in ubuntu. $3.$ Write a program to print characters starting from $15$ th character till the $20$ th character of file F$1.$txt into file F$4.$txt$.4.$ Write a program to read a number$(\backslash ($ n$)\backslash )$ from the user. Print the first $\backslash ($ n $\backslash )$ characters from the file F$1.$txt$.5.$ Write a program to print the second half of a file.