userdata - interpolate stack variables into file

[brettswift]

It'd be great if we could interpolate variables within an external userdata file.

Problem statement:

given this:
UserData=userdata.from_file('./templates/userdata/userdata.sh'),

and a requirement to drop in a cfn-signal that contains parameters at the end of the script:

    "cfn-signal -e 0",
        "    --resource AutoscalingGroup",
        "    --stack ", Ref("AWS::StackName"),
        "    --region ", Ref("AWS::Region"), "\n"

I'd like to have a way to drop params and have them replaced within the external file. :)

Maybe there's a magical way I haven't come across?

[phobologic]

@brettswift In stacker we have the file lookup, which handles loading a file and replacing certain patterns with Ref statements. You can see that here:

https://github.com/remind101/stacker/blob/master/stacker/lookups/handlers/file.py

[markpeek]

I haven't looked at the code but could there be a variant that does dictionary based string formatting against the file being imported?

[brettswift]

@phobologic cool - I'm pretty new to trop but stacker is on the radar as well. I'm more familiar with ruby but I'm sure there's a robust templating language on the python side that would be a good candidate. At risk of bloating the code for a fringe case.

The simple one would be along the lines of @markpeek - pass a dict into the function and find / replace. Likely not a massively hard PR to throw in. Even for a ruby guy :)

[brettswift]

Another use case:
This would need to be a generic function for other files not just userdata.

For instance putting environment variables into a buildspec file for codebuild.

It's nicer to have a buildspec as a .yml file a opposed to inline with \n carriage returns all over :)

I haven't done the work around yet. I'm guessing someone has?

[jaekunchoi]

#724 should address this issue. I have tested it. It would be great if we can merge this one in @markpeek

[brettswift]

@hybmg57 that will still result in a Base64 string though?

#683 is another ticket I created that might solve this one, if we can bring files in that aren't base64 encoded, then you can wrap the result in a Sub() function and use template variables directly in the files.