Shell Script to Check Whether a String is

Palindrome or not
Palindrome: A palindrome is a word, phrase, number or other
sequence
of units that can be read the same way in either direction
(the adjustment of punctuation and spaces between words is generally
permitted).
Examples:
Phrases:

Dammit, I'm mad!

Quotations: Able was I ere I saw Elba.

Madam, I'm Adam.
Names:

Some people have names that are palindromes.Lon

Nol (19131985) was Prime Minister of Cambodia.

Palindromic names are very common in Finland.
Examples
include Emma Lamme,Sanna Rannas, Anni Linna and
Asko Oksa.
Words:

civic,radar,level,rotator,rececar,reviver.
The command "Level, madam, level!", composed only

of words
that are themselves palindromes, is both a
character-bycharacter and a word-by-word palindrome.
Numbers:

5335, 123454321

Dates:

01/02/2010 (dd/mm/yyyy format)

Method 1:

#!/bin/bash
# SCRIPT: palindrome1.sh
# USAGE: palindrome.sh
# PURPOSE:

or

palindrome.sh STRING

Script to test if a given string is a palindrome.

#
# In this script I uses the well known method, compare first
character
# with last character, up to middle of the string. One
mismatch in the
# scanning leads to immediate termination of the scanning as
it is
# not a palindrome. To extract character from string, I will
use cut
# command with the -c option with the position number.
#
##############################################################
#######
#

Arguments Checking

#
##############################################################
#######
if [ $# -eq 0 ]
then
echo -n "Enter a String: "
read orgstr
else
orgstr=$*
fi
# You can also use single statement
#[ $# -eq 0 ] && (echo -n "Enter a String:"; read String) ||
String=$*
##############################################################
#######

Variable Initialization

#
##############################################################
#######
# Remove all punctuations from input string and convert upper
case to
# lower or lower case to upper.
String="$(echo $orgstr | sed 's/[^[:alnum:]]//g' | \
tr '[:upper:]' '[:lower:]')"
Flag=0
# Find length of the string.
len=${#String}
#You can also calculate string length using bellow commands.
#len=`echo $str | wc -c`
#len=$((len-1))
#get the mid value up to which the comparison would be done.
mid=$((len/2))
##############################################################
#######
#

Main Script Starts Here

#
##############################################################
#######
for ((i=1;i<=mid;i++))
do
c1=`echo $String|cut -c$i`

# extracts from

beginning
c2=`echo $String|cut -c$len`

# extracts from last

if [ $c1 != $c2 ]
then
Flag=1

break 2

# break N breaks out of N levels

of loop.
fi
let len-done
if [ $Flag -eq 0 ]
then
echo "\"$orgstr\" is a Palindrome"
else
echo "\"$orgstr\" is not a Palindrome"
fi

OUTPUT:
[root@www ]# ./palindrome1.sh Dammit, I\'m mad!
"Dammit, I'm mad!" is a Palindrome
[root@www ]# ./palindrome1.sh
Enter a String: 01/02/2010
"01/02/2010" is a Palindrome
[root@www ]# ./palindrome1.sh Hello world
"Hello world" is not a Palindrome

Method 2:

#!/bin/bash
# SCRIPT: palindrome2.sh
# USAGE: palindrome.sh

or

palindrome.sh STRING

# PURPOSE: Script to test if a given string is a palindrome.

#
# In this script I uses the well known method, compare first
character
# with last character, up to middle of the string. One
mismatch in the
# scanning leads to immediate termination of the

scanning as

it is
# not a palindrome. To extract a character from the string, I
will use
# string manipulation operations.So you need to know how to
manipulate
# strings to understand this script. I will give little bit of
explan# tion at the end of this script.
#
##############################################################
#######
#

Arguments Checking

#
##############################################################
#######
[ $# -eq 0 ] && { echo -n "Enter a String: "; read orgstr ;}
|| \
orgstr=$*
##############################################################
#######
#

Variable Initialization

#
##############################################################
#######

# Remove all punctuations from input string and convert upper

case to
# lower or lower case to upper.
String="$(echo $orgstr | sed 's/[^[:alnum:]]//g' | \
tr '[:upper:]' '[:lower:]')"
# Find length of the string.
len=${#String}
#get the mid value up to which the comparison would be done
mid=$(($len/2))
i=0
Flag=0
##############################################################
#######
#

Main Script Starts Here

#
##############################################################
#######
while [ $i -lt $mid ]
do
fchar=${String:$i:1}
let i++
bchar=${String: -$i:1}
if [ "$fchar" != $bchar ]
then
Flag=1
break 2
loop.
fi
done

# break N breaks out of N levels of

if [ $Flag -eq 0 ]
then
echo "\"$orgstr\" is a Palindrome"
else
echo "\"$orgstr\" is not a Palindrome"
fi

Substring Extraction:
${string:position}
Extracts substring from $string at $position.
${string:position:length}
Extracts $length characters of substring from $string at
$position
Bash numbers first character of string as '0'.
${string: 0: 1} will extracts one character from the 0th
character of
the string, ie it will only get the 0th character. ${string:
2: 1}
will get the third character. Also ${string: -1: 1} will
extracts the
last one character, ${string: -3:1} will get the third last
character.
Note:

${string: -1:1} in this construct don't forget to give

space
before -1, otherwise you will get full string.
For example
[root@localhost www]# tempvar=madam
[root@localhost www]# echo ${tempvar: -1:1}
m

[root@localhost www]# echo ${tempvar:-1:1}

madam
You can also use following command
[root@localhost www]# echo ${tempvar:(-1):1}
m
OUTPUT:
[root@www ]# ./palindrome2.sh Able was I ere I saw Elba
"Able was I ere I saw Elba" is a Palindrome
[root@www ]# ./palindrome2.sh 123454321
"123454321" is a Palindrome
[root@www ]# ./palindrome2.sh
Enter a String: 12345654321
"12345654321" is a Palindrome
[root@www ]# ./palindrome2.sh
Enter a String: 1234564321
"1234564321" is not a Palindrome

Method 3:
#!/bin/bash
# SCRIPT: palindrome3.sh
# USAGE: palindrome.sh

or

palindrome.sh STRING

# PURPOSE: Script to test if a given string is a palindrome.

#
# This simply uses the 'rev' utility which is used to reverse
lines of
# a file. Then check if the reverse of the string is same as
the
# original.rev command is part of util-linux-ng or util-linux

package.
#
if `which rev &>/dev/null`

# Checks rev command exist or not

then
[ $# -eq 0 ] && { echo -n "Enter a String: "; read orgstr
;} || \
orgstr=$*
String="$(echo $orgstr | sed 's/[^[:alnum:]]//g' | \
tr '[:upper:]' '[:lower:]')"
if [ "$(echo $String | rev)" = "$String" ]
then
echo "\"$orgstr\" is a palindrome"
else
echo "\"$orgstr\" is not a palindrome"
fi
else
echo "Install util-linux or util-linux-ng package"
fi

OUTPUT:
[root@www ]# ./palindrome3.sh 01/02/2010
"01/02/2010" is a palindrome
[root@www ]# ./palindrome3.sh 01/03/2010
"01/03/2010" is not a palindrome
[root@www ]# ./palindrome3.sh
Enter a String: Hello World
"Hello World" is not a palindrome
[root@www ]# ./palindrome3.sh
Enter a String: rotator
"rotator" is a palindrome

Method 4:
#!/bin/bash
# SCRIPT: palindrome4.sh
# USAGE: palindrome.sh
# PURPOSE:

or

palindrome.sh STRING

Script to test if a given string is a palindrome.

#
# In this method we are not using 'rev' command to reverse the
string.
# Using Substring Removal method or Substring Extraction
method we
# will reverse the string, then compare it with oldstring.
#
##############################################################
#######
#

Arguments Checking

#
##############################################################
#######
[ $# -eq 0 ] && { echo -n "Enter a String: "; read orgstr ;}
|| \
orgstr=$*
##############################################################
#######
#

Variable Initialization

#
##############################################################
#######
String="$(echo $orgstr | sed 's/[^[:alnum:]]//g' | \

tr '[:upper:]' '[:lower:]')"
oldstring=$String
newstring=
##############################################################
#######
#

Main Script Starts Here

#
##############################################################
#######
while [ -n "$String" ]
do
temp=${String#?}
letter=${String%"$temp"}
String=$temp
newstring=${letter}${newstring}
done
if [ "$oldstring" = "$newstring" ]
then
echo "\"$orgstr\" is a palindrome"
else
echo "\"$orgstr\" is not a palindrome"
fi
# ${string#substring} is a Substing Removal operation. If you
want to
# use Substring Extraction method, use bellow code.
#i=0
#while [ $i -lt ${#String} ]
#do
#

letter=${String:$i:1}

newstring=${letter}${newstring}

let i++;

#done
#
#if [ "$String" = "$newstring" ]
#then
#

echo "\"$orgstr\" is a palindrome"

#else
#

echo "\"$orgstr\" is not a palindrome"

#fi

Substring Removal:
${string#substring}
Strips shortest match of $substring from front of $string.
Example:
[root@www]# tempvar=madam
[root@www]# echo ${tempvar#m}
adam
[root@www]# echo ${tempvar#ma}
dam
[root@www]# echo ${tempvar#?}
adam

${string%substring}
Strips shortest match of $substring from back of $string.
Example:
[root@www]# temp=${tempvar#?}
[root@www]# echo $temp
adam

[root@www]# echo ${tempvar%$temp}

m
OUTPUT:
[root@www ]# ./palindrome4.sh Madam, I\'m Adam
"Madam, I'm Adam" is a palindrome
[root@www ]# ./palindrome4.sh Madam I Adam
"Madam I Adam" is not a palindrome
[root@www ]# ./palindrome4.sh
Enter a String: 123454321
"123454321" is a palindrome
[root@www ]# ./palindrome4.sh
Enter a String: 1234564321
"1234564321" is not a palindrome