To complement marsze’s helpful answer:
${...}
(enclosing the variable name in {
and }
) is indeed always necessary if a variable name contains special characters, such as spaces, .
, or -
.
- Not special are
_
and – surprisingly and problematically –?
. - Note:
:
is invariably interpreted as terminating a PowerShell drive reference, in the context of namespace variable notation, irrespective of whether{...}
enclosure is used or required (e.g., in$env:USERNAME
or${env:USERNAME}
,env
refers to the PowerShell drive representing all environment variables).
In the context of string expansion (interpolation) inside "..."
, there is another reason to use ${...}
, even if the variable name itself doesn’t need it:
If you need to delineate the variable name from directly following non-whitespace characters, notably including :
:
$foo = 'bar' # example variable # INCORRECT: PowerShell assumes that the variable name is 'foobarian', not 'foo' PS> "A $foobarian." A . # Variable $foobarian doesn't exist -> reference expanded to empty string. # CORRECT: Use {...} to delineate the variable name: PS> "A ${foo}barian." A barbarian. # INCORRECT: PowerShell assumes that 'foo:' is a *namespace* (drive) reference # (such as 'env:' in $env:PATH) and FAILS: PS> "$foo: bar" Variable reference is not valid. ':' was not followed by a valid variable name character. Consider using ${} to delimit the name. # CORRECT: Use {...} to delineate the variable name: PS> "${foo}: bar" bar: bar
See this answer for a comprehensive overview of PowerShell string-expansion rules.
Note that you need the same technique when string expansion is implicitly applied, in the context of passing an unquoted argument to a command; e.g.:
# INCORRECT: The argument is treated as if it were enclosed in "...", # so the same rules apply. Write-Output $foo:/bar # CORRECT Write-Output ${foo}:/bar
Finally, a somewhat obscure alternative is to `
-escape the first character after the variable name, but the problem is that this only works as expected with characters that aren’t part of escape sequences (see about_Special_Characters
):
# OK: because `: is not an escape sequence. PS> "$foo`: bar" bar: bar # NOT OK, because `b is the escape sequence for a backspace character. PS> "$foo`bar" baar # The `b "ate" the trailing 'r' of the variable value # and only "ar" was the literal part.