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In the previous Get-Command example, the command s defintion was truncated to fit the book-formatting requirements. You can control how this information is described by using the Format-List and FormatTable commands.
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This is useful as a kind of lightweight help, but in addition to displaying information, the object returned by Get-Command can be used with the call operator to invoke that command. This is pretty significant. This extra degree of flexibility, invoking a command indirectly, is the first step on the road to metaprogramming. Let s try this out we ll get the CommandInfo object for the Get-Date command.
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PS (1) > $d = get-command get-date PS (2) > $d.CommandType Cmdlet PS (3) > $d.Name Get-Date
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As we can see from this example, the name get-date resolves to a cmdlet with the name get-date . Now let s run this command using the CommandInfo object with the call operator:
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PS (4) > & $d Sunday, May 21, 2006 7:29:47 PM
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It s as simple as that. So why do we care about this Because it s a way of getting a handle to a specific command in the environment. Say we defined a function get-date .
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PS (1) > function get-date {"Hi there"} PS (2) > get-date Hi there
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Our new get-date command outputs a string. Because PowerShell looks for functions before it looks for cmdlets, this new function definition hides the Get-Date cmdlet. Even using & with the string get-date still runs the function:
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PS (3) > & "get-date" Hi there
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SCRIPTBLOCK BASICS
Since we created a second definition for get-date (the function), now if you use Get-Command you will see two definitions. So how do we unambiguously select the cmdlet Get-Date
PS (4) > get-command get-date CommandType ----------Cmdlet Function Name ---Get-Date get-date Definition ---------Get-Date [[-Date] <D... "Hi there"
One way is to select the CommandInfo object based on the type of the command:
PS (5) > get-command -commandtype cmdlet get-date CommandType ----------Cmdlet Name ---Get-Date Definition ---------Get-Date [[-Date] <D...
Let s put the result of this command into a variable.
PS (6) > $ci = get-command -commandtype cmdlet get-date
and then run it using the call operator.
PS (7) > &$ci
Sunday, May 21, 2006 7:34:33 PM
The Get-Date cmdlet was run as expected. Another way to select which command to run, since Get-Command returns a collection of objects, is to index into the collection to get the right object:
PS (8) > &(get-command get-date)[0] Sunday, May 21, 2006 7:41:28 PM
Here we used the result of the index operation directly with the call operator to run the desired command. This is all interesting, but what does it have to do with scriptblocks We ve demonstrated that you can invoke a command through an object reference instead of by name. This was the problem we set out to work around. Scriptblocks are functions that don t have names; so, as you might expect, the way to call a scriptblock is to use the call operator. Here s what that looks like:
PS (1) > & {param($x,$y) $x+$y} 2 5 7
In this example, the scriptblock is
{param($x,$y) $x+$y}
We called it with two arguments, 2 and 5, so the result of executing the scriptblock is 7.
SCRIPTBLOCKS AND OBJECTS
The ScriptBlock literal Now we ll take a detailed look at the syntax for creating a scriptblock. We ll cover how to define a scriptblock that acts as a function, how to define one that acts like a filter, and finally how to define a scriptblock cmdlet. What we ve been writing to create scriptblocks is called a scriptblock literal in other words, a chunk of legitimate PowerShell script surrounded by braces. The syntax for this literal is shown in figure 8.1. The definition of a scriptblock looks more or less like the definition of a function, except the function keyword and function name are missing. If the param statement is not present then the scriptblock will get its arguments through $args, exactly as a function would.
AUTHOR S NOTE
The param statement in PowerShell corresponds to the lambda keyword in other languages. For example, the PowerShell expression
& {param($x,$y) $x+$y} 2 5
is equivalent to the LISP expression
(lambda (x y) (+ x y)) 2 5)
or the Python expression
(lambda x,y: x+y)(2,5)
Also note that, unlike Python lambdas, PowerShell scriptblocks can contain any collection of legal PowerShell statements.
Scriptblocks, like regular functions or scripts, can also behave like cmdlets. In other words, they can have one or all of the begin, process, or end clauses that you can
List of statements that make up the scriptblock body param Keyword The list of parameters for the function
{ param ( <parameter list> ) <statementList> }
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