PowerShell Deep Dive: Using $MyInvocation and Invoke-Expression to support dot-sourcing and direct invocation in shared PowerShell scripts

When creating PowerShell script (ps1) files to share with the community, there are a few different ways you can configure their intended use.  You can configure a ps1 file so that it contains one or more functions and/or variables that must be loaded into PowerShell before they can be used.  This loading is done via a technique called dot sourcing.  Alternatively you can make the body of the ps1 file be the script itself that you want to share with the community without encapsulating it in a function.  Using this configuration, your script consumers will be required to invoke the script using the absolute or relative path to your ps1 file, prefixing it with the call operator (&) and wrapping it in quotation marks if the path contains a space.  Let’s look at each of these in more detail and some advantages to each approach.

Dot-sourcing a ps1 file is like running the PowerShell script it contains inline in the current scope.  You can pass in parameters when you dot-source a ps1 file, or you can dot-source it by itself.  To dot-source a ps1 file you must use the full absolute or relative path to that file.  Aside from the handling of any parameters, the PowerShell script inside the ps1 file is run as if you typed it in manually into the current scope.  An advantage to this approach is that the variables and functions within the ps1 file that use the default scope will be declared in the current scope and therefore they will be available afterwards without requiring users to know the location of the script file.  This allows users to dot-source a ps1 file in their profile and have the functions and or variables they contain available to them in every PowerShell session they open.  If you had a ps1 file with the path ‘C:\My Scripts\MyScript.ps1’, you would dot-source it like this:

. ‘C:\My Scripts\MyScript.ps1’

Before I get to invoking scripts directly, I need to make an important note about dot-sourcing script files.  Users need to be careful when dot-sourcing script files, because while it is possible to dot-source a script that was intended to be invoked and have it appear to function the same as if you had invoked it, passing parameters and having the script within appear to run as expected, this is not a good practice.  Only dot-source ps1 files containing functions and variables you want available in your current session.  If the ps1 file you are using was intended to be invoked and not dot-sourced, steer clear of the dot-source operator.  Otherwise you risk leaving crumbs (variables and functions) of the script files you dot source behind in your current session, some of which may have been intended to be deleted when they went out of scope (secure strings used to temporarily store passwords, for example).  Since the current scope is the root scope, these won’t go out of scope until you close PowerShell.  I have seen users dot-source ps1 files while passing parameters many times in the online community, and those users should be using the call operator instead — not a good idea.  Now back to invoking scripts directly…

Invoking a script directly is akin to calling a function.  You can pass in parameters when you invoke a ps1 file, or you can invoke the ps1 file by itself.  To invoke a ps1 file you must use the full absolute or relative path to that file.  If that path contains one or more spaces in it, it must be wrapped in quotation marks and the call operator (&) must be used.  Otherwise it will just be treated as a string and output to the console (note: it is a good practice to always use the call operator when invoking a script this way so that it doesn’t matter if spaces are in the path or not — it will just work).  When you invoke a ps1 file, a child scope is created and the contents of that ps1 file are executed within that child scope.  An advantage to this approach is that the script file doesn’t leave anything behind after it is run unless it explicitly declares a function or variable as global.  This keeps the PowerShell environment clean.  If you had a ps1 file with the path ‘C:\My Scripts\MyScript.ps1’, you would call it like this:

& ‘C:\My Scripts\MyScript.ps1’

Between these two approaches, there is no best practice indicating which is the right one to use.  It seems to simply be a matter of preference.  Unfortunately, for the most part it is the script author’s preference, not the script consumer’s.  For script consumers to get ps1 files they find online in the community working they way they want, they may have to modify the file to get it to dot-source correctly, or to run correctly when invoked using the call operator, or they may just copy and paste the script into their own ps1 file or profile to get it running the way they like.  The end result is that each time a ps1 file is updated by its author, the script consumer may have manual steps to take to get that update in their own environment.

What if ps1 files could be created so that they could support both of these configuration approaches.  What if they would always work as expected whether they were dot-sourced or invoked directly?  And what if you want the functionality that the ps1 file provides to work inside of a pipeline, whether you dot-source it and use a function call or invoke it directly inside your pipeline?  Fortunately, PowerShell’s a rich enough scripting language to allow you to do just that.

The first thing you need to do to make this work is to determine how the script file was used.  PowerShell includes a built-in variable called $MyInvocation that allows your script to look at the way it was used.  Among other things, $MyInvocation includes two properties you’ll need to understand when making this work: InvocationName and MyCommand.  InvocationName contains the name of the command that was used to invoke the script.  If you dot-sourced the script, this will contain ‘.’.  If you invoked the script using the call operator, this will contain ‘&’.  If you invoked the script using the path to the script itself, this will contain the exact path you entered, whether it was relative or absolute, UNC or local.  MyCommand contains information that describes the script file itself: the path under which it was found, the name of the script file, and the type of the command (always ExternalScript for ps1 files).  These two pieces of information can be used together to determine how the script was used.  For example, consider a script file called Test-Invocation.ps1 at the root of C on a computer PoShRocks that contains the following script:

if ($MyInvocation.InvocationName -eq ‘&‘) {
    “Called using operator“
} elseif ($MyInvocation.InvocationName -eq ‘.‘) {
    “Dot sourced“
} elseif ((Resolve-Path -Path `
    $MyInvocation.InvocationName).ProviderPath -eq `
    $MyInvocation.MyCommand.Path) {
    “Called using path $($MyInvocation.InvocationName)“
}

Regardless of whether you dot-source Test-Invocation.ps1 or invoke it directly, and regardless of whether you use a relative local path, an absolute local path, or an absolute remote (UNC) path, this script will output how it was used.  Here are a few examples of how you might use this script, with the associated output:

PS C:\> . .\Test-Invocation.ps1
Dot sourced
PS C:\> . C:\Test-Invocation.ps1
Dot sourced
PS C:\> . \\PoShRocks\c$\Test-Invocation.ps1
Dot sourced
PS C:\> & .\Test-Invocation.ps1
Called using operator
PS C:\> & C:\Test-Invocation.ps1
Called using operator
PS C:\> & \\PoShRocks\c$\Test-Invocation.ps1
Called using operator
PS C:\> .\Test-Invocation.ps1
Called using path .\Test-Invocation.ps1
PS C:\> C:\Test-Invocation.ps1
Called using path C:\Test-Invocation.ps1
PS C:\> \\PoShRocks\c$\Test-Invocation.ps1
Called using path \\PoShRocks\c$\Test-Invocation.ps1

As you can see, each time our script knows exactly how it was used, so we can use that to make it behave appropriately in any situation.

Now that we’re armed with that knowledge, let’s add a function to our script that will do something simple, like output the definition of another PowerShell function.  First, we’ll need to write our function:

function Get-Function {
    param(
        [string]$name = $(throw ‘Name is required‘)
    )
    if (-not $name) { throw ‘Name cannot be empty‘ }
    if ($name -match “[^a-z0-9-]“) {
        Write-Error ‘Unsupported character found.‘
    } elseif ($function = Get-Item -LiteralPath “function:$name“) {
    “function $name {
`t$($function.Definition)
}“
    }
}

This function is pretty straightforward.  You call it passing in the name of a function and it outputs the function definition, including the name, to the console.

The next step is to follow up that function definition with a slightly modified version of our Test-Invocation.ps1 script.  Basically we just want to know if the file was invoked or dot-sourced.  If it was invoked, we want to automatically call our Get-Function function and pass the parameters used during the invocation directly through to the Get-Function function call.  If it was dot-sourced, we don’t want to do any additional work because the function will be imported into the current session so that we can use it without the script file, as intended.  This has the added benefit of preventing users from executing script through dot-sourcing that wasn’t intended to be executed.  Here’s the start of the additional script that we’ll need to put after our Get-Function call:

if ($MyInvocation.InvocationName -ne ‘.‘) {
    Get-Function # How do we pass arguments here?
}

This additional piece of script uses a simple if statement to compare $MyInvocation .InvocationName against the dot-source operator.  If they are equal, this portion of the script does nothing, allowing the function to be dot-sourced into the current session without invoking it.  If they are not equal, we know that the script was invoked either directly or using the call operator, so we need to call Get-Function so that the invocation uses the internal function automatically.  But as noted in the comment in the snippet above, how do we pass the arguments that were used during the invocation into the internal function?  There are two possible approaches that I can think of to resolve this.  We could use the param statement at the top of the script to identify the same parameters that are in the Get-Function function.  The problem with this approach is that it’s duplicating code unnecessarily, and I really don’t like duplicating code.  Another approach is to use Invoke-Expression inside of our if statement to pass the parameters received from the invocation of the script directly into the internal function.  The only special trick required in this approach is to only evaluate parameters that start with ‘-‘.  This is necessary so that the parameters of the internal function can be used by name, just like they could if you dot-sourced the script first and then invoked the function.  I think that’s a much better approach, so here’s our updated if statement:

if ($MyInvocation.InvocationName -ne ‘.‘) {
    Invoke-Expression “Get-Function $($passThruArgs = $args; for ($i = 0; $i -lt $passThruArgs.Count; $i++) {if ($passThruArgs[$i] -match ‘^-‘) {$passThruArgs[$i]} else {`”`$passThruArgs[$i]`”}})“
}

In this implementation, if the script file was invoked, Invoke-Expression is used to invoke the Get-Function function, passing arguments received by the script directly through to Get-Function.  And as just mentioned, I use the -match operator to determine if a given argument starts with -, in which case I evaluate it so that I end up calling Get-Function using named variables.  This is a trick that I find applies itself nicely to quite a few situations in PowerShell scripting I do.

At this point, we have a complete script file that can be invoked to execute the internal function directly or dot-sourced to import the internal function into PowerShell, all with a little help from $MyInvocation and Invoke-Expression.  This script can be seen below.

Get-Function.ps1 listing #1:

function Get-Function {
    param(
        [string]$name = $(throw ‘Name is required‘)
    )
    if (-not $name) { throw ‘Name cannot be empty‘ }
    if ($name -match “[^a-z0-9-]“) {
        Write-Error ‘Unsupported character found in $name.‘
    } elseif ($function = Get-Item -LiteralPath “function:$name“) {
        “function $name {
`t$($function.Definition)
}“
    }
}
if ($MyInvocation.InvocationName -ne ‘.‘) {
    Invoke-Expression “Get-Function $($passThruArgs = $args; for ($i = 0; $i -lt $passThruArgs.Count; $i++) {if ($passThruArgs[$i] -match ‘^-‘) {$passThruArgs[$i]} else {`”`$passThruArgs[$i]`”}})“
}

Now, I’m sure you’re thinking that’s great, flexible, etc., but where’s the pipeline support that you mentioned would work as well?  Well, as mentioned earlier, this is also possible in PowerShell although it adds another layer of complexity to the script.  The nice part though is that it will work whether it is used in a pipeline as an invoked ps1 file or as an invoked function that was previously imported by dot-sourcing the ps1 file.  The trick is to use the Begin, Process and End blocks and the $_ variable both in the ps1 file at the root level and in the internal Get-Function function.

At the root scope of the script file, the Begin block is used to declare any functions and variables used in the script.  The process block actually calls the function that is being exposed through the script (in a pipeline if appropriate), and the End block is used for cleanup (although we don’t have any cleanup to do).  Similarly, inside the Get-Function function, the Begin block is used to check parameters that don’t support pipeline input, the Process block is used to check the state of some parameters and actually do the work (using the objects coming down the pipeline if appropriate), and the End block is used for cleanup (although again, we don’t have any).  The end result of adding these to our script and making a few modifications so that users can invoke the script file or the function with -? and get the syntax can be found in Get-Function.ps1 listing #2.

Get-Function.ps1 listing #2:

BEGIN {
  function Get-Function {
    param(
      [string]$name = $null
    )
    BEGIN {
      if (($name -contains ‘-?‘) -or ($args -contains ‘-?‘)) {
        “SYNTAX“ | Write-Host
        “Get-Function [-name] <string>“ | Write-Host
        break
      }
    }
    PROCESS {
      if ($name -and $_) {
        throw ‘Ambiguous parameter set‘
      } elseif ($name) {
        $name | Get-Function
      } elseif ($_) {
        if ($_ -match “[^a-z0-9-]“) {
          throw ‘Unsupported character found.‘
        } elseif ($function = Get-Item -LiteralPath “function:$_“) {
          “function $_ {
`t$($function.Definition)
}“
        }
      } else {
        throw ‘Name cannot be null or empty‘
      }
    }
    END {
    }
  }
}
PROCESS {
  if ($MyInvocation.InvocationName -ne ‘.‘) {
    if ($_) {
      Invoke-Expression “`$_ | Get-Function $($passThruArgs = $args; for ($i = 0; $i -lt $passThruArgs.Count; $i++) {if ($passThruArgs[$i] -match ‘^-‘) {$passThruArgs[$i]} else {`”`$passThruArgs[$i]`”}})“
    } else {
      Invoke-Expression “Get-Function $($passThruArgs = $args; for ($i = 0; $i -lt $passThruArgs.Count; $i++) {if ($passThruArgs[$i] -match ‘^-‘) {$passThruArgs[$i]} else {`”`$passThruArgs[$i]`”}})“
    }
  }
}
END {
}

And there you have it.  Now you know how to create versatile ps1 files that you can share with the community that:

1. Automatically discourage unrecommended usage (executing internal code and processing parameters when dot-sourcing script files not meant to be dot-sourced).
2. Support importing functions and variables via dot-sourcing.
3. Support direct invocation via the path and the call operator (if necessary).
4. Output syntax when called with -?.
5. Work in the pipeline as both a ps1 file and an imported function.

This all may seem very complicated at first, but once you learn how it works it’s really not that complicated at all.  And hopefully the consumers of your script will thank you for all of your hard work in making it possible.

Thanks for reading!

Kirk out.

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