A literate programming tangler written in Go
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4.6 KiB

Randomly Colored Cat

Josias Allestad

This program will provide a cat-like utility for printing text from a file to stdout. Except that output will be randomly colorized.

Thus the name: Randomly Colored Cat.

Here is the basic structure of the program:





Now for the includes:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

unistd.h, stdlib.h, and time.h are required for randomness, stdio.h for printf, and string.h for strcmp and strlen.

Some constants that will be the same throughout the program:

#define BUF_SIZE 10


Now the colors:

#define KNRM "\x1B[0m"
#define KRED "\x1B[31m"
#define KGRN "\x1B[32m"
#define KYEL "\x1B[33m"
#define KMAG "\x1B[35m"
#define KCYN "\x1B[36m"
#define KWHT "\x1B[37m"

const char *colors[] = {KNRM, KRED, KGRN, KYEL, KMAG, KCYN};

These are all the codes for the colors we want, and an array to order them nicely to randomly index them.

Print Colors

int lastcolor = 0;
void printColors(char *text)
	if (text) {

If the output is piped (and thus isn't a tty), the colors don't make much sense.

if (!(isatty(STDOUT_FILENO))) {
	printf("%s", text);

Go through the text character by character and print it with a random color between 0 and 6 (the index of colors). It also makes sure that the color is not the same as the last one.

for (int i = 0; i < strlen(text); ++i) {
	int color = 0;
	do {
		color = rand() % 6;
	} while (color == lastcolor);
	lastcolor = color;
	printf("%s%c", colors[color], text[i]);

Now we have to reset the terminal to the normal colors.

printf("%s", KWHT);


The main function that every C program must have.

int main(int argc, char *argv[])


Initialize random and buffer, and loop through the arguments.

char buffer[BUF_SIZE] = {""};

for (int i = 0; i < argc; ++i) {

Determine whether or not the argument is -. If so, get the input from stdin instead of a file. In addition, it should not try to read the file from argv[0], since that is itself.

if (strcmp(argv[i], "-") == 0 || argc == 1) {
} else if (i >= 1) {

This reads the file section by section in chucks the size of BUF_SIZE and prints them with printColors. If the section is unreadable, it prints an error to stdout.

size_t got;
while ((got = fread(buffer, 1, BUF_SIZE -1, fp))) {
	buffer[got] = '\0';
	if (ferror(fp))
		fprintf(stderr, "Error: can't read %s\n", argv[1]);

Basically the same as above, but gets text from stdin instead of a file pointer. In addition, it needs to call clearerr to be able to reopen the input stream for multiple - arguments.

while (fgets(buffer, BUF_SIZE, stdin) != NULL) {

Prepare a file pointer and make sure the file exists. If not, write an error to stderr.

FILE *fp;
fp = fopen(argv[i], "r");
if (!(fp)) {
	fprintf(stderr, "rccat: %s: no such file or directory.\n", argv[i]);


To compile with your C compiler directly, you can use gcc -o rccat rccat.c, but that isn't as useful as a Makefile.

The complete Makefile:

PREFIX ?= /usr/local
INSTALL ?= install


PREFIX determines where you want the program to be installed when make install is run.

If you just want to run make, it automatically calls build.

all: build

Compiling the program with your C compiler.

	$(CC) -o rccat rccat.c

A simple way to delete the rccat file, and any other generated files (in the future).

	$(RM) rccat

Install the program to PREFIX (as mentioned above). It first prepares the install and then copies over the file to PREFIX/bin.

	$(INSTALL_PROGRAM) -m755 rccat $(PREFIX)/bin

Uninstall the program from PREFIX/bin.

	$(RM) $(PREFIX)/bin/rccat