Write a script that manages a telephone directory(file). 
Each entry in the directory will contain first name, last name and telephone number.
The script should be called "tel" and have the possible command line commands and arguments -

tel -a Harry Jones 521-2703 where -a means add the following first name, last name and telephone number to the directory

tel -f Jones where -f means find the entry for the last name and display it

tel -d Jones where -d means delete the entry for the last name

tel -s where -s means displays the directory sorted by last name

Your script should handle all error conditions gracefully i.e. invalid number of arguments, 
missing file, invalid option etc.