How to expand variable in powershell?

Question

Get-WmiObject -Class win32_logicaldisk -Filter \'DeviceID=\"C:\"\'

does what I want,

$var=\"C:\"
Get-WmiObject -Class win32_logical         


        

Answer 1

If, for instance, you're getting data from a file with select-string, the return is a single quote string. If that string contains variables they won't expand. Invoke-Expression can be used if the variable is clean - not mixed in with other text:

$abc = 123
$a = '$abc'
iex $a -> 123

If the variable is part of a path name, this doesn't work. Use $ExecutionContext.InvokeCommand.ExpandString($var)

$path = '$Home/HiMum'
$ExecutionContext.InvokeCommand.ExpandString($path)
-> /home/JoeBlogs/HiMum

You lucky sods on Windows might be able to use Convert-String to change singles to doubles.

Answer 2

When you start a string literal with ' (single-quotes), you're creating a verbatim string - that is, every character inside the string is interpreted literally and variable references and expressions won't expand!

Use " if you want the variable to be expanded:

$var = 'C:'
Get-WmiObject -Class win32_logicaldisk -Filter "DeviceID='$var'"

If your variable name has weird characters, or is followed by a word character, you can qualify the variable name with curly brackets {} immediately after the $:

$var = 'C:'
Get-WmiObject -Class win32_logicaldisk -Filter "DeviceID='${var}'"

Answer 3

I didn't find expanding expressions in PowerShell, but here's what I found.

# Let's set some varaible
$ComputerName = 'some-value'

# Let's store this variable name
$name = 'ComputerName'

# Get value by `Get-Item`.
(Get-Item variable:$name).Value # throws an exception for undefined variables.

# Get value by `Invoke-Expression`
Invoke-Expression "`$variable:$name"
Invoke-Expression "`$$name"

The same but for environment variables

(Get-Item env:$name).Value # throws an exception for undefined environments.
Invoke-Expression "`$env:$name"

I prefer Get-Item as it "fails loudly".

And Get-Item allows to use additional literals during this process, as well as Invoke-Expression.
I.e. see the Computer literal below before the $ sign.

$otherName = 'Name'

(Get-Item variable:Computer$otherName).Value
(Get-Item env:Computer$otherName).Value