Powershell Command Processing (Passing in Variables)

Question

I\'m creating a Powershell script to deploy some code and part of the process is to call a command-line compression tool called RAR.EXE to back-up some folders.

I\'m at

Answer 1

The call operator '&' is unnecessary in this case. It is used to invoke a command in a new scope. This is typically used to invoke a command specified by a string or scriptblock. It also has the side benefit that any variables created in say a PowerShell script are discarded after the command finishes and the scope goes away.

However since the cmd is an EXE it executes in a completely different process. FWIW, you get similar output directly from cmd.exe:

> cmd "/c echo foo"
foo"

So the extra quote on the end is a cmd.exe issue. Typically you need to keep the command separate from the parameters when PowerShell is doing the parsing to invoke the command e.g.

45> & { $foo = "foo" }
46> $foo  # Note that $foo wasn't found - it went away with the scope
47> . { $foo = "foo" } # dotting executes in the current scope
48> $foo 
foo

The notable exception here is that Invoke-Expression behaves like an "evaluate this string" function. Use with care, especially if the user provides the string. Your day could suck if they provided "ri C:\ -r".

In this case, as others have suggested I would pull the /c out of the string $param string and specify it e.g.:

cmd /c $param

Or use Invoke-Expression but use with care. BTW when you are trying to debug issues with sending arguments to EXE from PowerShell, check out the echoargs utility in PowerShell Community Extensions (http://pscx.codeplex.com). It is very handy:

49> $param = "/c echo foo"
50> echoargs $param
Arg 0 is 

This shows that cmd.exe receives "/c echo foo" as a single argument. "/c" should be a separate argument from "echo foo" (the command to execute).