So You Like Color--The Mysterious ^[[ Characters

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Learn how to make the text in your xterm blink magenta against a cyan background--even in your C programs.

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.

Terminal Codes

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.

The Color Code: <ESC>[{attr};{fg};{bg}m

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:

  • 0 Reset All Attributes (return to normal mode)

  • 1 Bright (usually turns on BOLD)

  • 2 Dim

  • 3 Underline

  • 5 Blink

  • 7 Reverse

  • 8 Hidden

{fg} needs to be one of the following:

  • 30 Black

  • 31 Red

  • 32 Green

  • 33 Yellow

  • 34 Blue

  • 35 Magenta

  • 36 Cyan

  • 37 White

{bg} needs to be one of the following:

  • 40 Black

  • 41 Red

  • 42 Green

  • 43 Yellow

  • 44 Blue

  • 45 Magenta

  • 46 Cyan

  • 47 White

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"

With printf()

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.

______________________

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Bypass the need for CTRL-V ESC

Themilkman's picture

If you want to omit the CTRL-V ESC key sequence you can use \033 instead.

So the lines would read echo "\033[0;31;40m In Color"
and echo "\033[0;37;40m"

This makes it easier to type and to have in scripts.

Excellent post

Chris Hampson's picture

A fantastic post, I'm just about to do some scripts involving interpreting these colour codes from the logs from the screen utility, this page was invaluable.

Keep up the good work :-)

Cool Tutorial, but...

Matt Cudmore's picture

Cool tutorial, but, I noticed that in the two coding examples you #define BLINK as 4, when it's actually supposed to be '5'

It's just a small type-o, but I thought I'd let you know just incase someone (such as myself) tries to learn from one of the examples without first looking to the list that correctly defines BLINK as 5.

Inspired me to write a library

John Holden's picture

It's been a slow day before exams started, so I went and wrote a C header I've named SimpleColors after reading this article: CVS (tarball). Great information!

This link is broken!

Anonymous's picture

This link is broken!

awesome

herbmaster's picture

OMG, this is awesome. I've been wondering for about three years how to do colors in c(besides using ncurses). I just happened to be bored and decided to checkout lj's website since I have a subscription and really like the mag.

I think the editors are retarded for not putting this stuff in the magazine. I mean, I have to look at a computer monitor 12 hours a day and I'd really rather go home and read this stuff from the print mag.

But anyway, awsome article Pradeep. Thanks!

bugs in code

Anonymous's picture

In your while loop, the second and third if statements are incorrect. It looks like you copied/pasted the first statement and did not change the conditions -- you are comparing attr when it should be fg and then bg.

RE: bugs in code

Anonymous's picture

Sorry, forgot to mention that is for the "Demo of Colors" program.

Will Linux make computers soo

Anonymous's picture

LINUX is no brand

webdesign hamburg's picture

and never will be. So Linux as a organization could not exist. And you cannot build real material things over the web, cause your hands doesnt fit into the cables :)

Re:

A@A's picture

In USSR was not any commercical company. But this counrty was better than USA.
Communism rulezzz!

Such a thing could be

gentoo user's picture

Such a thing could be written by the person who never lived in both countries nor US nor USSR. It seems the author of the previous comment is a teen who grown up in Ukraine as an independent country and considers that USSR was a communist state... Funny :)

look it up

Anonymous's picture

Actually the Ukraine beeing a part of the USSR is not that long ago! They became independant 1991.

cool article. i finally under

Anonymous's picture

cool article. i finally understood colors... although a msg about that it is not possible to paste the escape sequence into your terminal would be good.

But what about outputting to a file?

Boom's picture

Hi, I use escape sequences in my program but then if I redirect the output to a file, I start seeing all these escape characters in there. Is there a way that I can disable this functionality when outputting to a file?

istty

sde's picture
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