#!/bin/bash s='@{SOMETEXT_A} becomes $<SOMETEXT_A> @{SOMETEXT_B} becomes $<SOMETEXT_B> @{SOMETEXT_C} becomes $<SOMETEXT_C>' sed 's/@{\([^{}]*\)}/$<\1>/g' <<< "$s"
Standard input is empty
$<SOMETEXT_A> becomes $<SOMETEXT_A> $<SOMETEXT_B> becomes $<SOMETEXT_B> $<SOMETEXT_C> becomes $<SOMETEXT_C>