When to use Single and Double quotes with PowerShell

Here is another one of those enduring questions from my PowerShell classes. When to use single and double quotes.

First off we need to establish the object type that is created by both single and double quotes. Take a look at the code below. Both single and double quotes produce a System.String object type. At this point there is no difference between the two.

PS C:\> $Single = ''

 

PS C:\> $Double = ""

 

PS C:\> $Single.GetType()

 

IsPublic IsSerial Name                                     BaseType                                         

-------- -------- ----                                     --------                                         

True     True     String                                   System.Object                                    

 

 

 

PS C:\> $Double.GetType()

 

IsPublic IsSerial Name                                     BaseType                                         

-------- -------- ----                                     --------                                         

True     True     String                                   System.Object 

The difference comes when you create strings that contain variables.

PS C:\> $Var1 = "Hello"

 

PS C:\> $Var2 = 'World'

 

PS C:\> Write-Output "$Var1 $Var2"

Hello World

 

PS C:\> Write-Output '$Var1 $Var2'

$Var1 $Var2

Here two strings are produce from variables. The first utilizes double quotes while the second utilizes single quotes. In the first, the value of the variables are placed in the string. In the second string, utilizing single quotes, the characters are treated as literal. In other words, they are no longer treated as variable names.

You may be thinking “Big deal”. Well, here is where it can become a big deal. Consider the following.

PS C:\> Write-Output 'The price of gas is $4.35 a gallon.'

The price of gas is $4.35 a gallon.

 

PS C:\> Write-Output "The price of gas is $4.35 a gallon."

The price of gas is .35 a gallon.

Do you see the difference? In the string created with single quotes, you get accurate output. In the other string, PowerShell is looking for the value of the variable $4. Well, $4 has a value of NULL. Now you just set the price of gas at $.35 a gallon. We call this a RGE. Otherwise known as a Resume Generating Event.

You could write your string this way.

PS C:\> Write-Output "The price of gas is `$4.35 a gallon."

The price of gas is $4.35 a gallon.

What you may not see is in front of the “$” is a “`” back tick. This is the PowerShell Escape Character. Be careful if you use it. It is hard to see. The Escape Character makes the dollar sign be treated as a character instead of the name of a variable. This will give us the correct output. Remember, this is not the single quote, but the back tick.  The back tick is on the left of a US keyboard where the single quote is on the right.