about_functions - PowerShell command help and examples

Describes how to create and use functions in Windows PowerShell. (about_functions)

TOPIC
about_Functions
SHORT DESCRIPTION
Describes how to create and use functions in Windows PowerShell.
LONG DESCRIPTION
A function is a list of statements that has a name that you assign. When you run a function, you type the function name. The statements in the list run as if you had typed them at the command prompt. Like cmdlets, functions can have parameters. The parameters can be named, positional, switch, or dynamic parameters. Function parameters can be read from the command line or from the pipeline. Functions can return values that can be displayed, assigned to variables, or passed to other functions or cmdlets. The function's statement list can contain different types of statement lists with the keywords Begin, Process, and End. These statement lists handle input from the pipeline differently. A filter is a special kind of function that uses the Filter keyword. Functions can also act like cmdlets. You can create a function that works just like a cmdlet without using C# programming. For more information, see about_Functions_Advanced. Syntax The following is the syntax for a function: function [<scope:>]<name> [([type]$parameter1[,[type]$parameter2])] { param([type]$parameter1 [,[type]$parameter2]) dynamicparam {<statement list>} begin {<statement list>} process {<statement list>} end {<statement list>} } A function includes the following items: - A Function keyword - A scope (optional) - A name that you select - Any number of named parameters (optional) - One or more Windows PowerShell commands enclosed in braces ({}) For more information about the Dynamicparam keyword and dynamic parameters in functions, see about_Functions_Advanced_Parameters. Simple Functions Functions do not have to be complicated to be useful. The following function gets the environment variables that do not belong to the System account of the current system: function other_env { get-wmiObject win32_environment | where {$_.username -ne "<System>"} } To run the function, type "other_env". You can create a toolbox of useful small functions. Add these functions to your Windows PowerShell profile, as described in about_Profiles and later in this topic. Functions with Parameters You can use parameters with functions, including named parameters, positional parameters, switch parameters, and dynamic parameters. For more information about dynamic parameters in functions, see about_Functions_Advanced_Parameters. Named Parameters You can define any number of named parameters. You can include a default value for named parameters, as described later in this topic. You can define parameters inside the braces using the Param keyword, as shown in the following sample syntax: function <name> { param ([type]$parameter1[,[type]$parameter2]) <statement list> } You can also define parameters outside the braces without the Param keyword, as shown in the following sample syntax: function <name> [([type]$parameter1[,[type]$parameter2])] { <statement list> } There is no difference between these two methods. Use the method that you prefer. When you run the function, the value you supply for a parameter is assigned to a variable that contains the parameter name. The value of that variable can be used in the function. The following example is a function called Small_files. This function has a $size parameter. The function displays all the files that are smaller than the value of the $size parameter, and it excludes directories: function small_files { param ($size) Get-ChildItem c:\ | where { $_.length -lt $size -and !$_.PSIsContainer} } In the function, you can use the $size variable, which is the name defined for the parameter. To use this function, type the following command: C:\PS> function small_files -size 50 You can also enter a value for a named parameter without the parameter name. For example, the following command gives the same result as a command that names the Size parameter: C:\PS> function small_files 50 To define a default value for a parameter, type an equal sign and the value after the parameter name, as shown in the following variation of the Small_files example: function small_files ($size = 100) { Get-ChildItem c:\ | where { $_.length -lt $size -and !$_.PSIsContainer} } If you type "small_files" without a value, the function assigns 100 to $size. If you provide a value, the function uses that value. Positional Parameters A positional parameter is a parameter without a parameter name. Windows PowerShell uses the parameter value order to associate each parameter value with a parameter in the function. When you use positional parameters, type one or more values after the function name. Positional parameter values are assigned to the $args array variable. The value that follows the function name is assigned to the first position in the $args array, $args[0]. The following Extension function adds the .txt file name extension to a file name that you supply: function extension { $name = $args[0] + ".txt" $name } C:\PS> extension myTextFile myTextFile.txt Functions can take more than one positional parameter. The following example displays any values entered with the function name. function repeat { foreach ($arg in $args) { "The input is $arg" } } C:\PS>repeat one The input is one C:\PS> repeat one two three The input is one The input is two The input is three This function can be used with any number of values. The function assigns each value to a position in the $args array. Switch Parameters A switch is a parameter that does not require a value. Instead, you type the function name followed by the name of the switch parameter. To define a switch parameter, specify the type [switch] before the parameter name, as shown in the following example: function switchExample { param ([switch]$on) if ($on) { "Switch on" } else { "Switch off" } } When you type the On switch parameter after the function name, the function displays "Switch on". Without the switch parameter, it displays "Switch off". C:\PS> SwitchExample -on Switch on C:\PS> SwitchExample Switch off You can also assign a Boolean value to a switch when you run the function, as shown in the following example: C:\PS> SwitchExample -on:$true Switch on C:\PS> SwitchExample -on:$false Switch off Piping Objects to Functions Any function can take input from the pipeline. You can control how a function processes input from the pipeline using Begin, Process, and End keywords. The following sample syntax shows the three keywords: function <name> { begin {<statement list>} process {<statement list>} end {<statement list>} } The Begin statement list runs one time only, at the beginning of the function. The Process statement list runs one time for each object in the pipeline. While the Process block is running, each pipeline object is assigned to the $_ automatic variable, one pipeline object at a time. After the function receives all the objects in the pipeline, the End statement list runs one time. If no Begin, Process, or End keywords are used, all the statements are treated like an End statement list. The following function uses the Process keyword. The function displays examples from the pipeline: function pipelineFunction { process {"The value is: $_"} } To demonstrate this function, enter an array of numbers created with commas, as shown in the following example: C:\PS> 1,2,4 | pipelineFunction The value is: 1 The value is: 2 The value is: 4 When you use a function in a pipeline, the objects piped to the function are assigned to the $input automatic variable. The function runs statements with the Begin keyword before any objects come from the pipeline. The function runs statements with the End keyword after all the objects have been received from the pipeline. The following example shows the $input automatic variable with Begin and End keywords. function PipelineBeginEnd { begin {"Begin: The input is $input"} end {"End: The input is $input" } } If this function is run by using the pipeline, it displays the following results: C:\PS> 1,2,4 | PipelineBeginEnd Begin: The input is End: The input is 1 2 4 When the Begin statement runs, the function does not have the input from the pipeline. The End statement runs after the function has the values. If the function has a Process keyword, the function reads the data in $input. The following example has a Process statement list: function PipelineInput { process {"Processing: $_ " } end {"End: The input is: $input" } } In this example, each object that is piped to the function is sent to the Process statement list. The Process statements run on each object, one object at a time. The $input automatic variable is empty when the function reaches the End keyword. C:\PS> 1,2,4 | PipelineInput Processing: 1 Processing: 2 Processing: 4 End: The input is: Filters A filter is a type of function that runs on each object in the pipeline. A filter resembles a function with all its statements in a Process block. The syntax of a filter is as follows: filter [<scope:>]<name> {<statement list>} The following filter takes log entries from the pipeline and then displays either the whole entry or only the message portion of the entry: filter ErrorLog ([switch]$message) { if ($message) { out-host -inputobject $_.Message } else { $_ } } Function Scope A function exists in the scope in which it was created. If a function is part of a script, the function is available to statements within that script. By default, a function in a script is not available at the command prompt. You can specify the scope of a function. For example, the function is added to the global scope in the following example: function global:get-dependentsvs { get-service | where {$_.dependentservices} } When a function is in the global scope, you can use the function in scripts, in functions, and at the command line. Functions normally create a scope. The items created in a function, such as variables, exist only in the function scope. For more information about scope in Windows PowerShell, see about_Scope. Finding and Managing Functions Using the Function: Drive All the functions and filters in Windows PowerShell are automatically stored in the Function: drive. This drive is exposed by the Windows PowerShell Function provider. When referring to the Function: drive, type a colon after Function, just as you would do when referencing the C or D drive of a computer. The following command displays all the functions in the current session of Windows PowerShell: C:\PS> dir function: The commands in the function are stored as a script block in the definition property of the function. For example, to display the commands in the Help function that comes with Windows PowerShell, type: (dir function:help).definition For more information about the Function: drive, see Function. Reusing Functions in New Sessions When you type a function at the Windows PowerShell command prompt, the function becomes part of the current session. It is available until the session ends. To use your function in all Windows PowerShell sessions, add the function to your Windows PowerShell profile. For more information about profiles, see about_Profiles. You can also save your function in a Windows PowerShell script file. Type your function in a text file, and then save the file with the .ps1 file name extension. Writing Help for Functions The Get-Help cmdlet gets help for functions, as well as for cmdlets, providers, and scripts. To get help for a function, type Get-Help followed by the function name. For example, to get help for the MyDisks function, type: get-help MyDisks You can write help for a function by using either of the two following methods: -- Comment-Based Help for Functions Create a help topic by using special keywords in the comments. To create comment-based help for a function, the comments must be placed at the beginning or end of the function body or on the lines preceding the function keyword. For more information about comment-based help, see about_Comment_Based_Help. -- XML-Based Help for Functions Create an XML-based help topic, such as the type that is typically created for cmdlets. XML-based help is required if you are localizing help topics into multiple languages. To associate the function with the XML-based help topic, use the .ExternalHelp help comment keyword. For more information about the ExternalHelp keyword, see about_Comment_Based_Help. For more information about XML-based help, see "How to Write Cmdlet Help" in MSDN. SEE ALSO about_Automatic_Variables about_Comment_Based_Help about_Functions_Advanced about_Functions_CmdletBindingAttribute about_Parameters about_Profiles about_Scopes about_Script_Blocks Function (provider) C:\Windows>powershell get-help about_functions_advanced -full

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