How do I run a command using the output of another function in powershell?

Question

I have the following, perfectly working ps1 script:

function Get-ScriptDirectory {
    $Invocation = (Get-Variable MyInvocation -Scope 1).Value
    Split-Path $Invocation.MyCommand.Path
}

$currentDir = Get-ScriptDirectory

node $currentDir/yarn.js $args

I would like to eliminate the $currentDir variable. I am trying to run:

function Get-ScriptDirectory {
    $Invocation = (Get-Variable MyInvocation -Scope 1).Value
    Split-Path $Invocation.MyCommand.Path
}

node (Get-ScriptDirectory)/yarn.js $args

But this fails - it looks like Get-ScriptDirectory output becomes the first argument. I've tried using quotes, and that doesn't work either - the brackets no longer expand.

How do I run a command using the output of a function?

@PetSerAl Thanks! Whats the difference between $() and () anyway? Add it as an answer and I'll mark it correct. — mikemaccana
– mikemaccana, Commented Jul 12, 2018 at 9:40

mklement0 · Accepted Answer · 2018-07-14 04:03:03Z

^{Based on past experience, there's no point in waiting for PetSerAl to post an answer, though his pointers are always appreciated.}

Your options are:

Either: Use an expression to construct the path:

node  ((Get-ScriptDirectory) + '/yarn.js') $Args

Or: Use an expandable string (string interpolation):
```
node  "$(Get-ScriptDirectory)/yarn.js" $Args
```

Background:

(...) is required to enclose a single expression or command in a context where a command argument or subexpression is expected (which could be a command invocation that itself needs to be wrapped in (...) or an embedded expression whose evaluation precedence needed clarification).
$(...)is the - more flexible, but slower - subexpression operator:
- In order to embed an expression or command(s) in an expandable string ("..."), you must use $(...) - (...) wont' work.
- Outside of an expandable string, while $(...) generally also works, there's generally no need to use it, unless you need to execute multiple statements.
  - For a single expression / command, (...) is not only sufficient but performs better.

Additional information:

To learn about PowerShell's fundamental parsing modes, see Get-Help about_Parsing
To learn about PowerShell's string expansion (string interpolation) rules, see this answer of mine.
For a comprehensive discussion of how unquoted command-line tokens are parsed in argument mode (as command arguments), see this answer of mine.

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