OSSEC is an Open Source Host-based Intrusion Detection System. It performs log analysis, file integrity checking, policy monitoring, rootkit detection, real-time alerting and active response.

These instructions will download the ossec archive, unzip it, install/update the app, and then remove the files...

go to the installed directory:

cd /var/ossec;

mkdir installation;

cd installation;

nano install-update-ossec.sh

paste this:

wget http://www.ossec.net/files/ossec-hids-2.3.tar.gz;
tar -zxvf ossec-hids-2.3.tar.gz;
cd ossec-hids-2.3;
./install.sh;
cd ..
ls -l;
rm -fv ossec-hids*;
ls -l

this did not provide me with the latest version (but would be the best way):

wget http://www.ossec.net/files/ossec-hids-latest.tar.gz;
tar -zxvf ossec-hids-*.tar.gz;
cd ossec-hids-*;
./install.sh;
cd ..
ls -l;
rm -fv ossec-hids*;
ls -l

make sure you as a user have execute rights

chmod u+x nano install-update-ossec.sh

then to run the script:

./install-update-ossec.sh