how to set the shell to bash if there is bash otherwise set to ksh

You can set your default shell per user in /etc/passwd.  Just change the last option on the line from /bin/sh to /bin/bash or /bin/ksh

This link explains the /etc/passwd layout
http://www.cyberciti.biz/faq/understanding-etcpasswd-file-format/

Omit the shebang line #!...

and call your script like

[[ -x /bin/bash ]] && INTP=/bin/bash || INTP=/bin/ksh; $INTP /path/to/script

If your default login shell is either bash or ksh then you can do

$SHELL /path/to/script

Here too you must omit #!...

This is another trick.

Prefix your script (again without the shebang) with the following code (I took the first part from your other question):

function shellselect {
if [[ -e "/bin/bash" ]];  then
                INTERP="/bin/bash"
                else
                                INTERP="/bin/ksh"
fi; }

if [[ $1 != "12345selected54321" ]];  then
   shellselect
   $INTERP $0 "12345selected54321" "$@"
   exit
   else shift
fi
# actual code starts here, next two lines just for demonstration ...
echo $BASH # line will be empty if ksh is used, else it will contain the path to bash.
echo $@ # to show that the params are still there
# ....
# ....