Install Nagios4 on Ubuntu 16.x and probably other Debian Jessie -based OS's. Works on Raspberry Pi too!

install_nagios4.sh

#!/bin/sh

# Downloads and installs Nagios4, Plugins, and NRPE
# https://gist.github.com/davecozz/fb94e2e68eb8709c7ff0d654b754e677

set -e

## check latest version here: https://sourceforge.net/projects/nagios/files/nagios-4.x/
NAGIOS_SOURCE_URL=http://downloads.sourceforge.net/project/nagios/nagios-4.x/nagios-4.2.4/nagios-4.2.4.tar.gz
## check latest version here: https://www.nagios.org/downloads/nagios-plugins/
NAGIOS_PLUGINS_URL=https://nagios-plugins.org/download/nagios-plugins-2.1.4.tar.gz
## check latest version here: https://github.com/NagiosEnterprises/nrpe/releases/
NAGIOS_NRPE_URL=https://github.com/NagiosEnterprises/nrpe/releases/download/3.0.1/nrpe-3.0.1.tar.gz
NAGIOS_SOURCE_DIR=/usr/local/src/nagios4
NAGIOS_TMP_DIR=/tmp/nagios4
NAGIOS_PREREQ_PKGS="build-essential libgd2-xpm-dev openssl libssl-dev xinetd apache2 apache2-utils libapache2-mod-php curl unzip"
NAGIOS_USER=nagios
NAGIOS_GROUP=nagcmd
APACHE_DIR=/etc/apache2
APACHE_SITES_DIR=${APACHE_DIR}/sites-available
APACHE_ENABLED_DIR=${APACHE_DIR}/sites-enabled
CPU_CORES=`nproc`

apt-get update && apt-get install --yes $NAGIOS_PREREQ_PKGS

useradd $NAGIOS_USER || true
groupadd $NAGIOS_GROUP || true
usermod --append --groups $NAGIOS_GROUP $NAGIOS_USER
usermod --groups $NAGIOS_GROUP www-data

a2enmod rewrite
a2enmod cgi

curl --location --create-dirs --output ${NAGIOS_TMP_DIR}/nagios4.tar.gz $NAGIOS_SOURCE_URL

mkdir --parents $NAGIOS_SOURCE_DIR

tar xvzf ${NAGIOS_TMP_DIR}/nagios4.tar.gz --directory $NAGIOS_SOURCE_DIR

cd ${NAGIOS_SOURCE_DIR}/nagios-4*
./configure --with-nagios-group=$NAGIOS_USER --with-command-group=$NAGIOS_GROUP --with-httpd-conf=$APACHE_SITES_DIR
make -j${CPU_CORES} all
make install
make install-commandmode
make install-init
make install-config
make install-webconf
ln -s ${APACHE_SITES_DIR}/nagios.conf ${APACHE_ENABLED_DIR}/
cd -

NAGIOS_HTPASSWD_FILE=`grep AuthUserFile ${APACHE_SITES_DIR}/nagios.conf | tail -1 | awk '{print $NF}'`

tee /etc/systemd/system/nagios.service <<-'EOF'
[Unit]
Description=Nagios
BindTo=network.target

[Install]
WantedBy=multi-user.target

[Service]
User=nagios
Group=nagios
Type=simple
ExecStart=/usr/local/nagios/bin/nagios /usr/local/nagios/etc/nagios.cfg
EOF

echo "-----------------"
read -p "Install Nagios Plugins? (Y/n) " yn
if [ -z "$yn" ] || [ "$yn" = "y" ] || [ "$yn" = "Y" ]; then
  curl --location --create-dirs --output ${NAGIOS_TMP_DIR}/nagios_plugins.tar.gz $NAGIOS_PLUGINS_URL

  tar xvzf ${NAGIOS_TMP_DIR}/nagios_plugins.tar.gz --directory $NAGIOS_SOURCE_DIR

  cd ${NAGIOS_SOURCE_DIR}/nagios-plugins-*
  ./configure --with-nagios-user=$NAGIOS_USER --with-nagios-group=$NAGIOS_GROUP --with-openssl
  make -j${CPU_CORES} all
  make install
  cd -
else
  echo "Not installing plugins."
fi

echo "-----------------"
read -p "Install NRPE? (Y/n) " yn
if [ -z "$yn" ] || [ "$yn" = "y" ] || [ "$yn" = "Y" ]; then
  curl --location --create-dirs --output ${NAGIOS_TMP_DIR}/nagios_nrpe.tar.gz $NAGIOS_NRPE_URL

  tar xvzf ${NAGIOS_TMP_DIR}/nagios_nrpe.tar.gz --directory $NAGIOS_SOURCE_DIR

  ARCH=`arch`
  if [ `echo $ARCH | grep x86_64` ]; then
    SSL_LIB=/usr/lib/x86_64-linux-gnu
  elif [ `echo $ARCH | grep arm` ]; then
    SSL_LIB=/usr/lib/arm-linux-gnueabihf
  else
    echo "UNABLE TO DETERMINE ARCHITECTURE, NEXT STEP WILL PROBABLY FAIL!"
  fi

  cd ${NAGIOS_SOURCE_DIR}/nrpe-*
  ./configure --enable-command-args --with-nagios-user=$NAGIOS_USER --with-nagios-group=$NAGIOS_GROUP --with-ssl=/usr/bin/openssl --with-ssl-lib=$SSL_LIB
  make -j${CPU_CORES} all
  make install
  make install-config
  make install-init
  cd -
  systemctl enable nrpe
  systemctl restart nrpe
else
  echo "Not installing NRPE."
fi

systemctl enable apache2
systemctl enable nagios

systemctl restart apache2
systemctl restart nagios

echo ""
echo ""
NAGIOS_HTPASSWD_DIR=`dirname $NAGIOS_HTPASSWD_FILE || true`
if [ -d $NAGIOS_HTPASSWD_DIR ]; then
  echo "Please type a password for the 'nagiosadmin' user."
  htpasswd -c $NAGIOS_HTPASSWD_FILE nagiosadmin
else
  echo "Could not determine the dir to contain the htpasswd file to use for 'nagiosadmin' auth (I got this: ${NAGIOS_HTPASSWD_DIR})."
  echo "Please set the password for 'nagiosadmin' manually!"
fi
echo ""
echo "DONE!"

Install NRPE should be from https://github.com/NagiosEnterprises/nrpe/releases/download/nrpe-3.1.1/nrpe-3.1.1.tar.gz