How to expand variable in powershell?

Question

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

does what I want,

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

does nothing. I tried to change the quotes, to concatenate strings, and other 1000 things, but they didn't work. Why the above example doesn't work, and what would work?

Goodnatured

-Filter "DeviceID='$var'" (notice the double-quotes around the filter string) – Location

@MathiasR.Jessen as mentioned, I tried to reverse the quotes, it doesn't work – Goodnatured

Did you actually try it? In your example you've put {} around the variable, which will surely make the query return nothing – Location

didn't notice the {}. Now it works. I can't believe this did not happen to be one of the 1000 things I've tried, because for sure I tried 'DeviceID="$var"', and I thought that ' 'and " " are the same, but each one goes inside the other. Still, do you know why the {} didn't work? I used {$var} inside strings a lot of times, and just now it didn't work. Also, you can add your solution as an answer. – Goodnatured

${var} would have worked, and resulted in the string C:, but {$var} would result in the string {C:} – Location

Here I provide a few different methods, to expand commandlet arguments, using variables. – Spohr

Answer 1

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}'"

Location

Answer 2

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.

Raiment

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

Womanize

to use in command arguments do it like this "$env:temp\*.tmp" | ls -filter $0 – Otti