Here's the process for upgrading Nagios Core 4

Make sure you have a good backup of your existing Nagios Core installation and configuration files. If anything goes wrong or doesn't work, this will allow you to rollback to your old version.

Become the nagios user. Debian/Ubuntu users should use sudo -s nagios.

su -l nagios

Removed the following old HTML files that were used by the web frontend. They have been replaced by PHP equivalents.

rm /usr/local/nagios/share/{main,side,index}.html

Download the source code tarball of the latest version of Nagios Core (visit https://www.nagios.org/download/ for the link to the latest version).

wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-x.y.z.tar.gz

Extract the Nagios Core source code tarball.

tar xzf nagios-x.y.z.tar.gz
cd nagios-x.y.z

Run the Nagios Core configure script, passing the name of the group used to control external command file permissions like so:

./configure --with-command-group=nagcmd

Compile the Nagios Core source code.

make all

Install updated binaries, documentation, and web web interface. Your existing configuration files will not be overwritten by this step.

make install

Verify your configuration files. Correct any errors shown here before proceeding with the next step.

/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg

Restart Nagios Core. Debian/Ubuntu users should use /etc/init.d/nagios restart.

/sbin/service nagios restart

Important: If you are upgrading from version 4.2.4 or before, to version 4.3.0 or after, there are changes that need to be made to the 'nagios.conf' apache configuration file. You can either update it manually, copying the new parts from 'sample-config/httpd.conf' or run

cd /etc/httpd/conf.d
cp nagios.conf nagios.conf.bak
cd /<the nagios build directory>
make install-webconf
service httpd restart

Important: If you are upgrading from version 4.3.3 or before, to version 4.3.4 or after, there are changes that need to made to the initialization script, and to your main Nagios configuration file. You should be able to simply run make install-init to update your init file - depending on if you've made any customizations. For your configuration file, you'll need to update the lock_file directive to point to the new one in use by the initialization file. First, let's update our init file:

make install-init

If you don't know where your initialization script lives, you can find out the value of your new lock file with the following command:

INITFILE=$(grep ^INIT_DIR= Makefile | sed 's/.*=//' | sed 's/.*/&\/nagios/') && grep ^NagiosRunFile $INITFILE

If you do know where your init script lives, you can find the value of your lock file with the following command:

grep ^NagiosRunFile INITFILE

Now that you have the value of the lock file, find the part of your nagios.cfg file that has the lock_file directive, and update it with your new value:

lock_file=[NEW_LOCK_FILE_LOCATION]

That's it - you're done!