M4 is so impotent and crippling difficult to use, that it led me to create my own dumb templating engine[0] because there is no standard POSIX tool that will let you, simply and in the dumbest way possible, substitute strings within a file.

If you try using awk or sed, you will need to figure out a magical way to escape whatever arbitrary value you are trying to replace, so you don't break sed's regex or awk's syntax.

M4 was one of the last things I tried, and it was an utter disaster. If any of M4's magical characters collides with the syntax of the file you're trying to template: tough luck. And there is no way to escape your file for M4 either, it's syntax doesn't support that.

I eventually discovered that Bash 4.4+ has some built-in functions for "dumb" string substitution[1], which is exactly what's missing from the default POSIX tools.

Since then I wrote this little wrapper around that functionality, as a simple, single-file, dependency-free Bash script. If you find something like that useful, it's on the link below:

[0] https://github.com/luizberti/templ

[1] https://www.tldp.org/LDP/abs/html/string-manipulation.html