detecting installer iptables and firewalld with if conditions / bash - bash

Question

Creating script to detect
installer (yum or apt-get)
iptables
firewalld
Current system:
Debian 8
iptables NOT installed
firewalld NOT installed
Theoretically it must be working, but missing something:
#!/bin/bash
installer_check () {
if [[ $(apt-get -V >/dev/null 2>&1) -eq 0 ]]; then
installer=apt
elif [[ $(yum --version >/dev/null 2>&1) -eq 0 ]]; then
installer=yum
fi
}
frw_det_yum () {
if [[ $(rpm -qa iptables >/dev/null 2>&1) -ne 0 ]]; then
ipt_status_y=installed_none
elif [[ $(rpm -qa firewalld >/dev/null 2>&1) -ne 0 ]]; then
frd_status_y=installed_none
fi
}
frw_det_apt () {
if [[ $(dpkg -s iptables >/dev/null 2>&1) -ne 0 ]]; then
ipt_status_a=installed_none
elif [[ $(dpkg -s firewalld >/dev/null 2>&1) -ne 0 ]]; then
frd_status_a=installed_none
fi
}
echo "checking installer"
installer_check
echo -e "$installer detected"
if [ "$installer" = "yum" ]; then
echo "runing firewallcheck for yum"
frw_det_yum
echo $ipt_status
fi
if [ "$installer" = "apt" ]; then
echo "checking installer for apt"
frw_det_apt
echo $frd_status_a
fi
output I'm getting:
~# ./script
checking installer
apt detected
checking installer for apt
So in this current system I'm not getting any value for $frd_status_a

Answer 1

You expect the body of the following to be invoked if firewalld is not installed:
if [[ $(dpkg -s firewalld >/dev/null 2>&1) -ne 0 ]]; then
frd_status_a=installed_none
fi
However, let's look at what this actually does:
redirect stdout and stderr of the command dpkg -s firewalld to /dev/null
capture the stdout of that command, and compare it numerically to the value 0
If the stdout of that command (which has no stdout because you redirected it) has a numeric value other than 0, then we set the flag.
Of course that flag will never be set, no matter what the dpkg command does when it's invoked, and no matter what its output is.
Consider instead:
if ! dpkg-query -l firewalld; then
frd_status_a=installed_none
fi