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About Parameters

JuanPablo Jofre|Last Updated: 11/17/2016
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2 Contributors

about_Parameters

SHORT DESCRIPTION

Describes how to work with command parameters in Windows PowerShell�.

LONG DESCRIPTION

Most Windows PowerShell commands, such as cmdlets, functions, and scripts, rely on parameters to allow users to select options or provide input. The parameters follow the command name and have the following form:

-<parameter_name> <parameter_value>

The name of the parameter is preceded by a hyphen (-), which signals to Windows PowerShell that the word following the hyphen is a parameter name. Some parameters do not require or accept a parameter value. Other parameters require a value, but do not require the parameter name in the command.

The type of parameters and the requirements for those parameters vary. To find information about the parameters of a command, use the Get-Help cmdlet. For example, to find information about the parameters of the Get-ChildItem cmdlet, type:

Get-Help Get-ChildItem

To find information about the parameters of a script, use the full path to the script file. For example:

Get-Help $home\Documents\Scripts\Get-Function.ps1

The Get-Help cmdlet returns various details about the command, including a description, the command syntax, information about the parameters, and examples showing how to use the parameters in a command.

You can also use the Parameter parameter of the Get-Help cmdlet to find information about a particular parameter. Or, you can use the Parameter parameter with the wildcard character (*) value to find information about all parameters of the command. For example, the following command gets information about all parameters of the Get-Member cmdlet:

Get-Help Get-Member -Parameter *

DEFAULT PARAMETER VALUES

Optional parameters have a default value, which is the value that is used or assumed when the parameter is not specified in the command.

For example, the default value of the ComputerName parameter of many cmdlets is the name of the local computer. As a result, the local computer name is used in the command unless the ComputerName parameter is specified.

To find the default parameter value, see help topic for the cmdlet. The parameter description should include the default value.

You can also set a custom default value for any parameter of a cmdlet or advanced function. For information about setting custom default values, see about_Parameters_Default_Values.

PARAMETER ATTRIBUTE TABLE

When you use the Full, Parameter, or Online parameters of the Get-Help cmdlet, Get-Help displays a parameter attribute table with detailed information about the parameter.

This information includes the details you need to know to use the parameter. For example, the help topic for the Get-ChildItem cmdlet includes the following details about its Path parameter:

-path <string[]>  
    Specifies a path of one or more locations. Wildcard characters are  
    permitted. The default location is the current directory (.).
Required?                    false  
Position?                    1  
Default value                Current directory  
Accept pipeline input?       true (ByValue, ByPropertyName)  
Accept wildcard characters?  true

The parameter information includes the parameter syntax, a description of the parameter, and the parameter attributes. The following sections describe the parameter attributes.

Parameter Required?

This setting indicates whether the parameter is mandatory, that is, whether all commands that use this cmdlet must include this parameter. When the value is "True" and the parameter is missing from the command, Windows PowerShell prompts you for a value for the parameter.

Parameter Position?

This setting indicates whether you can supply a parameter's value without preceding it with the parameter name. If set to "0" or "named," a parameter name is required. This type of parameter is referred to as a named parameter. A named parameter can be listed in any position after the cmdlet name.

If the "Parameter position?" setting is set to an integer other than 0, the parameter name is not required. This type of parameter is referred to as a positional parameter, and the number indicates the position in which the parameter must appear in relation to other positional parameters. If you include the parameter name for a positional parameter, the parameter can be listed in any position after the cmdlet name.

For example, the Get-ChildItem cmdlet has Path and Exclude parameters. The "Parameter position?" setting for Path is 1, which means that it is a positional parameter. The "Parameter position?" setting for Exclude is 0, which means that it is a named parameter.

This means that Path does not require the parameter name, but its parameter value must be the first or only unnamed parameter value in the command. However, because the Exclude parameter is a named parameter, you can place it in any position in the command.

As a result of the "Parameter position?" settings for these two parameters, you can use any of the following commands:

Get-ChildItem -path c:\techdocs -exclude *.ppt  
Get-ChildItem c:\techdocs -exclude *.ppt  
Get-ChildItem -exclude *.ppt -path c:\techdocs  
Get-ChildItem -exclude *.ppt c:\techdocs

If you were to include another positional parameter without including the parameter name, that parameter would have to be placed in the order specified by the "Parameter position?" setting.

Parameter Type

This setting specifies the Microsoft .NET Framework type of the parameter value. For example, if the type is Int32, the parameter value must be an integer. If the type is string, the parameter value must be a character string. If the string contains spaces, the value must be enclosed in quotation marks, or the spaces must be preceded by the escape character (`).

Default Value

This setting specifies the value that the parameter will assume if no other value is provided. For example, the default value of the Path parameter is often the current directory. Required parameters never have a default value. For many optional parameters, there is no default because the parameter has no effect if it is not used.

Accepts Multiple Values?

This setting indicates whether a parameter accepts multiple parameter values. When a parameter accepts multiple values, you can type a comma-separated list as the value of the parameter in the command, or save a comma-separated list (an array) in a variable, and then specify the variable as the parameter value.

For example, the ServiceName parameter of the Get-Service cmdlet accepts multiple values. The following commands are both valid:

get-service -servicename winrm, netlogon
$s = "winrm", "netlogon"  
get-service -servicename $s

Accepts Pipeline Input?

This setting indicates whether you can use the pipeline operator (|) to send a value to the parameter.

Value                    Description  
-----                    -----------  
False                    Indicates that you cannot pipe a value to the   
                         parameter.
True (by Value)          Indicates that you can pipe any value to the   
                         parameter, just so the value has the .NET   
                         Framework type specified for the parameter or the  
                         value can be converted to the specified .NET   
                         Framework type.
When a parameter is "True (by Value)", Windows   
PowerShell tries to associate any piped values   
with that parameter before it tries other methods  
to interpret the command.
True (by Property Name)  Indicates that you can pipe a value to the   
                         parameter, but the .NET Framework type of the   
                         parameter must include a property with the same  
                         name as the parameter.
For example, you can pipe a value to a Name   
parameter only when the value has a property   
called "Name".

Accepts Wildcard Characters?

This setting indicates whether the parameter's value can contain wildcard characters so that the parameter value can be matched to more than one existing item in the target container.

Common Parameters

Common parameters are parameters that you can use with any cmdlet. For more information about common parameters, see about_CommonParameters

SEE ALSO

about_Command_syntax

about_Comment_Based_Help

about_Functions_Advanced

about_Parameters_Default_Values

about_Pipelines

about_Wildcards

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