UNIX for Dummies Questions & Answers

#!/bin/bash

# A LIST OF THE ERROR MESSAGES AND THE PROPER SYNTAX:
error_0="Correct amount of arguments provided."
error_10="Error: No arguments present at all."
error_11="You must input the first argument."
error_12="Error: Expecting \"-i\" for the first argument."
error_13="You must enter a filename for the second argument."
error_14="$2 does not exist.  Please enter a real file."
error_15="You must enter a third argument."
error_16="Error: Expecting \"-o\" for the third argument."
error_17="You must enter a filename for the fourth argument."
error_18="Error: Too many arguments."
error_19="No overwrite selected...exiting program"

proper="Usage:  ./Lab_14.sh -i <infile> -o <outfile> \n \n-i <infile>   The filename containing the existing data. \n-o <outfile>  The filename you want the output"

hit_enter="Press <ENTER> to continue"
ow="1"
menu="1"
menu_choice="Menu option "$choice" selected"
menu_content="\n     MAIN MENU \n     --------- \n     1) Menu Option 1 \n    2) Menu Option 2 \n    3) Menu Option 3 \n    4) Menu Option 4 \n \n     5) Quit \n \n     Please enter a menu option:"
			

#CHECK IF THERE ARE NO ARGUMENTS PRESENT:
if [ "$#" == "0" ] ;then
	echo
	echo $error_10
	echo
	echo -e $proper
	echo
	exit 10
fi

#CHECK IF FIRST ARGUMENT IS BLANK:
if [ "$1" == "" ] ;then
	echo
	echo $error_11
	echo
	echo -e $proper
	echo
	exit 11
fi 

#CHECK IF FIRST ARGUMENT IS -i:
if [ "$1" != "-i" ] ;then
	echo
	echo $error_12
	echo
	echo -e $proper
	echo
	exit 12
fi

#CHECK IF SECOND ARGUMENT IS BLANK:
if [ "$2" == "" ] ;then
	echo
	echo $error_13
	echo
	echo -e $proper
	echo 
	exit 13
fi

#CHECK IF SECOND ARGUMENT (INPUT FILENAME) EXISTS:
if [ ! -e "$2" ] ;then
	echo
	echo $error_14
	echo
	echo -e $proper
	echo
	exit 14
fi

#CHECK IF THIRD ARGUMENT IS BLANK:
if [ "$3" == "" ] ;then
	echo
	echo $error_15
	echo
	echo -e $proper
	echo
	exit 15
fi

#CHECK IF THIRD ARGUMENT IS -o:
if [ "$3" != "-o" ] ;then
	echo
	echo $error_16
	echo
	echo -e $proper
	echo
	exit 16
fi

#CHECK IF FOURTH ARGUMENT IS BLANK:
if [ "$4" == "" ] ;then
	echo
	echo $error_17
	echo
	echo -e $proper
	echo
	exit 17
fi

#CHECK IF THERE ARE TOO MANY (MORE THAN 4) ARGUMENTS:
if [ "$#" -ge "5" ] ;then
	echo
	echo $error_18
	echo
	echo -e $proper
	echo
	exit 18
fi

#IF FOURTH FILE EXISTS, ASK IF USER WANTS IT OVERWRITTEN:
if [ "$1" == "-i" ] && [ -e "$2" ] && [ "$3" == "-o" ] && [ -e "$4" ] ;then
	while [ $ow != "5" ]
	do
		echo
		echo "File \"$4\" exists already"
		echo -n "Do you wish to overwrite file contents (y/n)?"
		read overwrite
	
		case $overwrite in
		y)
	        ow="5"	
			clear
			while [ $menu != "quit" ] 
			do 
				echo
				echo -en $menu_content
				read choice
	
				case $choice in
				1)
					echo 
					echo $menu_choice
					echo $hit_enter
					read continue
					clear
				;;
				2)
					echo
					echo $menu_choice
					echo $hit_enter
					read continue
					clear
				;;
				3)
					echo
					echo $menu_choice
					echo $hit_enter
					read continue
					clear
				;;
				4)
					echo
					echo $menu_choice
					echo $hit_enter
					read continue
					clear
				;;
				5)
					echo
					echo $menu_choice
					echo $hit_enter
					read continue
					clear 
					menu="quit"
					exit 0
				;;
				*)	
					echo
					echo "Try picking an option from the menu"
					echo $hit_enter
					read continue
					clear
				;;
				esac
    		done
		;;
		n)
			echo
			echo $error_19
			exit 19
		;;
		*)
			echo "Please select y or n"
			echo $hit_enter
			read continue
		;;
		esac
	done
fi

#IF FOURTH FILE DOES NOT EXIST, GO STRAIGHT TO MENU:
if [ "$1" == "-i" ] && [ -e "$2" ] && [ "$3" == "-o" ] && [ ! -e "$4" ] ;then
	clear
	while [ $menu != "quit" ] 
	do 
		echo
		echo -en $menu_content
		read choice
	
		case $choice in
		1)
			echo 
			echo $menu_choice
			echo $hit_enter
			read continue
			clear
		;;
		2)
			echo
			echo $menu_choice
			echo $hit_enter
			read continue
			clear
		;;
		3)
			echo
			echo $menu_choice
			echo $hit_enter
			read continue
			clear
		;;
		4)
			echo
			echo $menu_choice
			echo $hit_enter
			read continue
			clear
		;;
		5)
			echo
			echo $menu_choice
			echo $hit_enter
			read continue
			clear 
			menu="quit"
			exit 0
		;;
		*)	
			echo
			echo "Try picking an option from the menu"
			echo $hit_enter
			read continue
			clear
		;;
		esac
    done
fi

menu_choice="Menu option "$choice" selected"
echo $menu_choice