Expanding variables and subexpressions in strings

In PowerShell, you can define a string with single or double quotes. There is a difference between these two methods. In a string with single quotes, variables and subexpressions are not expanded; while in a string with double quotes, variables and subexpressions are expanded.

Let's look at an example of variable expansion in a double-quoted string:

PowerCLI C:\> $Number = 3
PowerCLI C:\> "The number is: $Number"
The number is: 3

In the preceding example, the string is defined with double quotes and the $Number variable is expanded. Let's see what happens if you use single quotes:

PowerCLI C:\> $Number = 3
PowerCLI C:\> 'The number is: $Number'
The number is: $Number

Using a single-quoted string, PowerShell doesn't expand the $Number variable. Let's try to put the number of virtual CPUs of a virtual machine in a double-quoted string:

PowerCLI C:\> $vm = Get-VM -Name dc1
PowerCLI C:\> "The number of vCPU's of the vm is: $vm.NumCpu"
The number of vCPU's of the vm is: dc1.NumCpu

The output is not what you intended. What happened? The $ in front of a variable name tells PowerShell to evaluate the variable. In the string used in the preceding example, $vm evaluates the variable vm but does not evaluate $vm.NumCpu. To evaluate $vm.NumCpu, you have to use another $ sign before and parentheses around the code that you want to evaluate, like so: $($vm.NumCpu). This is called a subexpression notation.

In the corrected example, you will get the number of virtual CPUs:

PowerCLI C:\> $vm = Get-VM -Name dc1
PowerCLI C:\> "The number of vCPU's of the vm is: $($vm.NumCpu)"
The number of vCPU's of the vm is: 2

You can use subexpression evaluation to evaluate any PowerShell code. In the next example, you will use PowerShell to calculate the sum of 3 and 4:

PowerCLI C:\> "3 + 4 = $(3+4)"
3 + 4 = 7

PowerCLI C:\> $Number = 3
PowerCLI C:\> $String = "The number is: $Number"
PowerCLI C:\> $String
The number is: 3
PowerCLI C:\> $Number = 4
PowerCLI C:\> $String
The number is: 3

PowerCLI C:\> $Number = 3
PowerCLI C:\> $String = 'The number is: $Number'
PowerCLI C:\> $ExecutionContext.InvokeCommand.ExpandString($String)
The number is: 3
PowerCLI C:\> $Number = 4
PowerCLI C:\> $ExecutionContext.InvokeCommand.ExpandString($String)
The number is: 4