Unix Utilities

In this project, you'll build a few different UNIX utilities, simple versions of commonly used commands like cat, ls, etc. We'll call each of them a slightly different name to avoid confusion; for example, instead of cat, you'll be implementing my-cat.

Objectives:

Re-familiarize yourself with the C programming language
Re-familiarize yourself with a shell / terminal / command-line of UNIX
Learn (as a side effect) how to use a proper code editor such as emacs
Learn a little about how UNIX utilities are implemented

While the project focuses upon writing simple C programs, you can see from the above that even that requires a bunch of other previous knowledge, including a basic idea of what a shell is and how to use the command line on some UNIX-based systems (e.g., Linux or macOS), how to use an editor such as emacs, and of course a basic understanding of C programming. If you do not have these skills already, this is not the right place to start.

my-cat

The program my-cat is a simple program. Generally, it reads a file as specified by the user and prints its contents. A typical usage is as follows, in which the user wants to see the contents of main.c, and thus types:

prompt> my-cat main.c
#include <stdio.h>
...

As shown, my-cat reads the file main.c and prints out its contents.

You'll need to learn how to use a few library routines from the C standard library (often called libc) to implement the source code for this program, which we'll assume is in a file called my-cat.c. All C code is automatically linked with the C library, which is full of useful functions you can call to implement your program. Learn more about the C library here and perhaps here¹.

For this project, we recommend using the following routines to do file input and output: fopen(), fgets(), and fclose(). Whenever you use a new function like this, the first thing you should do is read about it -- how else will you learn to use it properly?

On UNIX systems, the best way to read about such functions is to use what are called the man pages (short for manual). In our HTML/web-driven world, the man pages feel a bit antiquated, but they are useful and informative and generally quite easy to use.

To access the man page for fopen(), for example, just type the following at your UNIX shell prompt:

prompt> man fopen

Then, read! Reading man pages effectively takes practice; why not start learning now?

We will also give a simple overview here. The fopen() function "opens" a file, which is a common way in UNIX systems to begin the process of file access. In this case, opening a file just gives you back a pointer to a structure of type FILE, which can then be passed to other routines to read, write, etc.

Here is a typical usage of fopen():

FILE *fp = fopen("main.c", "r");
if (fp == NULL) {
    printf("cannot open file\n");
    exit(1);
}

A couple of points here. First, note that fopen() takes two arguments: the name of the file and the mode. The latter just indicates what we plan to do with the file. In this case, because we wish to read the file, we pass "r" as the second argument. Read the man pages to see what other options are available.

Second, note the critical checking of whether the fopen() actually succeeded. This is not Java where an exception will be thrown when things goes wrong; rather, it is C, and it is expected (in good programs, you know, the only kind you'd want to write) that you always will check if the call succeeded. Reading the man page tells you the details of what is returned when an error is encountered; in this case, the macOS man page says:

Upon successful completion fopen(), fdopen(), freopen() and fmemopen() return
a FILE pointer.  Otherwise, NULL is returned and the global variable errno is
set to indicate the error.

You can use the functions perror() or strerror() to print out more about why the error occurred; learn about those on your own (using ... you guessed it ... the man pages!).

my-grep

my-zip and my-unzip

my-sort

my-uniq

Footnotes

1: Unfortunately, there is a lot to learn about the C library, but at some point, you've just got to read documentation to learn what is available. Why not now, when you are young? Or, if you are old, why not now, before it's ... ahem ... too late?