PowerShell scope clash when dot sourcing scripts

Question

I'm having some scope issues when dot sourcing powershell scripts. Suppose I have one script 'A.ps1':

$VERSION = "1.0"

# Dot source B.ps1
. .\B.ps1

function Write-Version { Write-Host "A.ps1 version $VERSION" }
Write-Version

And a script B.ps1

$VERSION = "2.0"
function Write-Version { Write-Host "B.ps1 version $VERSION" }
Write-Version

The output of running A.ps1 will be:

B.ps1 version 2.0
A.ps1 version 2.0

Why this happens is quite obvious. The $VERSION variable from B.ps1 is put into the scope of A.ps1 and overwrites that variable. Indeed, this happens with Write-Version as well, but here A.ps1 overwrites B's version, but because Write-Version is called in B.ps1 before that happens, we can still see the output of B's Write-Version function.

The question, of course, is how to prevent this?? I've tried various scope options, but this doesn't seem to work when dot-sourcing. And since there are functions in B.ps1 that I do need in A's scope, just invoking B.ps1 is probably not an option.

Does anyone have any ideas?

Interesting question. If it were me I would not reuse the same variable name in both scripts if they are used in the same session... — JNK
– JNK, Commented Dec 15, 2011 at 13:54
Everything about your question is valid and so are both answers (that you should use modules) -- except for your premise. You state that B outputs 2.0 and A outputs 1.0. (If that were in fact true, you would have no need to have even posted the question!) In reality, A outputs 2.0, which is your whole point. ( Yes, I know it is just a typo, but what a difference one character makes :-) — Michael Sorens
– Michael Sorens, Commented Dec 15, 2011 at 17:38
@msorens Yes, thank you for spotting that. You're absolutely correct in your reasoning. I'll modify the post. — wensveen
– wensveen, Commented Dec 16, 2011 at 8:45

Rynant · Accepted Answer · 2011-12-15 14:49:34Z

3

You can do it by making B.ps1 a module and renaming it to B.psm1. Add Export-ModuleMember to make your functions available to other scripts.

This would be B.psm1:

$VERSION = "2.0"
function Write-Version { Write-Host "B.ps1 version $VERSION" }
Write-Version

# Only items specified here will be exported. If Export-ModuleMember is not used,
# ALL members (functions, variables, and aliases) will be exported. In this case
# if $VERSION was exported, $VERSION will be set to "2.0" in script A.ps1
Export-ModuleMember -Function Write-Version

And A.ps1 would be:

$VERSION = "1.0"

# Import B.psm1
Import-Module .\B.psm1

function Write-Version { Write-Host "A.ps1 version $VERSION" }
Write-Version

# Use B.psm1's `Write-Version` function
B\Write-Version

edited Dec 15, 2011 at 14:49

answered Dec 15, 2011 at 14:32

Rynant

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2 Comments

Andy Arismendi Over a year ago

Some helpful information about Export-ModuleMember than can prevent confusion in trying to figure out why specific things aren't being imported: If a script module does not include an Export-ModuleMember command, the functions in the script module are exported, but the variables and aliases are not. When a script module includes an Export-ModuleMember command, only the members specified in the Export-ModuleMember command are exported.

Rynant Over a year ago

Good point! Thanks for making note of that. I'll add a comment to the example.

Andy Arismendi · Accepted Answer · 2011-12-15 14:25:16Z

3

Modules were created in Powershell V2 to address these kinds of issues with dot sourcing. Save your script with a psm1 extension and use the Import-Module cmdlet instead of dot sourcing in the code.

answered Dec 15, 2011 at 14:25

Andy Arismendi

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1 Comment

wensveen Over a year ago

Thanks for this answer. I'll mark Rynant's answer because it has a little more information. I wish I could mark both though.

Community · Accepted Answer · 2017-05-23 12:14:09Z

As guys abouve mentioned one of the solution is to convert your script to PS Module.

But, as soon as you don't need to dot-source functions in functions to global source (I face that problem and not sure if there is a way to solve it: question) you can solve your issue this way:

ScopesA.ps1:

$VERSION = "1.0"
$overridenFromAntotherFile = "original"

# Invoke ScopesB.ps1 via &
& .\ScopesB.ps1

Function Write-Version { Write-Host "ScopesA.ps1 version $VERSION" }
Write-Version

Write-Host $overridenFromAntotherFile

ScopesB.ps1:

$VERSION = '2.0'
$global:overridenFromAntotherFile = 'overriden'
function Write-Version { Write-Host "ScopesB.ps1 version $VERSION" }
Write-Version

Output:

ScopesB.ps1 version 2.0
ScopesA.ps1 version 1.0
overriden

The idea is to use & invocation instead of dot-sourcing (you can read about them in my article but it is not much to say more than & invokes something without adding it to current scope and . invokes and adds to scope).

And still, you can access your global scope from ScopeB.ps1 via scope modifier (this is also mentioned in same article with examples). This is explained with $overridenFromAntotherFile variable in script above.

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