Differentiate Between Automatic Variables and User Created Variables


I was delivering a PowerShell Work Shop the other week. One of the questions asked by a Mr T. T. was this:

"…How do you show variables created by you in a session?…"


So… Mr T. T. wanted to know how to distinguish between Automatic Variables and user-created variables. A jolly good question and one that's not as easy to answer as one might think!

Here's what I came up with:

powershell.exe -command {(Get-Variable).name > C:\Users\ianfarr\Desktop\automatic_variables.txt} -noprofile

Compare-Object -ReferenceObject (Get-Variable).name -DifferenceObject (Get-Content C:\Users\ianfarr\Desktop\automatic_variables.txt)

 

Line 1

Right, we create a temporary powershell.exe session and make sure it doesn't load a profile script. A profile script may add user-created variables to the session so we don't want those. Get-Variable is executed in the session to generate variable objects. The variables names are written to a text file for future parsing. The variables captured should only be Automatic variables.

 

Line 2

Right, we now use the Compare-Object cmdlet to compare the names of the variables in the current session with those from the temporary session. The output shows just user-created variables:

 

"But, wait, PoSh Chap!", I hear you cry.

"What's that, gentle reader?", I respond.

"Well, isn't LASTEXITCODE an Automatic Variable?"

 

And, you would be right, my erudite friends! This one is only generated when we call a 'windows-based program', e.g. ipconfig or ping. It's still an automatic variable though. How to remove it from the results:

powershell.exe -command {cmd /c; (Get-Variable).name > C:\Users\ianfarr\Desktop\automatic_variables.txt} -noprofile

 

Notice the addition of cmd /c in the script block executed by the -command parameter. This populates LASTEXITCODE for us. Which means it's matched by Compare-Object:

 

New outcome:

 

There you go Mr T.T. – it ain't pretty, but it works!

Comments (1)

  1. turbomcp says:

    very interesting
    thanks

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