#!/bin/bash s='fruit_id "Apple"; salad_id ""; fruit_id "Mango"; salad_id ""; fruit_id "Kiwi"; salad_id "";' sed 's/\(fruit_id "\([^"]*\)"; salad_id \)"";/\1"\2";/g' <<< "$s"
Standard input is empty
fruit_id "Apple"; salad_id "Apple"; fruit_id "Mango"; salad_id "Mango"; fruit_id "Kiwi"; salad_id "Kiwi";