Shell Programming and Scripting

#!/bin/sh
        if [ -f $1 ] && [ -f $2 ] && [ -f $3 ]
                then
                        sort < $1 > mySortedFile.txt
                        tail -1 < mySortedFile.txt > myTailed1.txt
                        sort < $2 > mySortedFile1.txt
                        tail -1 < mySortedFile1.txt > myTailed2.txt
                        sort < $3 > mySortedFile2.txt
                        tail -1 < mySortedFile2.txt > myTailed3.txt
                        sort -r < myTailed1.txt > finalSort.txt
                        sort -r < myTailed2.txt >> finalSort.txt
                        sort -r < myTailed3.txt >> finalSort.txt
                        cat finalSort.txt
                else
                        if [ "$1" == colours.txt ] && [ "$2" == fruit.txt ]
                                then
                                        echo "Error number.txt missing"
                        elif [ "$1" == colours.txt ] && [ "$3" == number.txt ]
                                then
                                        echo "Error fruit.txt missing"
                        elif [ "$2" == fruit.txt ] && [ "$3" == number.txt ]
                                then
                                        echo "Error colour.txt missing"
                        elif [ "$1" == colour.txt ]
                                then
                                        echo "Fruit and Number is missing"
                        elif [ "$2" == fruit.txt ]
                                then
                                        echo "Colour and Number is missing"
                        elif [ "$3" == number.txt ]
                                then
                                        echo "Colour and Fruit is missing"
                        fi
        fi