Date command to get the current date.

$ date=$(date +%Y-%m-%d)
$ echo $date

Date command to get yesterday’s date.

$ date=$(date --date=yesterday +%Y-%m-%d)
$ echo $date

Date command to get tomorrow’s date.

$ date=$(date --date=tomorrow +%Y-%m-%d)
$ echo $date

$ vim swap-usage.sh

#!/bin/sh
free -m | grep Swap | while read output;
do
  swap=$(echo $output | awk '{print $2}' )
  used=$(echo $output | awk '{ print $3 }' )
  freed=$(echo $output | awk '{ print $4 }' )
  echo "Swap : $swap"
  echo "Used : $used"
  echo "Free : $freed"
  usep=`expr $used \* 100 / $swap`
  echo $usep
  if [ $usep -ge 70 ]; then
    echo "Swap Usage Alert Total Swap: \"$swap\" Used: \"$used ($usep%)\" Free:
\"$freed\" on $(hostname) as on $(date)" |
     mail -s "Alert: Swap Usage space $usep%" mail1@example.com,mail2@example.com
  fi
done

This script will send mails to the specified mails, if the system started to use more than 70% of  swap area. For continuously check the swap usage we need to set a cronjob for executing this script. So that the system should monit the swap usage by executing this script.

Edit the corntab file.

$ sudo crontab -e

Set the cronjob to to execute the script in every 15 Mints.

*/15 * * * * ~/scripts/swap-usage.sh

Restart the Cron

$ sudo /etc/init.d/cron restart

vim disk-usage.sh

#!/bin/sh
df -H | grep -vE '^Filesystem|tmpfs|cdrom' | awk '{ print $5 " " $1 }' | while read output;
do
  echo $output
  usep=$(echo $output | awk '{ print $1}' | cut -d'%' -f1  )
  partition=$(echo $output | awk '{ print $2 }' )
  if [ $usep -ge 85 ]; then
    echo "Running out of space \"$partition ($usep%)\" on $(hostname) as on $(date)" |
     mail -s "Alert: Almost out of disk space $usep%" example@example.com
  fi
done

Set a cronjob to run this script.

*/30 * * * * /bin/sh /path-to-file/disk-usage.sh

1. Create a Firewall script

 vim /etc/set_iptables.bash

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -t nat -A PREROUTING -p tcp -i eth0 -d 192.168.0.10 --dport 555 -j DNAT --to 192.168.0.12:22
iptables -A FORWARD -p tcp -i eth0 -d 192.168.0.12 --dport 22 -j ACCEPT

2. Change the file permission mod

sudo chmod o+x /etc/set_iptables.bash

3. Create a start.stop script

vim /etc/init.d/iptables

#!/bin/bash

RETVAL=0

# To start the firewall
start() {
  echo -n "Iptables rules creation: "
  /etc/set_iptables.bash
  RETVAL=0
}

# To stop the firewall
stop() {
  echo -n "Removing all iptables rules: "
  /sbin/iptables -F
  RETVAL=0
}

case $1 in
  start)
    start
    ;;
  stop)
    stop
    ;;
  restart)
    stop
    start
    ;;
  status)
    /sbin/iptables -L
    /sbin/iptables -t nat -L
    RETVAL=0
    ;;
  *)
    echo "Usage: iptables {start|stop|restart|status}"
    RETVAL=1
esac
exit

4. Change the file permission mod

sudo chmod o+x /etc/init.d/iptables

5. The final step is to make your script running on each boot of your computer:

sudo update-rc.d iptables defaults

Now you can use these commands to start/stop/restart/status your iptables.

sudo /etc/init.d/iptables start
sudo /etc/init.d/iptables stop
sudo /etc/init.d/iptables restart
sudo /etc/init.d/iptables status

One of the more powerful tricks of the .htaccess is the ability to rewrite URLs. This enables us to do some mighty manipulations on our links; useful stuff like transforming very long URL’s into short, cute URLs,  redirecting URL to some other URL, transforming dynamic ?generated=page&URL’s into /friendly/flat/links, redirect missing pages, preventing hot-linking, performing automatic language translation, and much, much more.

When you use rewrite rule you have to enable rewrite module in the apache.conf file and you have to restart apache again.

In Ubuntu you can use a2enmod to enable rewrite mod

#sudo a2enmod rewrite

#sudo /etc/init.d/apache2 restart

begin rewriting… 

First you have to add following lines to the Apache Virtualhost.

<Directory / Your Document Root>
Options FollowSymLinks
AllowOverride all
</Directory>

Simple rewriting

Simply put, Apache scans all incoming URL requests, checks for matches in our .htaccess file and rewrites those matching URLs to whatever we specify. something like this..

Example 1 :

all requests to whatever.htm will be sent to whatever.php:

# .htaccess file

Options +FollowSymlinks
RewriteEngine on
RewriteRule ^(.*)\.htm$ $1.php [nc]

Example 2 :

all requests to example.com will be sent to xyz.com

# .htaccess file

Options +FollowSymLinks
RewriteEngine On
RewriteBase /

# require non-empty HOST header
RewriteCond %{HTTP_HOST} !^$

# require case-insensitive HOST to be www.example.net
RewriteCond %{HTTP_HOST} !^www\.example\.net$ [NC]

# 301 redirect everything to correct www.example.net
RewriteRule ^(.*)$ http://www.xyz.com/$1 [R=301,L]

# or you can use this rewriterule
#RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]