re-implementation of PowerShell via the Mono framework. Pash was created by Igor Moochnick, written in C# and was released under the GNU General Public License. Pash development stalled in 2008, but was restarted in 2012.
Cmdlets are the heart-and-soul of Windows PowerShell, Microsoft's new command shell/scripting language. This series provides a task-based introduction to Windows PowerShell cmdlets: rather than focusing on the individual cmdlets themselves, the emphasis is on the tasks you can carry out using those cmdlets. These tasks include everything from reading and writing text files to managing event logs to sorting and filtering data.
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