In a powershell script, if I try to use an undefined variable, it continues on its way, not indicating by any warning or error that I did so.
For example, the following script
echo "a"
echo $nonexistent_variable
echo "c"
gives the output
a
c
Is there any way to get powerShell to let me know that the variable I'm trying to use is undefined?
You could consider using the strict mode:
Set-strictmode -version latest
This will give you warning and errors if you do common mistakes (such as using an undeclared variable etc).
You could also use PSDebug -Strict
Set-PSDebug -Strict
Unlike the Set-PSDebug cmdlet, Set-StrictMode affects only the current scope and its child scopes. Therefore, you can use it in a script or function without affecting the global scope. Source: learn.microsoft.com/en-us/powershell/module/…
Set-StrictMode -Version 1 is sufficient; higher versions include additional (potentially unwanted) strictness checks and -Version Latest is best avoided; as the docs (now) note: "Caution: Using Latest for -Version in scripts isn't deterministic. The meaning of Latest can change in new releases of PowerShell. A script written for an older version of PowerShell that uses Set-StrictMode -Version Latest is subject to more restrictive rules when run in a newer version of PowerShell."Use Set-PSDebug's -Strict switch:
-Strict
Specifies that the interpreter should throw an exception if a variable is referenced before a value is assigned to the variable
#I belong in your PowerShell (ISE)-profile
Set-PSDebug -Strict
While PowerShell does have Set-PSDebug and Set-StrictMode, fundamentally it is a bad language and is not parsing scripts properly before executing them. It will gladly ignore code that is invalid that it has not run. There is no way to validate Powershell code without executing it.
Set-PSDebug -strict
Set-StrictMode -version latest
$ErrorActionPreference = 'Stop'
if($false)
{
Write-Output $fnord #undeclared variable
}
#powershell should halt because of an error and not get here.
write-output "this language is clownshoes"
yields
this language is clownshoes
Contrast this to parse errors, which behave reasonably:
Set-PSDebug -strict
Set-StrictMode -version latest
$ErrorActionPreference = 'Stop'
if($false)
{
() #parseerror
}
#powershell should halt because of an error and not get here.
write-output "this text will not show"
yields
At line:9 char:4
+ () #parseerror
+ ~
An expression was expected after '('.
+ CategoryInfo : ParserError: (:) [], ParentContainsErrorRecordException
+ FullyQualifiedErrorId : ExpectedExpression
So, in short, while you can have powershell error out when it attempts to read from an undeclared variable, there is no way to have powershell properly check your script for undeclared variables at parse-time.
Using Set-StrictMode -Version 1 or higher makes PowerShell report a by default statement-terminating error whenever a nonexistent variable is referenced at runtime.
Note that this means that execution continues by default, unless $ErrorActionPreference = 'Stop' is in effect or - for code inside an advanced function or script - if -ErrorAction Stop was passed to it, in which case any error results in a script-terminating (runspace-terminating) error.
Note:
PowerShell's dynamic scoping makes it impossible to detect references to nonexistent variables at parse time; thus, errors only surface at runtime.
-Version 2 and above include additional strictness checks, whose ramifications must be considered; -Version Latest
Caveat:
The specified script mode affects the calling scope and its descendent scopes, which requires careful management of when the desired mode is in effect, possibly selectively reverting to no strict mode with Set-StrictMode -Off
Specifically, third-party PowerShell code that is invoked while a strict mode is in effect can break if it wasn't designed with that mode in mind. De facto, much existing code relies on no script mode being in effect (without explicitly turning it off) and instead relying on references to nonexistent variables to default to $null.
Set-StrictMode