-----------------test2.sh-------------------------

#!/bin/sh

# If bash or ksh is not the current shell, try to set
[ $SHELL != /bin/ksh -a $SHELL != /bin/bash ] && {
        if [ -x /bin/ksh ]
        then
                SHELL=/bin/ksh
        elif [ -x /bin/bash ]
        then
                SHELL=/bin/bash
        else
                echo "Error: Unable to locate ksh or bash shell."
                exit 1
        fi
        export SHELL
        exec $SHELL $0 "$@"
}

function foo
{
        echo "Im in $SHELL"
}

foo

--------------------------test.sh--------------------
#!/bin/sh

# If bash or ksh is not the current shell, try to set
[ $SHELL != /bin/ksh -a $SHELL != /bin/bash ] && {
        if [ -x /bin/ksh ]
        then
                SHELL=/bin/ksh
        elif [ -x /bin/bash ]
        then
                SHELL=/bin/bash
        else
                echo "Error: Unable to locate ksh or bash shell."
                exit 1
        fi
        export SHELL
        exec $SHELL $0 "$@"
}

./test2.sh || exit 1

[ $? == 0 ] && {
        echo "Worked!!!" 
        exit 0
}
exit 1