#apt-get install monit

Once installed, you’ll find the main configuration file located at /etc/monit/monitrc. Below is sample global configuration file, which will also explain some keywords. for more information do man monit

# poll at 2-minute intervals. Monit will wakeup every two minute to
# monitor things. Time must be given in seconds.
set daemon  120

#If port 25 broken for primary SMTP server,
#all alerts will redirect to secondary SMTP server or
#localhost after 15 seconds timeout.
set mailserver mail.bar.com, mail.foo.com port 10025,
    localhost with timeout 15 seconds

# You can define your mail-notification format. Do man monit
mail-format {
   from: foo@bar.com
   subject: $SERVICE $EVENT at $DATE
   message: Monit $ACTION $SERVICE at $DATE on $HOST,

   Yours sincerely,
   Foo Bar.
   }

# Make monit start its web-server. So you can access it from webrowser.
set httpd port 2812 and
use address 192.168.0.1 # or bar.com

##Monit web-server ACL.
allow localhost       # allow localhost to connect to the server and
allow 192.168.1.2     # allow 192.168.1.2 to connect to the server,
                     # You can give only one entry per line.

allow foo:bar     # user name and password for authentication.

allow bar:bar   # set multiple user to access through browser.

Below is some of the checks that can monitor the various services on the server.

check process apache with pidfile /var/run/apache2.pid
  start program = "/etc/init.d/apache2 start"
  stop  program = "/etc/init.d/apache2 stop"
  if failed host 127.0.0.1 port 80
       protocol http then restart
  if 5 restarts within 5 cycles then timeout

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

check process sshd with pidfile /var/run/sshd.pid
  start program  "/etc/init.d/ssh start"
  stop program  "/etc/init.d/ssh stop"
  if failed port 22 protocol ssh then restart
  if 5 restarts within 5 cycles then timeout

check process named with pidfile /var/run/named.pid
  start program = "/etc/init.d/bind start"
  stop program = "/etc/init.d/bind stop"
  if failed host 127.0.0.1 port 53 type tcp then alert
  if failed host 127.0.0.1 port 53 type udp then alert
  if 5 restarts within 5 cycles then timeout

check process exim4 with pidfile /var/run/exim4/exim.pid
  start program = "/etc/init.d/exim4 start"
  stop program = "/etc/init.d/exim4 stop"
  if failed host 127.0.0.1 port 25 protocol smtp then alert
  if 5 restarts within 5 cycles then timeout

check process clamavd with pidfile /var/run/clamav/clamd.pid
  start program = "/etc/init.d/clamav-daemon start"
  stop  program = "/etc/init.d/clamav-daemon stop"
  if failed unixsocket /var/run/clamav/clamd.ctl then restart
  if 5 restarts within 5 cycles then timeout

You can also include other configuration files via include directives:

include /etc/monit/default.monitrc
include /etc/monit/mysql.monitrc

After modifying the configuration file you should check for the syntax to make sure they are correct. To do this run:

# monit -t

Now you can run monit directly:

# monit

Once monit is running you can check for activity with your web-browser. You can of course use the Debian init script to start the monitoring:

/etc/init.d/monit start

For this to work you must enable the service by changing the file /etc/default/monit:

# You must set this variable to for monit to start
startup=1

# To change the intervals which monit should run uncomment
# and change this variable.
# CHECK_INTERVALS=180

For more example of monit check the monit homepage.