Shell Script Basics

Mathematical Operator in  Shell Script	Meaning	Normal Arithmetical/ Mathematical Statements	But in Shell
			For test statement with if command	For [ expr ] statement with if command
-eq	is equal to	5 == 6	if test 5 -eq 6	if [ 5 -eq 6 ]
-ne	is not equal to	5 != 6	if test 5 -ne 6	if [ 5 -ne 6 ]
-lt	is less than	5 < 6	if test 5 -lt 6	if [ 5 -lt 6 ]
-le	is less than or equal to	5 <= 6	if test 5 -le 6	if [ 5 -le 6 ]
-gt	is greater than	5 > 6	if test 5 -gt 6	if [ 5 -gt 6 ]
-ge	is greater than or equal to	5 >= 6	if test 5 -ge 6	if [ 5 -ge 6 ]

NOTE: == is equal, != is not equal.

For string Comparisons use

Operator	Meaning
string1 = string2	string1 is equal to string2
string1 != string2	string1 is NOT equal to string2
string1	string1 is NOT NULL or not defined
-n string1	string1 is NOT NULL and does exist
-z string1	string1 is NULL and does exist

Shell also test for file and directory types

Test	Meaning
-s file	Non empty file
-f file	Is File exist or normal file and not a directory
-d dir	Is Directory exist and not a file
-w file	Is writeable file
-r file	Is read-only file
-x file	Is file is executable

Logical Operators

Logical operators are used to combine two or more condition at a time

Operator	Meaning
! expression	Logical NOT
expression1  -a  expression2	Logical AND
expression1  -o  expression2	Logical OR

Qmail Log Analysis.....

sooooonnnnnnnn...............

Play

click to play Basket Ball

Removing old keys in your known_hosts file

If you happen to have changed server IP address and while at it you get this message doing a remote connnection:

ssh ants@192.168.1.1
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
eb:e8:de:47:00:77:2c:14:28:20:db:18:4c:9f:69:02.
Please contact your system administrator.
Add correct host key in /home/ants/.ssh/known_hosts to get rid of this message.
Offending key in /home/ants/.ssh/known_hosts:13
RSA host key for 192.168.1.1 has changed and you have requested strict checking.
Host key verification failed.

You will have to remove the entry by editing your known_hosts file. Remove the old entry, in this case its on line "13" save the file and try connecting again, accept the new one when prompted.

Done.

Learn IPTABLES and be Secure

IPTABLES TUTORIAL

Traceroute shell script for linux/unix

Before you run the below script you need to save server list in /tmp/servlist. The below mentioned bash script you need to put in crontab to run as per the schedule. This script will send the traceroute whenever node/server is down. Please comment if this script is useful for you.

#!/bin/bash
#Author : Tejas Vino
#Purpose : To send the traceroute whenever server/node is down.
#Version : V1.0
#Date :April 19, 2011

Hostname=`/bin/hostname`
Trace="/bin/traceroute -I"

for i in `cat /tmp/servlist`
do
test=`fping $i | grep unreachable | awk '{print $1'}`
echo $test
if [ -z "$test" ]; then
sleep 1
else
$Trace $test | mail -s "$test is Down!!! Traceroute from $Hostname to $test" youremailid@domain.com &
fi
done

Linux Foundation Is Celebrating 20 Years Of Linux - Are you ?

Linux is turning 20. And to commemorate this happy occasion, the Linux Foundation is celebrating 20 years of Linux with a slew of special events which includes Gala dinner, Video contest,a T-Shirt contest, Linux Scholarships et al.

The Linux Foundation has also released this nice infographic which depicts the major milestones of Linux development.