    Note to the next instructor:

    This assignment tacitly assumes that $COLUMNS is usually in your
    environment.  This is an error on my part.  I assumed that $COLUMNS
    was, by default, exported.  However, it’s not.

    One possible fix is to use a different variable: $WIDTH instead
    of $COLUMNS.  If $WIDTH isn’t set, use 80.

CS155 HW4: Rulers!

Summary

Alas, this assignment concerns neither Prime Minister Justin Trudeau of Canada or Queen Jetsun Pema of Bhutan.

It it sometimes useful to be able to quickly count columns in a terminal window. For this assignment, you will write a bash script called ruler which will display a ruler.

Arguments

ruler will take one optional command-line argument:

ruler: If no arguments are given, use the environment variable COLUMNS to determine the length of the ruler. If COLUMNS is not set, assume eighty columns.
ruler half: If the word “half” is given, display a ruler half as wide (round down) as would be otherwise displayed, if no arguments were given.
ruler n: If a single numeric argument is given, display a ruler that wide.

Examples

Here are some examples, where % is my prompt:

% echo $COLUMNS
80
% ./ruler
12345678901234567890123456789012345678901234567890123456789012345678901234567890
         1         2         3         4         5         6         7         8
% ./ruler half
1234567890123456789012345678901234567890
         1         2         3         4
% ./ruler 27
123456789012345678901234567
         1         2

Requirements

You may not change, add, or remove from the required output.
- This implies that extra messages are forbidden.
Your program should emit a helpful error message if:
- too many arguments are given
- an argument < 1 is given
- a bad argument is given
If your program emits an error message, it should not emit anything else. For example, given the argument “bonehead”, it would be wrong for the program to complain about that argument, and then go on to display a ruler.
In cases where multiple requirements are violated, it doesn't matter which requirement you complain about. Pick one, complain, and that should be all.
This must be a pure bash script, and not use any other languages such as perl or awk. You may use programs such as sed or grep.
Don’t be concerned about leading zeroes in numbers. They are used by bash to indicate octal (base 8). We will not test your program with leading zeroes.
If the requested length is wider than the terminal, it will look bad. That’s ok.
Ruler lengths ≥ 100 will not be tested.
The first line of your script must be:

    #! /bin/bash

Your script must include a comment block after the first line that contains: your name, course, assignment, and date.

Hints

There’s a convenient mechanism to get a default value for a Bash variable. Look for it if you’re interested.

How to submit your homework:

Via web checkin, or:

    ~cs155/bin/checkin HW4 ruler

How to receive negative points:

Turn in someone else’s work.

CS155: Introduction to Unix

Spring 2018

HW 4