How to properly use quotes when working with strings
| Executive Summary:|
Windows PowerShell cmdlets often include strings. The rules that govern how to use quotes in PowerShell strings are referred to as quoting rules. In this lesson, you’ll learn about these rules. You also learn how to obtain and use the methods and properties of PowerShell's System.String object.
Most PowerShell statements include string values. Usually, these strings are passed to cmdlets as arguments. In some cases, the strings are enclosed in single quotes. In other cases, they’re enclosed in double quotes. And sometimes they’re not enclosed in quotes at all. It’s important to understand how to properly handle strings. The rules that govern how to do so are often referred to as quoting rules. In this lesson, you’ll learn about these rules. Specifically, you’ll learn when to enclose string values in quotes and whether to use single or double quotes. In addition, you’ll learn how to flag, or escape, special characters.
Working with String Values
Whenever you enclose text in quotes, PowerShell treats that text as a string value. So, as long as the text doesn’t contain any special characters (you’ll learn more about these characters shortly) or reference variables (I’ll discuss how to reference variables in strings in the next lesson), you can enclose the text in either single or double quotes. For example, the following statements achieve the same results:
Write-Output “String in quotes.”
Write-Output ‘String in quotes.’
In these examples, the Write-Output cmdlet sends a string object down the pipeline or, in this case, directly to the PowerShell console. As you can see in Figure 1, the outputted value is the same for both statements.
In addition to the Write-Output cmdlet, PowerShell’s Out-Host and Write-Host cmdlets output information to the console. Their differences lie in the details. For example, the Write-Output cmdlet sends output down the pipeline to the next cmdlet. When Write-Output is the last cmdlet in the pipeline, the output is displayed in the console. The Out-Host cmdlet sends output directly to the console and offers an optional parameter that lets you view the output one screen at a time, which can be helpful if there is a lot of output. This is the default output cmdlet, so if you don’t specify an output cmdlet, Out-Host cmdlet is used. The Write-Host cmdlet also sends output directly to the console. However, Write-Host has two optional parameters that let you change the color of the text or text background, thereby creating a customized console.
For basic quoted string values that you want to output directly to the console window, all three cmdlets behave in similar ways. For example, the following four commands all display the string in the console window in the same way as in Figure 1:
“String in quotes.”
Write-Output “String in quotes.”
Write-Host “String in quotes.”
-InputObject “String in quotes.”
Notice that no cmdlet is specified in the first command. As a result, the Out-Host cmdlet is used.
For many of the examples in this lesson, I use the Write-Output cmdlet because it outputs an object in a way similar to many other cmdlets. This lets me demonstrate different principles about quoted values. Keep in mind, however, that the Write-Output, Out- Host, and Write-Host cmdlets can behave differently in different circumstances. For more information about these cmdlets, see their Help files.
If you want to include quotes within a string, you can use single quotes within double quotes or double quotes within single quotes:
Write-Output “String ‘in’ quotes.”
Write-Output ‘String “in” quotes.’
If you refer again to Figure 1, you’ll see that inside quotes in both cases are carried to the output. This isn’t the case when you use the same type of quotes throughout the string:
Write-Output “String “in” quotes.”
Write-Output ‘String ‘in’ quotes.’
As Figure 1 shows, the results are quite different. In both cases, the quotes are not displayed and a new line is added. This is because PowerShell interprets the one string as multiple strings and consequently adds a line break. For example, PowerShell interprets String as the first string (so it adds a line break after that string), then interprets the rest as a different string. You can use double quotes within double quotes, but you must escape the inside quotes, which I’ll describe how to do later.
Whenever you work with quotes, be careful not to mix up the type of quotes or forget to include one. Otherwise, you might get stuck in a loop that continues to prompt you for an entry—but nothing you enter gets you out of the loop. If you run into that situation, press Ctrl+C to return to the command prompt.
Another issue to take into account when defining cmdlet arguments is how Power- Shell treats numerical values. As I said earlier, PowerShell treats any values within quotes as strings, even if the value consists of all numbers:
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This statement uses the GetType method of the object’s type, which in this case is String, or more specifically System.String, also shown in Figure 2. For more information about System.String and GetType, see the sidebar “Getting and Using the System.String Object’s Members.”
If you don’t enclose a numerical value in quotes, PowerShell treats the value as a numerical object. For example, the following statement returns an integer object:
Again, you can verify the object’s type by using the GetType method:
As you can see in Figure 2, the object’s type is Int32.
If a value includes both numbers and letters, PowerShell treats it as a string, whether or not it’s in quotes. For example, the following statement returns a string, as verified by the second statement:
Once again, Figure 2 shows the output of these statements.
In most cases, you can omit quotes if your argument is a string with no embedded spaces. For example, the following three statements use the Set-Location cmdlet to set the working folder to the C drive:
Set-Location ‘C:\’Now suppose you want to change the working folder to C:\Documents and Settings:
C:\Documents and Settings
This statement generates an error because it doesn’t know what to do with the tokens (words) after the first space. As you can see in Figure 3, the parser interprets “and” as a parameter, and because there’s no parameter by this name, the parser generates an error. You can easily fix this problem by enclosing the entire argument in quotes:
“C:\Documents and Settings”
Now when you run this statement, it changes the working folder, as shown in Figure 3.
One other important issue when working with strings is how to reference variables within a quoted string. If you enclose a string in double quotes, the variable’s value is used. If you enclose a string in single quotes, the literal value is used. As I mentioned earlier, I’ll cover variables and describe how to reference them in strings in the next lesson. However, if you’re anxious to learn more about using variables in your strings now, refer to the about_quoting_ rules Help file.
Escaping Special Characters in Strings
Up to this point, the arguments you’ve seen in the examples could have taken single or double quotes. As a result, it would appear that there’s no difference between the two. However, there’s one very important difference: Single quotes always treat a string literally, whereas double quotes allow you to escape special characters within the text. A special character is one that, when preceded by a backtick (`), takes a specific action that it would not have taken without the backtick. Table 1 lists PowerShell’s special characters.
The best way to explain this concept is through a few examples. In the following statement, several characters have been escaped in order to change how the text is displayed:
The first escaped character (the one preceded by the first backtick) is n, which in this context inserts a new line. The next escaped character is t, which inserts a tab. Note that the backtick at the end of the first line isn’t being used as an escape character but rather as a continuation character (see Lesson 2). In the second line, `n and `t are used several more times. In addition, backticks precede the double quotes surrounding the word escaped. As a result, the double quotes appear in the output. If you refer to Figure 4, you’ll see how the new lines, tabs, and double quotes appear. For more information about escaping characters, see the about_escape_character Help file.
If you try to escape characters in a string enclosed in single quotes, the backtick and special characters have no effect on the output, other than to be treated literally. For example, the statement
returns the exact string as originally typed, as Figure 4 shows. Note that, in pre-release versions of PowerShell, you could escape characters inside single-quoted strings, and PowerShell would correctly parse them. So, you might come across material that says that this is how PowerShell handles singlequoted strings.
In most cases, string values play an important role in creating PowerShell statements. The better you understand how to work with strings, the more effective your statements will be. I encourage you to spend time practicing the various ways to use strings. Try enclosing them in single and double quotes, then try running the commands without the quotes. And don’t forget to practice escaping special characters.