Master Linux Monitoring with Monit: Installation, Configuration, and Real‑World Examples

Monit is a lightweight monitoring tool for Linux that can watch system resources, processes, files, directories, and devices, automatically restart failed services, and provide a built‑in web UI with color‑coded status indicators and email notifications.

Installation

On Ubuntu, install Monit with: sudo apt-get install monit On CentOS, use: yum install monit For source installation:

wget http://mmonit.com/monit/dist/monit-5.5.tar.gz
tar zxvf monit-5.5.tar.gz
cd monit-5.5
./configure --prefix=/usr
make
make install

If configure reports missing PAM headers, install the development package: apt-get install libpam0g-dev For SSL library errors, install the SSL development package and, if necessary, point configure to the library directory:

sudo apt-get install libssl-dev
./configure --prefix=/usr --with-ssl-lib-dir=/usr/lib/i386-linux-gnu/

After compilation, copy the sample configuration file and set proper permissions:

mv monitrc /etc/monitrc
chown root:root /etc/monitrc
chmod 0700 /etc/monitrc

Starting Monit and Web UI

Start the daemon: monit The built‑in HTTP server listens on port 2812. A basic configuration looks like:

set httpd port 2812 and
    use address localhost
    allow localhost
    allow admin:monit
    allow @monit
    allow @users readonly

To allow remote access, modify the address and credentials:

set httpd port 2812 and
    use address 10.64.74.99
    allow 0.0.0.0/0.0.0.0
    allow manu:manu

Now the UI can be reached at http://10.64.74.99:2812.

Basic Settings

set daemon 60               # check every 60 seconds
set logfile /var/log/monit.log

Email Alerts

set mailserver xxx.xx.xxx.xxx port 25 with timeout 15 seconds
set mail-format {
    from: [email protected]
    subject: monit alert -- $EVENT $SERVICE
    message: $EVENT Service $SERVICE
    Date: $DATE
    Action: $ACTION
    Host: $HOST
    Description: $DESCRIPTION
    Your faithful employee,
    bean
}
set alert [email protected]

Monitoring Examples

1. Process Monitoring

check process apache with pidfile /var/run/httpd.pid
    start program = "/etc/init.d/rcWebServer.sh start https"
    stop program = "/etc/init.d/rcWebServer.sh stop https"
    if changed pid then alert

For processes without a PID file, use matching on the command line:

check process test_abc with MATCHING test_abc
    if changed pid then alert

2. Filesystem / Device Monitoring

check device VAR_LOG with path /var/log
    if space usage > 85% then alert

check filesystem tmpfs with path /var
    if space usage > 80% then alert

3. File Monitoring

check file monit_target with path /var/log/monit_target
    if failed permission 700 then alert
    if size > 1000 MB then exec "/usr/local/bin/rotate /var/log/monit_target"

4. Program Monitoring (Custom Script)

check program COREDUMP_EXIST with path "/var/log/tool/coredump_exist.sh"
    if status != 0 then alert

#!/bin/sh
corefile_num=`ls /home/manu/core/core* 2>/dev/null | wc -l`
if [ $corefile_num -eq 0 ]; then
    exit 0
else
    exit 1
fi

5. Host Monitoring

check host my_brother with address XX.XX.XX.XX
    if failed icmp type echo count 3 with timeout 3 seconds then alert

For detailed options, refer to the official Monit documentation at http://mmonit.com/monit/documentation/monit.html#connection_testing .