Trial #20: Passing a Scriptblock with Arguments to a New Powershell Instance

Problem:

Its quite hard to Pass a Scriptblock with Arguments to a new powershell instance.

In a normal process the following works perfectly:

$arg = "HAM"
$command = {param($ham) write-host $ham}
& $command $arg

However the following and hundred of similar more complex variations produced odd string based output and useless errors.

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

& : The term 'param' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the
spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:3
+ & param($ham) write-host $ham HAM
+   ~~~~~
+ CategoryInfo          : ObjectNotFound: (param:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException

I spent at least an hour trying lots of slightly different syntax arrangements, escaped characters. I even tried passing a command “-“ as I haven’t bothered learning all the meanings of this syntax.

PowerShell[.exe]
         [-Command { - | <script-block> [-args <arg-array>]
                       | <string> [<CommandParameters>] } ]

Also SS64 have a more descriptive but equally elusive, to me, description:

-Command          Execute the specified commands (and any parameters) as though they
                  were typed at the PowerShell prompt, and then exit, unless NoExit is specified.
                  The value of Command can be "-", a string. or a script block.

                  If the value of Command is "-", the command is read from standard input.

Anyway there are lots of ways to get this wrong and lots of bad workarounds.

Solution:

I finally found an acceptable syntax for passing arguments to a new powershell instance. It is actually in line with the above quoted syntax -Command <script-block> [-args <arg-array>].

After many different variations with odd output and useless errors. I did finally get a useful type error using an variation including an “Invoke-Command”.

$arg = "HAM"
$command = {param($ham) write-host $ham}
Start-Process powershell -ArgumentList "-noexit -command invoke-command -scriptblock $command -argumentlist $arg"


Invoke-Command : Cannot bind parameter 'ScriptBlock'. Cannot convert the "param" value of type "System.String" to type
"System.Management.Automation.ScriptBlock".

As this stated that my $command parameter was of type string and must be a script-block I tried to through on another set of curly braces.

In addition to getting better error messages, using the Invoke-Command gives you the recognisable “-ArgumentList” parameter to operate against the given Command that you are missing with the standard powershell.exe parameters.

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. It also works without the invoke-command as follows

$arg = "HAM"
$command = {param($ham) write-host $ham}

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

Update 10/9/2020

Following a comment from Boris Andonov.

I should reinforce that the second last example using Invoke-Command is more powerful than & and will allow you to pass several arguments for example

$text = "'This is a test message.'"

$cmd = { param([string]$msg, [int]$proc); Write-Host "$msg FROM PID: $proc" }

Start-Process powershell -ArgumentList "-noexit -command (Invoke-Command -ScriptBlock {$cmd} -ArgumentList $text, $PID)"

Furthermore although $arg = "HAM" is acceptable $arg = "HAM SANDWICH" is not. “If the string contain spaces it must be encapsulated with double quotes and single quotes.” So we must write, $arg = "'HAM SANDWICH'".

And amazingly if you want to escape double quotes inside a string requires 8 double quotes:

$text = "'This is a """"""""test"""""""" message.'"