Another simple option is envsubst which does one thing well.

I used envsubst when I can but as far as I know you cannot do loop using envsubst. When you use the c preprocessor or php, m4, etc..., you can.

Nah, that's what I meant. It does $-expansion and that's it. If you need conditionals and loop you need templates and macros like others are saying: m4, cpp, handlebars, whatever.

I use it as a low-brain template engine to do only that.

I use envsubst a lot, but without - and + forms it's hard to say it does it well.