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USING THE IF/ELSEIF/ELSE STATEMENT
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The first statement we ll look at is the if statement. This is the basic conditional statement found in all languages. Figure 6.1 shows the structure of this statement. The statement shown in figure 6.1 is somewhat hard to read, so here s an example to clarify how the if statement works
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if ($x gt 100) { "It's greater than one hundred" } elseif ($x gt 50) { "It's greater than 50" } else { "It's not very big." }
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Executed when the if condition is true elseif Pipeline to test if Keyword elseif Keyword
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Executed when none of the preceding conditions are true
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if (<pipeline>) {<statementList>} elseif (<pipeline>) {<statementList>} else {<statementList>}
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Pipeline to test enclosed in parentheses
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Braces are mandatory around each statement list.
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Executed when the elseif condition is true
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else Keyword
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Figure 6.1 PowerShell s version of the if Statement, which is the basic conditional statement found in all software languages.
FLOW CONTROL IN SCRIPTS
In this example, if the variable $x holds a value greater than 100, the string It s greater than one hundred will be emitted. If $x is greater than 50 but less than 100, it will emit It s greater than 50 ; otherwise you ll get It s not very big . Of course, you can have zero or more elseif clauses to test different things. As you might have noticed, the PowerShell if statement is modeled on the if statement found in C-derived languages such as C#, but there are a couple of exceptions. First, elseif is a single keyword with no spaces allowed between the words. Next, the braces are mandatory around the statement lists, even if you only have a single statement in the list (or no statements for that matter, in which case you would have to type {} ). If you try to write something like
if ($x gt 100) "It's greater than one hundred"
you ll get a syntax error:
PS (1) > if ($x -gt 100) "It's greater than one hundred" Missing statement block after if ( condition ). At line:1 char:17 + if ($x -gt 100) " <<<< It's greater than one hundred" PS (2) >
AUTHOR S NOTE
The PowerShell grammar technically could support the construction shown in the preceding code segment. In fact, I did enable this construct at one point, but when we tried it out, the result was a lot of errors. The problem is that a newline or a semicolon is required to terminate a command. This leads to the situation where you write something like:
if ($x gt 3) write x is $x while ($x--) $x
and discover that, because you ve missed the semicolon before the while statement, it writes out the while statement instead of executing it. In the end, the cost of typing a couple of additional characters was more than offset by a decreased error rate. For this reason, we made the braces mandatory.
In general, the syntax of the if statement (and all of the PowerShell flow control statements) is freeform with respect to white space. In other words, you can lay out your code pretty much any way you want. You can write an if statement that looks like this:
if($true){"true"}else{"false"}
with no white space whatsoever. Alternatively, you could also write it like this:
if ( $true ) { "true" }
USING THE IF/ELSEIF/ELSE STATEMENT
else { "false" }
where each token is on a separate line. There is, however, one constraint on how you can format an if statement. When PowerShell is being used interactively, the else or elseif keywords have to be on the same line as the previous closing brace; otherwise the interpreter will consider the statement complete and execute it immediately. It s important to note that the PowerShell if statement allows a pipeline in the condition clause, not just a simple expression. This means that it s possible to do the following:
if ( dir telly*.txt | select-string penguin ) { "There's a penguin on the telly." }
In this example, the pipeline in the condition part of the if statement will scan all of the text files whose names start with telly to see whether they contain the word penguin . If at least one of the files contains this word, the statement block will be executed, printing out:
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