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ADVANCED OPERATORS AND VARIABLES
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All of these examples using variables to read and write files cause the entire contents of files to be loaded into memory as a collection of strings. On modern computers it s possible to handle moderately large files this way, but doing it with very large files is memory-intensive, inefficient, and might even fail under some conditions. Keep this in mind when using these techniques. When accessing a file using the variable namespace notation, PowerShell assumes that it s working with a text file. Since the notation doesn t provide a mechanism for specifying the encoding, you can t use this technique on binary files. You ll have to use the Get-Content and Set-Content cm dlets instead.
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When the filesystem provider reads the file, it returns the file as an array of strings.
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This provides a simple way to get the length of a file:
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${c:file.txt}.length
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The downside of this simple construct is that it requires reading the entire file into memory and then counting the result. It works fine for small files (a few megabytes) but it won t work on files that are gigabytes in size. This is all we re going to cover about variables here. In chapter 7, we ll return to variables and talk about how variables are scoped in the PowerShell language.
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In this chapter, we finished our coverage of PowerShell operators and expressions. We covered how to build complex data structures in PowerShell and how to use the redirection operators to write output to files. We covered arrays, properties, and methods. Finally, we covered the basics of PowerShell variable semantics and variable namespaces. The important points to remember are: The type operators allow you to write scripts that have polymorphic behavior. By using these operators to examine the types of objects, you can decide how to process those objects. The prefix and postfix operators ++ and -- are a convenient way of incrementing and decrementing variables. The subexpression operator $( ... ) allows you to use arbitrary PowerShell script code anywhere that you can use a value expression. The array subexpression operator @( ... ) allows you to guarantee that the result of an expression is always an array. PowerShell arrays support both jagged arrays that is, arrays that contain or reference other arrays and multi-dimensional arrays. Array slicing is also supported. Use the comma operator to build arrays and complex nested data structures.
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Use the dot operator . for accessing instance members and the double-colon :: for accessing static members. The PowerShell redirection operators allow you to control where the output and error objects are written. They also allow you to easily discard these objects if so desired. The format operator -f can be used to do complex formatting tasks when the default formatting doesn t produce the desired results. PowerShell variable namespaces let you access a variety of Windows data stores , including environment variables and the filesystem, not just PowerShell variables.
ADVANCED OPERATORS AND VARIABLES
Flow control in scripts
6.1 Using the if/elseif/else statement 148 6.2 The while loop 151 6.3 The do/while loop 152 6.4 The for loop 153 6.5 The foreach loop 155 6.6 Labels, break, and continue 159 6.7 The PowerShell switch statement 161 6.8 Flow control using cmdlets 169 6.9 The value of statements 175 6.10 Summary 176
I may not have gone where I intended to go, but I think I have ended up where I needed to be. Douglas Adams, The Long Dark Teatime of the Soul
Previous chapters showed you how to solve remarkably complex problems in PowerShell using only commands and operators. You can select, sort, edit, and present all manner of data by composing these elements into pipelines and expressions. In fact, commands and operators were the only elements available in the earliest prototypes of PowerShell. Sooner or later, however, if you want to write significant programs or scripts, you need to add some sort of custom looping or branch logic to your solution. This is what we re going to cover in this chapter: PowerShell s take on the traditional programming constructs that all languages possess. PowerShell has the usual flow control statements for branching and loops; however, there are some behavioral differences that even experienced shell users should be aware of. The most obvious difference is that PowerShell typically allows the use of pipelines in places where other programming languages only allow simple 147
expressions. An interesting implication of this pipeline usage is that the PowerShell switch statement is both a looping construct and a conditional statement, as you ll see in this chapter. This is also the first time we ve really dealt with keywords in PowerShell. Keywords are part of the core PowerShell language. This means that, unlike cmdlets, keywords cannot be redefined or aliased. Keywords are also case insensitive so you can write foreach, ForEach, or FOREACH and they will all be accepted by the interpreter. (By convention, however, keywords in PowerShell scripts are usually written in lowercase.) With these basics out of the way, let s look at the PowerShell flow control statements themselves.
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