No way to escape variables in transpiler parsing

[blakerouse]

There is no easy way to provide a raw ${var} to a computed unit configuration. At the moment the transpiler will determine that to be a variable and try to replace it, which could be something that the unit configuration would not want.

We should support the usage of $${var}, where the transpiler just ignores it and removes the initial $ resulting in the unit configuration get the constant value of ${var}.

At the moment the workaround is to use the pipe | to work around this and provide a default.

${vars|'$vars'}

This is not a great user experience and also has an issue where vars must also not exist or it will do the wrong thing.

For confirmed bugs, please report:

• Version: 8.6.0
• Operating System: All

[cmacknz]

Re-opening, needs to be added to the documentation, likely in https://www.elastic.co/guide/en/fleet/current/dynamic-input-configuration.html. The source for that is in https://github.com/elastic/ingest-docs/blob/5cd3270b3e2d589f2eff8cfaea59e308fcb907e0/docs/en/ingest-management/elastic-agent/elastic-agent-variable-substitution.asciidoc.

[ycombinator]

Re-opening, needs to be added to the documentation, likely in https://www.elastic.co/guide/en/fleet/current/dynamic-input-configuration.html. The source for that is in https://github.com/elastic/ingest-docs/blob/5cd3270b3e2d589f2eff8cfaea59e308fcb907e0/docs/en/ingest-management/elastic-agent/elastic-agent-variable-substitution.asciidoc.

@blakerouse could you please make a PR to https://github.com/elastic/ingest-docs and request @kilfoyle to review it? Thanks!

[blakerouse]

PR for docs is here - elastic/ingest-docs#1151