Writing a Language Server in Bash

  # Capture the content-length header value
  [[ "$line" =~ ^Content-Length:\ ([0-9]+) ]]

Be forewarned that the space after any http header is optional <https://www.rfc-editor.org/rfc/rfc9112#name-field-syntax> so require it in your match at your peril. And, of course, http headers are case insenstiive so checking for the upper in any position is rolling the dice

  id=$(echo -E "$json_payload" | jq -r '.id')

that answers my question about how "in Bash" was going to deal with all the json