Have you ever redirected the output of a curses program with colors and wondered what those mysterious ^[[ symbols are? Have you ever tried to produce colors with a printf command without using curses? If the answer to either of these questions is yes, read on. This article attempts to explain the mysterious characters that one finds in the output of a curses program that produces colors. Later, we extend this concept to produce colors with a mere printf command.

In the old days of teletype terminals, terminals were located far away from computers and were connected to them through serial cables. The terminals could be configured by sending a series of bytes to each one. All of the capabilities of terminals could be accessed through these series of bytes, which usually are called escape sequences because they start with an escape (0x1B) character. Even today, with vt100 emulation, we can send escape sequences to the emulator that have the same effect on the terminal window. Hence, in order to print color, we merely echo a control code.

To start, type this on your console:

echo "^[[0;31;40mIn Color"

The first character is an escape character, which looks like two characters, ^ and [. To be able to print this, you have to press CTRL+V and then the ESC key. All the other characters are normal printable characters, so you see the string In Color in red. The type stays that color until you revery back by typing this:

echo "^[[0;37;40m"

As you can see, it is easy to set and reset colors in a console or xterm. A myriad of escape sequences are available with which you can do a lot of things, including moving the cursor and resetting the terminal.

Now, I explain the escape sequence used to produce colors. The sequence to be printed or echoed to the terminal is

	<ESC>[{attr};{fg};{bg}m

The first character is ESC, which has to be entered by pressing CTRL+V and then ESC on the Linux console or in xterm, konsole, kvt and so on. Incidentally, CTRL+V ESC also is the combination used to embed an Esc character in a document in Vim. Then, {attr}, {fg} and {bg} have to be replaced with the correct value to achieve the corresponding effect. attr is the attribute, such as blinking or underlined text, while fg and bg are foreground and background colors, respectively. You don't have to put braces around the number; simply writing the number is sufficient.

{attr} needs to be one of the following:

{fg} needs to be one of the following:

{bg} needs to be one of the following:

So, to get a blinking line with a blue foreground and a green background, the combination should be:

	
echo "^[[5;34;42mIn color"

which actually is very ugly. So, revert back with

echo "^[0;37;40m"

What if you want to use this code color and attribute functionality in a C program? Well, that's simple. Before you printf something, print the escape sequence to produce it in the desired color. I have written a small routine, textcolor(), that does this automatically for you. You can use it in your C programs, along with the #define constants.

      |textcolor()|

#include <stdio.h>

#define RESET		0
#define BRIGHT 		1
#define DIM		2
#define UNDERLINE 	3
#define BLINK		4
#define REVERSE		7
#define HIDDEN		8

#define BLACK 		0
#define RED		1
#define GREEN		2
#define YELLOW		3
#define BLUE		4
#define MAGENTA		5
#define CYAN		6
#define	WHITE		7

void textcolor(int attr, int fg, int bg);
int main()
{	textcolor(BRIGHT, RED, BLACK);	
	printf("In color\n");
	textcolor(RESET, WHITE, BLACK);	
	return 0;
}

void textcolor(int attr, int fg, int bg)
{	char command[13];

	/* Command is the control command to the terminal */
	sprintf(command, "%c[%d;%d;%dm", 0x1B, attr, fg + 30, bg + 40);
	printf("%s", command);
}

The textcolor() program is modeled against the Turbo C API function. You call the function to set the color and then print it with sprintf(), a function used in Turbo C to produce console output in color.

The following program asks the user to play with attributes and colors and then shows a text string in those colors and with those attributes. I usually use it to find the best combination of colors for my GUIs.

#include <stdio.h>

#define RESET		0
#define BRIGHT 		1
#define DIM		2
#define UNDERLINE 	3
#define BLINK		4
#define REVERSE		7
#define HIDDEN		8

#define BLACK 		0
#define RED		1
#define GREEN		2
#define YELLOW		3
#define BLUE		4
#define MAGENTA		5
#define CYAN		6
#define	WHITE		7

#define ARRAY_SIZE(a) (sizeof(a) / sizeof(a[0]))

char *attrs[] = {"NORMAL", "BRIGHT", "DIM", "UNDERLINE", "BLINK",
		 "REVERSE", "HIDDEN", "EXIT"};
char *colors[] = {"BLACK", "RED", "GREEN", "YELLOW", "BLUE", "MAGENTA",
		 "CYAN", "WHITE", "EXIT"};
void textcolor(int attr, int fg, int bg);
int print_menu(char *array[], int n_options, char *title);
int main()
{	int attr, fg, bg;
	int attr_size, colors_size;
	
	attr_size = ARRAY_SIZE(attrs);
	colors_size = ARRAY_SIZE(colors);
	while(1)
	{	printf("\n");
		attr = print_menu(attrs, attr_size, "Choose the attr you want:");
		if(attr == attr_size - 1)
			break;
		fg = print_menu(colors, colors_size, "Choose the foreground you want:");
		if(attr == colors_size - 1)
			break;
		bg = print_menu(colors, colors_size, "Choose the background you want:");
		if(attr == colors_size - 1)
			break;
		printf("\n");
		textcolor(attr, fg, bg);	
		printf("This is what you get if you use the combination %s attribute %s foreground and %s
 background", attrs[attr], colors[fg], colors[bg]);
		textcolor(RESET, WHITE, BLACK);
		system("clear");
	}
	return 0;
}

int print_menu(char *array[], int n_options, char *title)
{	int choice, i;
	for(i = 0;i < n_options; ++i)
		printf("%d.%s\n", i, array[i]);
	printf("%s", title);
	scanf("%d", &choice);
	return choice;
}		
void textcolor(int attr, int fg, int bg)
{	char command[13];

	/* Command is the control command to the terminal */
	sprintf(command, "%c[%d;%d;%dm", 0x1B, attr, fg + 30, bg + 40);
	printf("%s", command);
}

So, what's the catch? If producing colors and attributes for text is so easy, why do people waste their time writing huge programs in curses, which in turn queries terminfo in a complex way? As we know, many terminals are out there that have few capabilities. Others don't recognize these escape codes or need different codes entered to achieve the same effect. So, if you want a portable program that can run on various terminals with the same or reduced functionalities, you should use curses. Curses uses terminfo to find the correct codes to accomplish these types of tasks across various terminals. Terminfo is a large database that contains information about the various functionalities of different terminals.

But, if you simply want to write a program that produces color on a Linux console or in an xterm window, you can use the escape sequences above to do it easily. The Linux console mostly emulates vt100, so it recognizes these escape sequences.

However, there is a way to query the terminfo database for the information you need and do the work yourself. tput is the command that queries the database and executes the functionality you specify. The two capabilities setf and setb are useful to set foreground and background colors. For example, use the following to set the foreground color to red and the background color to green:

	tput setf 4	# tput setf {fg color number}
	tput setb 2	# tput setb {bg color number}

These commands can be used in shell scripts wherever you want. See the tput man page for additional capabilities of tput. The terminfo man pages contain a lot of information regarding terminal capabilities, how to get and set their values and more. There are two terminfo man pages: man 5 terminfo describes the terminfo database, and man 3ncurses terminfo describes the C functions that use the database.

Here are the numbers to be passed as arguments to tput setf and tput setb and their corresponding colors:

The Text-Terminal-HOWTO

Man pages for tput and terminfo.

Copyright (c) 2001, Pradeep Padala. Originally published in Linux Gazette issue 65. Copyright (c) 2001, Specialized Systems Consultants, Inc.