Alternatives for heredocs in bashscripts? External templating? [duplicate]

Question

I currently use many heredocs in bash scripts.

These heredocs contain bash variables, e.g. ${MY_VAR}, which are substituted as the script executes and the heredoc is read.

Some of these heredocs have grown to hundreds of lines long, and so I would like to put these in their own files.

Unfortunately this breaks the variable substitution. I could add boilerplate to each file, turning each file into its own bash script (containing their own heredoc, which uses environment variables). This feels like pointless complexity.

Is there a way of making bash perform variable substitution on an external file as it is read in?

Answer 1

Assuming you fully trust these files, one option is to write a function that reads the file and adds the boilerplate that you mention, and then use the source built-in with process substitution to execute the result inside the main script's execution environment.

That is, something like this:

function boilerplatify() {
  local varname="$1"
  local filepath="$2"
  echo "$varname=<<EOF"
  cat -- "$filepath"
  echo EOF
}

source <(boilerplatify my_var /path/to/template)