PowerShell String Formatting: Why is the colon character causing my variable's value to be blank?

Question

I was looking over what's coming with the next WinRM and PowerShell 3 and I was looking through the list of breaking changes and saw something that I'd never seen before.

The example was:

$server = "msp42"
$status = "online"
"$server: $status"

The resulting output was:

online

OK, I'd never encountered that before and have no clue why the colon caused an issue. A solution suggested in the document was to put a space (which is silly because then you change the output):

"$server : $status"

Another suggestion was to use this format (new to me!):

"${server}: $status"

The final suggestion was to make an expression, which I am familiar with and use all the time:

"$($server): $status"

So, my questions to you PowerShell gurus out there are:

What the heck is up with that colon? Does it do something?
What the heck is the ${variable} syntax? Is it strictly to deal with the colon or does it have some neat features?

See also: PowerShell string interpolation syntax, which discusses the difference between ${foo} and $($foo) in detail. — mklement0
– mklement0, Commented Nov 13, 2024 at 12:55

Joey · Accepted Answer · 2011-12-07 13:08:03Z

67

The colon is a valid character for variable names, e.g. in $Env:PATH, etc.

You can use the following option, too

$server`: $status

or, for some cases a format string is more readable:

'{0}: {1}' -f $server, $status

Back to the colon. There is a special case for variable names that correspond to an item on a PSDrive:

$Env:Foo           # equivalent to the contents of Env:\Foo
$Function:C:       # equivalent to the contents of Function:\C:
${C:\autoexec.bat} # ... you get the picture

The syntax ${} exists to be able to specify variable names that otherwise use characters reserved for other parts of the syntax. You could see it as being similar (but more powerful) to C#'s @ in front of identifiers. See above where a \ is used in the variable name, since $Drive:Item only works for the current container on a drive (or the root for non-hierarchic ones like Env, Alias or Function).

Another example where the variable name would be normally invalid syntax:

PS> $+
The term '$+' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the
spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:1
+ $+
+ ~~
    + CategoryInfo          : ObjectNotFound: ($+:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException

PS> ${+} = 5
PS> ${+}
5
PS> Get-Variable +

Name                           Value
----                           -----
+                              5

edited Dec 7, 2011 at 13:08

answered Dec 5, 2011 at 13:46

Joey

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1 Comment

mklement0 Over a year ago

Re "variable names that correspond to an item on a PSDrive": The (unfortunately rarely seen) name of this feature is namespace variable notation, discussed in detail in this answer.

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