Use variable scope in Windows PowerShell scripts
Intuitively, you would assume that the variable named $computer that you set in a function could be accessed in the script when the function is complete. However, that's not the case. Variables have a specific scope and are limited in how they interact between scopes.
The following table describes the three scopes and how they affect variable use.
Table 1: Scopes
| Scope | Description |
|---|---|
| Global | The global scope is for the Windows PowerShell prompt. Variables set at the Windows PowerShell prompt can be reviewed in all the scripts started at that Windows PowerShell prompt. Variables created at a Windows PowerShell prompt don't exist in other Windows PowerShell prompts or in instances of the Windows PowerShell Integrated Scripting Environment (ISE). |
| Script | The script scope is for a single script. Variables set within a script can be reviewed by all the functions within that script. If you set a variable value in the script scope that already exists in the global scope, a new variable is created in the script scope. There are then two variables of the same name in two separate scopes. At this point, when you review the value of the variable in the script, the value of the variable in the script scope is returned. |
| Function | The function scope is for a single function. Variables set within a function aren't shared with other functions or the script. If you set a variable value in the function scope that already exists in the global or script scope, a new variable is created in the function scope. Then, there are two variables of the same name in two separate scopes. |
Note
To avoid confusion, it's a best practice to avoid using the same variable names in different scopes.
In addition to reviewing a variable in a higher-level scope, you can also modify that variable by specifically referencing the scope of the variable when you modify it. To modify a script scope variable from a function, use the following syntax:
$script:var = "Modified from function"
It's a best practice to avoid modifying variables between scopes, because doing so can cause confusion. Instead, set the script scope variable equal to the output of the function. If the data in the function is in a variable, you can use Return() to pass it back to the script.
The following is an example of using Return() at the end of a function to pass a variable value back to the script scope:
Return($users)
Note
Using Return() in a function adds the specified data to the pipeline of data being returned, but doesn't replace existing data in the pipeline. As part of script development, you need to verify exactly which data is being returned by a function.