[~/temp]$ cat deepak.sh 
#! /bin/sh

# reverse a string

[[ -z "$@" ]] && STR=" a b c d e f g h i j k l m n o p q r s t u v w x y z " || STR="$@"

len=${#STR}

REV=""
for (( i=$len ; i>0 ; i-- ))
do
        REV=$REV""${STR:$i-1:$i}
        STR=${STR%${STR:$i-1:$i}}
done

echo "Reversed string"
echo $REV

[~/temp]$ ./deepak.sh
Reversed string
z y x w v u t s r q p o n m l k j i h g f e d c b a

# remove leading white spaces
REV=${REV## }
# remove trailing white spaces
REV=${REV%% }

#! /usr/bin/ksh

str="abcd"
rev=""
typeset -R1 last

while ((${#str})) ; do
        last=$str
        rev=${rev}${last}
        if ((${#str} > 1)) ; then
                typeset -L$((${#str}-1)) rest=$str
                str=$rest
        else
                str=
        fi
done
echo rev = $rev
exit 0

str="program"
echo $str | awk '{
for(i=length($0);i>=1;i--)
printf("%s",substr($0,i,1));
}'

str="program"
i=${#str}
final=""

while [ $i -gt 0 ]
do
  rev=`echo $str | awk '{printf substr($0, '$i', 1)}'`
  final=$final$rev
  i=$(($i - 1))
done

echo "Reversed String: $final"

str="this script is to reverse"
i=${#str}
word=""
fin=""
final=""

while [ $i -ge 0 ]
do
temp=`echo $str | awk '{printf substr($0, '$i', 1)}'`
  if [ \( "$temp" = " " \) -o $i -eq 0 ]
  then
     wordlen=${#word}
     while [ $wordlen -gt 0 ]
     do
        revtemp=`echo $word | awk '{printf substr($0, '$wordlen', 1)}'`
        fin=$fin$revtemp
        wordlen=$(($wordlen -1))
     done
     final=$final$fin" "
     fin=""
     word=""
     temp=""
  else
     word=$word$temp
  fi
i=$(($i - 1))
done

echo "Reversed line: $final"