BASH script to modify /etc/ufw/before.rules

Question

I'm trying to write a Bash script to automate the process of modifying /etc/ufw/before.rules in Ubuntu Server 18.04

Basically I want to replace ACCEPT for DROP in certain lines.

What I tried to do is assign the original string to a variable and then replace it for a second variable with the desired output.

I failed miserably so here I am asking for help :)

This is what I wrote, it does absolutely nothing but I hope you get the idea:

#!/bin/bash

# makes a backup of the original before.rules file
sudo cp /etc/ufw/before.rules /etc/ufw/before.rules.bak

# ok icmp codes for INPUT
# Replaces ACCEPT for DROP

duA1="-A ufw-before-input -p icmp --icmp-type destination-unreachable -j ACCEPT"
duD1="-A ufw-before-input -p icmp --icmp-type destination-unreachable -j DROP"

teA1="-A ufw-before-input -p icmp --icmp-type time-exceeded -j ACCEPT"
teD1="-A ufw-before-input -p icmp --icmp-type time-exceeded -j DROP"

ppA1="-A ufw-before-input -p icmp --icmp-type parameter-problem -j ACCEPT"
ppD1="-A ufw-before-input -p icmp --icmp-type parameter-problem -j DROP"

erA1="-A ufw-before-input -p icmp --icmp-type echo-request -j ACCEPT"
erD1="-A ufw-before-input -p icmp --icmp-type echo-request -j DROP"

sudo sed -i -e 's/$duA1/$duD1/g' /etc/ufw/before.rules
sudo sed -i -e 's/$teA1/$teD1/g' /etc/ufw/before.rules
sudo sed -i -e 's/$ppA1/$ppD1/g' /etc/ufw/before.rules
sudo sed -i -e 's/$erA1/$erD1/g' /etc/ufw/before.rules


# ok icmp code for FORWARD
duA2="-A ufw-before-forward -p icmp --icmp-type destination-unreachable -j ACCEPT"
duD2="-A ufw-before-forward -p icmp --icmp-type destination-unreachable -j DROP"

teA2="-A ufw-before-forward -p icmp --icmp-type time-exceeded -j ACCEPT"
teD2="-A ufw-before-forward -p icmp --icmp-type time-exceeded -j DROP"

ppA2="-A ufw-before-forward -p icmp --icmp-type parameter-problem -j ACCEPT"
ppD2="-A ufw-before-forward -p icmp --icmp-type parameter-problem -j DROP"

erA2="-A ufw-before-forward -p icmp --icmp-type echo-request -j ACCEPT"
erD2="-A ufw-before-forward -p icmp --icmp-type echo-request -j DROP"

sudo sed -i -e 's/$duA2/$duD2/g' /etc/ufw/before.rules
sudo sed -i -e 's/$teA2/$teD2/g' /etc/ufw/before.rules
sudo sed -i -e 's/$ppA2/$ppD2/g' /etc/ufw/before.rules
sudo sed -i -e 's/$erA2/$erD2/g' /etc/ufw/before.rules

Answer 1

Replace the ' with " in the last 4 lines. Review how the shell processes a command line before evaluating it. In this case, the single quote prevents the shell from expanding the variables contained within the string.