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Is it possible to write self-modifying PowerShell scripts?

Suppose I have a script named foo.ps1:

"Write-Host `"Goodbye world`"" | Set-Content "foo.ps1"
Write-Host "Hello world"

This script prints out Hello world. What techniques could I use to get it to print out Goodbye world?

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2 Answers 2

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To me it does not make sense, after you have modified a script, you need to re-dotsource it in order to get new statments.

This script changes itself three times:

Write-Host "Hello world"
"Write-Host `"Goodbye world`"" | Set-Content "foo.ps1"
. .\foo.ps1
"Write-Host `"Is really useful?`"" | Set-Content "foo.ps1"
. .\foo.ps1

and prints:

Hello World
Goodbye World
Is really useful?

Another possibility is to write a script which accepts as input parameters script blocks.

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6 Comments

The example script I provided is trivial and contrived; there are lots of potential uses for self-modifying code (stackoverflow.com/questions/516688/…). Your idea to dot-source the script is one answer to my question. I'm curious if there are other techniques for changing a script as it runs.
You asked for a self-modifying script. There is no other ways to load new data in the current working session, than dot-source.
Powershell is not really the proper language here. You might be interested in this thread.
You could have some kind of master Driver script, that is kicked off by the user. It then calls a subscript and dot sources it.. That script could modify itself and then send a return code that it should be reran and re-dot sourced. As stated above, PowerShell isn't really build for these kinds of lofty concepts in mind.
It can be useful for persisting settings the next time you run the script. i.e. if([string]::IsNullOrWhiteSpace($projPath)) { $projPath = Read-Host -Prompt "Where is the project file?" } Now we can save that in the script so next time we don't need to ask. (sorry, comments are not meant for code)
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Yes you an however I would be there is a better way to do what you and to accomplish. 

C:(...)WindowsPowerShell >.\foo.ps1
Hello world
C:(...)WindowsPowerShell >.\foo.ps1
Goodbye world
C:(...)WindowsPowerShell >
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1 Comment

I would like to my script to be called once from the command line. With that in mind, you are basically suggesting that my script call itself after modifying itself.

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