[feature] a File() helper than can interpolate references for UserData scripts

[cfndsl]

[feature] a File() helper than can interpolate references for UserData scripts

[cmaxwellau]

Surely this can be closed now

[stevenjack]

@rprieto I know the pain you feel it's a real pain creating the userdata scripts line-by-line when you need to interpolate stack params inside the file.

Currently it's not something cfndsl supports natively. I think as you mentioned in your last comment, it's not that being able to turn a file into the string type cfndsl expects would be difficult but more how we express all the different types such as FindInMap in the file you'd like to parse being the harder part.

Let me have a think about this, there might be a more elegant way around it. @howech any thoughts?

[rprieto]

Thanks for getting back to me, keen to see how the conversation progresses!

[howech]

Cfndsl adds a fake function called "FnFormat" that turns interpolates strings with %{tag} style tags into Fn::Join and Fn::Ref functions as required. For instance FnFormat ( "Hello %{world}") would turn into the cfn jason {"Fn::Join":["", ["Hello ", {"Ref": "world"}]}. Between that an ruby ' File.read call, you can probably get a long way towards what you are looking for.

[herebebogans]

The addition of FnSub() now makes this pretty simple - variables in template get interpolated via ${} tags at CF runtime. So for your example.

cfndsl template

USER_DATA=File.read('userdata.sh')
....snipped...
UserData FnBase64(FnSub(USER_DATA))

userdata.sh

#!/bin/bash -vx
/opt/aws/bin/cfn-init -s ${AWS::StackId} -r LaunchConfig --region ${AWS::Region}
sh /tmp/setup.sh
sleep 5
/opt/aws/bin/cfn-signal -e $? --stack ${AWS::StackName} --resource ServiceFleet --region ${AWS::Region}