How to get exact command line string from shell?

Question

I know about $*, $@, \"$@\" and even ${1+\"@\"} and what they mean.

I need to get access to the EXACT

Answer 1

But I need to exactly reproduce a user-entered command line to be able to programmatically re-execute it in case of failure

You don't need your exact command line for that; you can reconstruct an equivalent (even if not identical!) shell command yourself.

#!/usr/bin/env bash
printf -v cmd_q '%q ' "$@"
echo "Executed with a command identical to: $cmd_q"

...though you don't even need that, because "$@" can be re-executed as-is, without knowing what the command that started it was:

#!/usr/bin/env bash
printf "Program is starting; received command line equivalent to: "
printf '%q ' "$@"
printf '\n'

if [[ ! $already_restarted ]]; then
  echo "About to restart ourselves:
  exec "$@" # restart the program
fi