Understanding Cmdlet positional parameter

问题

All, Forgive me I am a newbie for the Power Shell, Currently I was reading book Windows PowerShell CookBook try to get start with it. So far, Everything make sense to me except one thing, I was totally confused with the Positional Parameter to Cmdlet.

For example: Select-String

The syntax is following :

Select-String [-Pattern] <String[]> [-Path] <String[]> [-AllMatches] [-CaseSensitive] [-Context <Int32[]>]
[-Encoding <String>] [-Exclude <String[]>] [-Include <String[]>] [-List] [-NotMatch] [-Quiet] [-SimpleMatch]
[<CommonParameters>]

Select-String [-Pattern] <String[]> [-AllMatches] [-CaseSensitive] [-Context <Int32[]>] [-Encoding <String>]
[-Exclude <String[]>] [-Include <String[]>] [-List] [-NotMatch] [-Quiet] [-SimpleMatch] -InputObject <PSObject>
[<CommonParameters>]

Select-String [-Pattern] <String[]> [-AllMatches] [-CaseSensitive] [-Context <Int32[]>] [-Encoding <String>]
[-Exclude <String[]>] [-Include <String[]>] [-List] [-NotMatch] [-Quiet] [-SimpleMatch] -LiteralPath <String[]>
[<CommonParameters>]

I can pass the parameter value to the Cmdlet directly by ignoring the parameter name . like below:

"Hello World"|Select-String .

Based on the concept of Positional parameter,Because the parameter -Pattern is the first parameter. the value . can match the parameter -Pattern. It is ok for me to understand.

But When I try this command line "hello world" | Select-String -AllMatches . the value . is not in the first. Why the Select-String can know it and work out the result ? Could someone please tell me more to understand it well? thanks.

回答1:

It's the first unnamed paramter. The position value tells the cmdlet which argument belongs to which parameter.

The command Select-String -allmatches . links the -allmatches switch to its parameter, and sets the . as the first item in the $args-array (arguments-array) since it doesn't have -parametername in front of it.

Then because Select-String includes a positional value for its parameters, the cmdlet knows that the first item in the arguments-array ($args[0]) should be bound to the -Pattern parameter.

If you'd like to understand this better, read the part about Parameter Position? in the help-section by running:

Get-Help about_Parameters

Then notice that the -Pattern parameter has position 1, as seen here:

Get-Help select-string -Parameter pattern

-Pattern <String[]>
    Specifies the text to find. Type a string or regular expression. If you type a string, use the SimpleMatch parameter.

    To learn about regular expressions, see about_Regular_Expressions.

    Required?                    true
    Position?                    1
    Default value                
    Accept pipeline input?       false
    Accept wildcard characters?  false

Then run the following line to see how PowerShell binds the inputobject "hello world" and the parameters and arguments.

Trace-Command -Expression { "Hello World" | Select-String -allmatches . } -Name ParameterBinding -PSHost

来源：https://stackoverflow.com/questions/20760669/understanding-cmdlet-positional-parameter

标签

powershell

cmdlets