Monit is not available from the system base repositories, you need to add and enable third party epel repository to install monit package under your RHEL/CentOS systems.

# wget http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm
# sudo rpm -Uvh epel-release-6*.rpm

Install Monit

# yum install monit

Monit has it’s web interface that runs on port 2812 using web server. To enable web interface you need to make changes in monit configuration file.

# nano /etc/monit.conf

Uuncomment the following section and add the IP address or domain name of your server, allow anyone to connect and change monit user and password or you can use default ones.

 set httpd port 2812 and
     use address localhost  # only accept connection from localhost
     allow localhost        # allow localhost to connect to the server and
     allow admin:monit      # require user 'admin' with password 'monit'
     allow @monit           # allow users of group 'monit' to connect (rw)
     allow @users readonly  # allow users of group 'users' to connect readonly

Restart Monit

# /etc/init.d/monit start

Add Monit to start at boot

# chkconfig monit on

in /etc/monit.d

Add httpd

# nano /etc/monit.d/httpd
check process httpd with pidfile /var/run/httpd/httpd.pid
start program = /etc/init.d/httpd start
stop  program = /etc/init.d/httpd stop
if failed host 127.0.0.1 port 80
protocol http then restart
if 5 restarts within 5 cycles then timeout
#Use for specific host
#if failed host domain.com port 80 protocol HTTP
#    then restart
#if totalmem > 75% for 2 cycles then restart

Add mysql

#nano /etc/monit.d/mysql
check process mysqld with pidfile /var/run/mysqld/mysqld.pid
start program = "/etc/init.d/mysqldd start"
stop  program = "/etc/init.d/mysqld stop"
if failed host 127.0.0.1 port 3306 then restart
if 5 restarts within 5 cycles then timeout

Nginx

check process nginx with pidfile /var/run/nginx.pid
start program = "/etc/init.d/nginx start"
stop program = "/etc/init.d/nginx stop"

Check the syntax

# monit -t

Restart monit

# service monit restart

You can verify that monit service is started by checking log file.

# tail -f /var/log/monit
[BDT Apr  3 03:06:04] info     : Starting monit HTTP server at [localhost:2812]
[BDT Apr  3 03:06:04] info     : monit HTTP server started
[BDT Apr  3 03:06:04] info     : 'tecmint.com' Monit started
[BDT Apr  3 03:06:04] error    : 'nginx' process is not running
[BDT Apr  3 03:06:04] info     : 'nginx' trying to restart
[BDT Apr  3 03:06:04] info     : 'nginx' start: /etc/init.d/nginx