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before a single value is processed. In the Foreach-Object pipeline, each object is generated and then passed to the cmdlet for processing. The Foreach-Object cmdlet has an advantage over the foreach loop in the amount of space being used at a particular time. For example, if you are processing a large file, the foreach loop would have to load the entire file into memory before processing. When using the Foreach-Object cmdlet, the file will be processed one line at a time. This significantly reduces the amount of memory needed to accomplish a task. You ll end up using the Foreach-Object cmdlet a lot in command lines to perform simple transformations on objects (we ve already used it in many examples so far). Given the frequency of use, there are two standard aliases for this cmdlet. The first one is (obviously) foreach. But wait a second didn t we say earlier in this chapter that foreach is a keyword, and keywords can t be aliased This is true, but keywords are only special when they are the first unquoted word in a statement. If they appear anywhere else (for example as an argument or in the middle of a pipeline), they re just another token with no special meaning. Here s another way to think about this: the first word in a statement is the key that the PowerShell interpreter uses to decide what kind of statement it is processing, hence the term keyword . This positional constraint is how the interpreter can distinguish between the keyword foreach :
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foreach ($i in 1..10) { $i }
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and the aliased cmdlet foreach :
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1..10 | foreach ($_}
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When foreach is the first word in a statement, it s a keyword; otherwise it s the name of a command. Now let s look at the second alias. Even though foreach is significantly shorter than Foreach-Object, there have still been times when users wanted it to be even shorter.
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Actually users wanted to get rid of this notation entirely and have foreach be implied by an open brace following the pipe symbol. This would have made about half of our users very happy. Unfortunately, the other half were adamant that the implied operation be Where-Object instead of Foreach-Object.
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Where extreme brevity is required, there is a second alias that is simply the percent sign (%). Oh ho now people are really upset! You told us the percent sign is an operator! Well that s true but only when it s used as a binary operator. If it appears as the first symbol in a statement, it has no special meaning, so we can use it as an alias for Foreach-Object. This lets you write concise (but somewhat hard to read) statements such as the following, which prints out the numbers from 1 to 5, times two:
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PS (1) > 1..5|%{$_*2} 2 4 6 8 10 PS (2) >
Clearly this construction is great for interactive use where brevity is very important, but it probably shouldn t be used when writing scripts. The issue is that ForeachObject is so useful that a single-character symbol for it, one that is easy to distinguish, is invaluable for experienced PowerShell users. However, unlike the word foreach , % is not immediately meaningful to new users. So this notation is great for conversational PowerShell, but generally terrible for broad formal use. The last thing to know about the Foreach-Object cmdlet is that it can take multiple scriptblocks. If three scriptblocks are specified, the first one is run before any objects are processed, the second is run once for each object, and the last is run after all objects have been processed. This is good for conducting accumulation-type operations. Here s another variation, where we sum up the number of handles used by the service host svchost processes:
PS (3) > gps svchost |%{$t=0}{$t+=$_.handles}{$t} 3238
The standard alias for Get-Process is gps. This is used to get a list of processes where the process name matches svchost . These process objects are then piped into Foreach-Object, where the handle counts are summed up in $t and then emitted in the last scriptblock. We used the % alias here to show how concise these expressions can be. In an interactive environment, brevity is important. And now here s something to keep in mind when using Foreach-Object. The Foreach-Object cmdlet works like all cmdlets: if the output object is a collection, it gets unraveled. One way to suppress this behavior is to use the unary comma operator. For example, in the following, we assign $a an array of two elements, the second of which is a nested array.
PS (1) > $a = 1,(2,3)
Now when we check the length, we see that it is two as expected:
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