How to resolve variables in a Powershell script block

Question

Given I have:

$a = "world"
$b = { write-host "hello $a" }

How do I get the resolved text of the script block, which should be the entre string including write-host:

write-host "hello world"

UPDATE: additional clarifications

If you just print $b you get the variable and not the resolved value

write-host "hello $a"

If you execute the script block with & $b you get the printed value, not the contents of the script block:

hello world

This question is seeking a string containing the contents of the script block with the evaluated variables, which is:

write-host "hello world"

Possible duplicate of Pass arguments to a scriptblock in powershell — arco444
– arco444, Commented Mar 8, 2019 at 13:14
you have a scriptblock and you need to invoke/run the scriptblock. [grin] simply calling the $Var that holds the scriptblock will give the literal content, not run it. you can run it thus ... Invoke-Command -ScriptBlock $b output = hello world — Lee_Dailey
– Lee_Dailey, Commented Mar 8, 2019 at 14:43
I dont believe this is a duplicate because i am not executing the script block - i want a string of its syntax with evaluated variables — alastairtree
– alastairtree, Commented Mar 8, 2019 at 14:57
@Lee_Dailey - as you can see from the answer, you can do that using $ExecutionContext.InvokeCommand.ExpandString($b) — alastairtree
– alastairtree, Commented Mar 8, 2019 at 16:52
@alastairtree - ha! i learned something new! [grin] i will delete this comment soon - and immediately delete my wrong comment to avoid confusing folks. — Lee_Dailey
– Lee_Dailey, Commented Mar 8, 2019 at 17:00

AdminOfThings · Accepted Answer · 2019-03-08 16:59:44Z

15

As in the original question, if your entire scriptblock contents is not a string (but you want it to be) and you need variable substitution within the scriptblock, you can use the following:

$ExecutionContext.InvokeCommand.ExpandString($b)

Calling .InvokeCommand.ExpandString($b) on the current execution context will use the variables in the current scope for substitution.

The following is one way to create a scripblock and retrieve its contents:

$a = "world"
$b = [ScriptBlock]::create("write-host hello $a")
$b

write-host hello world

You can use your scriptblock notation {} as well to accomplish the same thing, but you need to use the & call operator:

$a = "world"
$b = {"write-host hello $a"}
& $b

write-host hello world

A feature to using the method above is that if you change the value of $a at any time and then call the scriptblock again, the output will be updated like so:

$a = "world"
$b = {"write-host hello $a"}
& $b
write-host hello world
$a = "hi"
& $b
write-host hello hi

The GetNewClosure() method can be used to create a clone of the scriptblock above to take a theoretical snapshot of the scriptblock's current evaluation. It will be immune to the changing of the $a value later the code:

$b = {"write-host hello $a"}.GetNewClosure()
& $b
write-host hello world
$a = "new world"
& $b
write-host hello world

The {} notation denotes a scriptblock object as you probably already know. That can be passed into Invoke-Command, which opens up other options. You can also create parameters inside of the scriptblock that can be passed in later. See about_Script_Blocks for more information.

edited Mar 8, 2019 at 16:59

answered Mar 8, 2019 at 13:19

AdminOfThings

25.2k4 gold badges22 silver badges30 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

alastairtree Over a year ago

Sorry, yeah meant $a not $b, although I think the question still stands. Updated the question

alastairtree Over a year ago

This is not correct and misses the crus of the question - printing $b just prints write-host "hello $a" and the question asks for write-host "hello world"

AdminOfThings Over a year ago

Found a short answer. Just run this $executioncontext.invokecommand.expandstring($b).

KrisG Over a year ago

This approach doesn't work for script blocks that contain assignment or local variables, e.g.: $ExecutionContext.InvokeCommand.ExpandString({$x=1; $x}) yields =1;. I am looking for a way to truly serialize a closure to include all referenced variables so that I can execute it outside of the current execution context. Not sure if this is possible as PowerShell uses dynamic scoping for closures according to what-exactly-is-a-powershell-scriptblock.

KrisG Over a year ago

A promising approach might be to extract variables from the SessionState Module and then use ScriptBlock.InvokeWithContext. Would probably need to walk the ScriptBlock AST to correctly substitute variables with strings.

|

Safwan · Accepted Answer · 2024-07-29 11:36:43Z

-1

Another way to do it is:

$a = "world"
$b = "write-host ""hello $a"""
$b

The output is:

write-host "hello world"

answered Jul 29, 2024 at 11:36

Safwan

3645 silver badges12 bronze badges

1 Comment

alastairtree Jun 24 at 16:18

Sorry this is a different question - I need a script block not a string

Collectives™ on Stack Overflow

How to resolve variables in a Powershell script block

2 Answers 2

8 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

8 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related