Can't pass a script block as a parameter to powershell.exe via -Command

Question

I'm trying this

$Global:commandBlock={
Start-Transcript -path $projectFolder\gruntLog.txt;
grunt $argList;
Stop-Transcript
}

$cmdProc=start-process powershell -ArgumentList ('-command `$Global:commandBlock') -WorkingDirectory $fwd -PassThru -NoNewWindow:$NoNewWindow

And keep getting $commandBlock : The term '$Global:commandBlock' is not recognized as the name of a cmdlet, function, script file, or operable program.

My guess was it has to do with scope. But making variable global didn't help. Adding -args $commandBlock like that:

-ArgumentList ('-command `$Global:commandBlock -args "-commandBlock:$commandBlock"') 
-ArgumentList ('-command `$Global:commandBlock -args $commandBlock"') 

didn't help

And I'm not sure that I escape variables correctly in the block, read this, but not sure how to apply to my script.

Answer 1

There's a few things which I think are keeping this from working. First, when you're using single quotes, ' you're instructing PowerShell to operate literally. This means that it won't expand variables. Not what you're looking for.

A better way to do this is to do it with an subexpression like this.

$Global:commandBlock={
'ham' >> C:\temp\test.txt
}

$cmdProc=start-process powershell -ArgumentList ("-command $($Global:commandBlock)") -PassThru -NoNewWindow:$NoNewWindow

This will give you the desired results.

Subexpressions are pretty sweet. It lets you embed a mini-scriptblock within a string, and it's then expanded out in the parent string.

"today's date is $(get-date), on system: $($env:COMPUTERNAME)"

today's date is 02/14/2017 11:50:49, on system: BEHEMOTH

Answer 2

There are two major issues (leaving the obvious mistake of attempting to reference a variable inside a single-quoted string aside):

• Any argument you want to pass to a new powershell instance via -Command must be escaped in non-obvious ways if it contains " and/or \ chars, which is especially likely if you're passing a piece of PowerShell source code.

  • The escaping issue can generally be solved by Base64-encoding the source-code string and passing it via the -EncodedCommand parameter - see this answer of mine to a related question for how to do that, but a more concise alternative is presented below.

• If the source code being passed references any variables that only exist in the calling session, the new instance won't see them.

  • The solution is not to reference session-specific variables in the source code being passed, but to pass their values as parameter values instead.

---

To solve the local-variable-not-seen-by-the-new-instance problem, we must rewrite the script block to accept parameters:

$scriptBlock={
  param($projectFolder, $argList)
  # For demonstration, simply *output* the parameter values.
  "folder: [$projectFolder]; arguments: [$argList]"
}

Now we can apply the necessary escaping, using PetSerAl's sophisticated -replace expression from his comment on the question.
We can then invoke the resulting string with & {...} while passing it parameter values (I'm omitting the -WorkingDirectory and -PassThru parameters for brevity):

# Parameter values to pass.
$projectFolder = 'c:\temp'
$argList='-v -f'

Start-Process -NoNewWindow powershell -ArgumentList '-noprofile', '-command', 
  (('& {' + $scriptBlock.ToString() + '}') -replace '\"|\\(?=\\*("|$))', '\$&'), 
    "'$projectFolder'", 
    "'$argList'"

^{For an explanation of the regular expression, again see this answer.}

Note how the variable values passed as parameters to the script block are enclosed in '...' inside a "..."-enclosed string in order to:

• pass the values as a single parameter value.
• protect them from another round of interpretation by PowerShell.

Note: If your variable values have embedded ' instances, you'll have to escape them as ''.

The above yields:

folder: [c:\temp]; arguments: [-v -f]

---

Alternative with a temporary, self-deleting script file:

Using -File with a script file has the advantage of being able to pass parameter values as literals, with no concern over additional interpretation of their contents.

Caveat: As of PowerShell Core v6-beta.3, there is a problem when passing parameter values that start with -: they are not bound as expected; see this GitHub issue.
To work around this problem, the sample script block below accesses only the first parameter by name, and relies on all remaining ones binding via the automatic $Args variable.

# Define the script block to be executed by the new PowerShell instance.
$scriptBlock={
  param($projectFolder)
  # For demonstration, simply *output* the parameter values.
  "folder: [$projectFolder]; arguments: [$Args]"
}

# Parameter values to pass.
$projectFolder = 'c:\temp'
$argList='-v -f'

# Determine the temporary script path.
$tempScript = "$env:TEMP\temp-$PID.ps1"

# Create the script from the script block and append the self-removal command.
# Note that simply referencing the script-block variable inside `"..."`
# expands to the script block's *literal* content (excluding the enclosing {...})
"$scriptBlock; Remove-Item `$PSCommandPath" > $tempScript

# Now invoke the temporary script file, passing the arguments as literals.
Start-Process -NoNewWindow powershell -ArgumentList '-NoProfile', '-File', $tempScript,
  $projectFolder,
  $argList

Again, the above yields:

folder: [c:\temp]; arguments: [-v -f]

Answer 3

I've messed around with the syntax for passing args to a new powershell instance and have found the following works. So many variations fail without a good error message. Maybe it would work in your case?

$arg = "HAM"
$command = {param($ham) write-host $ham}
 #please not its important to wrap your command 
 #in a further script block to stop it being processed to a string at execution
 #The following would normally suffice "& $command $arg"

Start-Process powershell -ArgumentList "-noexit -command & {$command}  $arg"

Also simply using the Invoke-Command gives you the -ArgumentList parameter to opperate against the given Command that you are missing with the standard powershell.exe parameters. This is probably a bit cleaner looking.

Start-Process powershell -ArgumentList "-noexit -command invoke-command -scriptblock {$command} -argumentlist $arg"

No need for any extra complex escaping or unwanted persisted variables. Just keep the script block in curly braces so it remains a script block on arrival in the new session. At least in this simple case...

If you have several string parameters that contain spaces. I found popping the string in a single parenthesis and separating with commas works well. You could also probably pass a predefined array as a single argument.

Start-Process powershell -ArgumentList "-noexit -command invoke-command -scriptblock {$command} -argumentlist '$arg1', '$arg2', '$arg3'"

Answer 4

Will this work:

$Global:commandBlock={
Start-Transcript -path $projectFolder\gruntLog.txt;
grunt $argList;
Stop-Transcript
}

& $Global:commandBlock