How do I determine if a terminal is color-capable? - unix

Question

I would like to change a program to automatically detect whether a terminal is color-capable or not, so when I run said program from within a non-color capable terminal (say M-x shell in (X)Emacs), color is automatically turned off.
I don't want to hardcode the program to detect TERM={emacs,dumb}.
I am thinking that termcap/terminfo should be able to help with this, but so far I've only managed to cobble together this (n)curses-using snippet of code, which fails badly when it can't find the terminal:
#include <stdlib.h>
#include <curses.h>
int main(void) {
int colors=0;
initscr();
start_color();
colors=has_colors() ? 1 : 0;
endwin();
printf(colors ? "YES\n" : "NO\n");
exit(0);
}
I.e. I get this:
$ gcc -Wall -lncurses -o hep hep.c
$ echo $TERM
xterm
$ ./hep
YES
$ export TERM=dumb
$ ./hep
NO
$ export TERM=emacs
$ ./hep
Error opening terminal: emacs.
$
which is... suboptimal.

Answer 1

A friend pointed me towards tput(1), and I cooked up this solution:
#!/bin/sh
# ack-wrapper - use tput to try and detect whether the terminal is
# color-capable, and call ack-grep accordingly.
OPTION='--nocolor'
COLORS=$(tput colors 2> /dev/null)
if [ $? = 0 ] && [ $COLORS -gt 2 ]; then
OPTION=''
fi
exec ack-grep $OPTION "$#"
which works for me. It would be great if I had a way to integrate it into ack, though.

Answer 2

You almost had it, except that you need to use the lower-level curses function setupterm instead of initscr. setupterm just performs enough initialization to read terminfo data, and if you pass in a pointer to an error result value (the last argument) it will return an error value instead of emitting error messages and exiting (the default behavior for initscr).
#include <stdlib.h>
#include <curses.h>
int main(void) {
char *term = getenv("TERM");
int erret = 0;
if (setupterm(NULL, 1, &erret) == ERR) {
char *errmsg = "unknown error";
switch (erret) {
case 1: errmsg = "terminal is hardcopy, cannot be used for curses applications"; break;
case 0: errmsg = "terminal could not be found, or not enough information for curses applications"; break;
case -1: errmsg = "terminfo entry could not be found"; break;
}
printf("Color support for terminal \"%s\" unknown (error %d: %s).\n", term, erret, errmsg);
exit(1);
}
bool colors = has_colors();
printf("Terminal \"%s\" %s colors.\n", term, colors ? "has" : "does not have");
return 0;
}
Additional information about using setupterm is available in the curs_terminfo(3X) man page (x-man-page://3x/curs_terminfo) and Writing Programs with NCURSES.

Answer 3

Look up the terminfo(5) entry for the terminal type and check the Co (max_colors) entry. That's how many colors the terminal supports.