CS537 Spring 2019, Project 1a



In this assignment, you will build a set of linux utilities but much simpler versions of common used commands like ls, cat etc. We say simpler because to be mildly put the original are quite complicated. For ex - you can find the source of cat command which concatenates files and writes out to standard output is around 700 lines. We will call each of these utilities slightly different to avoid confusion - my-cat, my-sed, my-uniq.


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, 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.

Summary of what gets turned in:


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. The “./” before the my-cat above is a UNIX thing; it just tells the system which directory to find my-cat in (in this case, in the “.” (dot) directory, which means the current working directory).

To create the my-cat binary, you’ll be creating a single source file, my-cat.c, and writing a little C code to implement this simplified version of cat. To compile this program, you will do the following:

prompt> gcc -o my-cat my-cat.c -Wall -Werror prompt>

This will make a single executable binary called my-cat which you can then run as above.

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, i.e., 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.  

Thus, as the code above does, please check that fopen() does not return NULL before trying to use the FILE pointer it returns.

Third, note that when the error case occurs, the program prints a message and then exits with error status of 1. In UNIX systems, it is traditional to return 0 upon success, and non-zero upon failure. Here, we will use 1 to indicate failure.

Side note: if fopen() does fail, there are many reasons possible as to why. 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!).

Once a file is open, there are many different ways to read from it. The one we’re suggesting here to you is fgets(), which is used to get input from files, one line at a time.

To print out file contents, just use printf(). For example, after reading in a line with fgets() into a variable buffer, you can just print out the buffer as follows:

printf("%s", buffer);

Note that you should not add a newline (\n) character to the printf(), because that would be changing the output of the file to have extra newlines. Just print the exact contents of the read-in buffer (which, of course, many include a newline).

Finally, when you are done reading and printing, use fclose() to close the file (thus indicating you no longer need to read from it).



The second utility you will build is called my-sed, a variant of the GNU/Linux utility called sed or streaming editor. For all purposes sed is the command line version of find and replace, the power of sed comes from matching regular expressions. Using regex’s you don’t just match strings but any possible pattern. But thankfully my-sed will only be used to find and replace it with the exact given string. It will find the first instance of a string in a line and substitute it with another. It will print the output to standard output. Instances following the first instance remain as is.
In case there is no candidate to substitute the line will be printed as is. Here is how a user will substitute word the foo with bar from baz.txt and qux.txt

prompt> ./my-sed foo bar baz.txt qux.txt



The last (phew !) utility you will build is called my-uniq, a version of the unix utility uniq (it should be pretty obvious by now). The original unix uniq utility detects adjacent duplicate lines from the input file and removes them and writes out to an output file. my-uniq will do exactly this, it finds out adjacent duplicate lines in files, and prints them.

For example, consider the following command: prompt> ./my-uniq foo.txt bar.txt This will read contents from file foo.txt and bar.txt, delete adjacent duplicate lines from each of them and print the output to shell.



The assignment borrows a considerable amount of content from assignment 1 of Prof. Remzi’s course in Spring 2018.


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?