powershell variable syntax $($a)?

Question

I have a simple question regarding why something works the way it does and I cant seem to readily find out why. I was trying to run the following command:

foreach($a in $list){set-mailboxcalendarpermissions -identity $($a):\calendar

while it works just fine, I don't know what adding the $( ) actually does. When I do ($a):\calendar it would return (variable):\calendar with the parenthesis, but adding the extra "$" fixes it. why?

Thank you all for your help with this terribly worded question.

Bacon Bits · Accepted Answer · 2014-11-18 05:26:57Z

15

$() is a subexpression operator. It means "evaluate this first, and do it separately as an independent statement".

Most often, its used when you're using an inline string. Say:

$x = Get-ChildItem C:\;
$x | ForEach-Object {
    Write-Output "The file is $($_.FullName)";
}

Compare that to:

$x = Get-ChildItem C:\;
$x | ForEach-Object {
    Write-Output "The file is $_.FullName";
}

You can also do things like $($x + $y).ToString(), or $(Get-Date).AddDays(10).

Here, without the subexpression, you'd get $a:\calendar. Well, the problem there is that the colon after a variable is an operator. Specifically, the scope operator. To keep PowerShell from thinking you're trying to look for a variable in the a namespace, the author put the variable in a subexpression.

As far as I've been able to tell using PS for the past few years, parentheses without the dollar sign are also essentially subexpressions. They won't be evaluated as a subexpression when within a string, but otherwise they usually will. It's kind of a frustrating quirk that there's no clear difference.

edited Nov 18, 2014 at 5:26

answered Nov 18, 2014 at 0:54

Bacon Bits

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2 Comments

Duncan Over a year ago

You could also mention that a simpler way to prevent the colon being interpreted as a namespace separator would have been to write ${a}:\calendar

Duncan Over a year ago

BTW, the main difference between (pipeline) and $(statement-list) is that the $() contains a statement-list which includes if, while etc. as well as a pipeline, but parentheses need a pipeline, i.e. an assignment, expression, or command. Also in strings only $() has meaning.

James World · Accepted Answer · 2019-03-05 09:36:04Z

The $() is the subexpression operator. It causes the contained expressions to be evaluated and it returns all expressions as an array (if there is more than one) or as a scalar (single value). Plain parentheses () are simply the mathematical precedence operator and therefore just work in arithmetic expressions to give precedence

Notably, $() is interpreted within a string, whereas () will be just be taken literally - it has no special meaning. So the following:

$a = "Hello"
"$($a.Length)"

gives

whereas

"($a.Length)"

gives

"(Hello.Length)"

As I said, the $() can consist of multiple expressions, all of which are returned as output. () does not do this. So this is an error as the contents are not an arithmetic expression:

(1;2)

whereas this

$(1;2)

evaluates to an array and outputs:

1
2

The expression $($a) is evaluated before the application of the trailing scope : operator and prevents the scope being applied directly to a, instead $a is evaluated first and then the scope is applied.

mklement0 · Accepted Answer · 2024-08-10 00:46:24Z

To add to the preexisting, helpful answers:

[...] $($a):\calendar while it works just fine ...

It actually doesn't: Unless you enclose $($a):\calendar as a command argument in "...", the value of $($a) and the literal :\calendar are passed as separate arguments.

A simple repro: Write-Output $($HOME):\foo: the value of $HOME and :\foo print on separate lines, because Write-Output sees them as separate arguments.
This perhaps surprising behavior is discussed in GitHub issue #6467.

The $(...) enclosure in $($a):\calendar is an apparent attempt to avoid the error that would result from $a:\calendar, in which case the : would be interpreted as a scope / namespace separator, with a getting interpreted as a scope / namespace name.
While this workaround is effective - inside "...", as noted - it is both syntactically cumbersome and inefficient, as it creates a nested pipeline:
- Enclosing a variable name in {...} is sufficient to disambiguate it from subsequent characters that would by default be considered part of the same variable reference, as Duncan notes:
```
${a}:\calendar
```
  - Note that "..." enclosure is not strictly necessary in this case.

$(...), the subexpression operator, is indeed primarily useful inside "...", i.e. expandable (interpolating) strings:

In command arguments, i.e., in argument-parsing mode, barewords, i.e. unquoted arguments are situationally treated implicitly like expandable strings, but - for the reasons discussed in the linked GitHub issue - result in separate arguments if the $(...) operation comes first.
As for distinguishing between $(...) (and, similarly, @(...), the array-subexpression operator) vs. (...), the grouping operator, again building on Duncan's comments:
- Among these three operators, only $(...) can be used inside (potentially implicitly) expandable strings ("...")
- Only $(...) and @(...) create a nested pipeline and can include language statements such as if and foreach and support multiple, ;-separated statements.
- (...) can only contain a single expression or pipeline.
- See this answer for additional information.

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