In PowerShell, syntax for if is as so:

Another syntax rule is that for subexpressions, you need to use parentheses like this:

So with these two rules combined, I would expect that the test for some path needs to be written with two sets of parentheses like this:

However, this also works:

and just for the sake of completeness, this doesn’t work:

(here, you would need to wrap the subexpression in parenthesis as usual).

Can anyone explain the syntax rules that apply here and how comes that I can use the IF test with one parenthesis only? Is it some PowerShell magic or am I misunderstanding the basic syntax rules?

Referring to C.2.2 from Appendix C: The PowerShell grammar in Bruce Payette’s Windows PowerShell in Action, we have:

This indicates the ( and ) tokens as part of the literal syntax for recognizing an if statement, and that the <test> from the about_If documentation refers to a pipeline that will be resolved to a Boolean.

Following the pipeline rules, we find:

• Test-Path ... parses to a <cmdletCall> of <name> <parameterArgumentToken>,
• !Test-Path ... results in an <expressionRule> of <UnaryOperatorToken> <propertyOrArrayReferenceRule>, which fails when the cmdlet call cannot match the simple property or array rule, whereas
• !(Test-Path ...) is able to match the parenthesized cmdlet call as a sub-expression.

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Edit: See also PowerShell 2.0 Language Specification (thanks to Roman’s answer to another question).