Let's say you're an xterm, terminal.app, iTerm2.app, etc. You open a pseudo terminal which gives you a pair of file descriptors - a master and a slave. The par

Let's say you're an xterm, terminal.app, iTerm2.app, etc. You open a pseudo terminal which gives you a pair of file descriptors - a master and a slave. The parent process (xterm, et alia) does all reading/writing through the master file descriptor. The child process sets stdin, stdout, and stderr to the slave process and execs bash. So when bash starts, stdin, stdout, and stderr are the same file descriptor. And those descriptors (the child pseudo terminal) are inherited by any programs launched from bash. (bash can redirect stdin, stdout, and stderr of course) Backing up a step, xterm (et alia) could open 2 pseudo terminals and use one for stdin/stdout and the second for stderr. And then stderr could be displayed in a separate window, always blinking red, or whatever. (You could also use a pipe for stderr. It would slightly less compatible since it's a pipe and not a pseudo terminal. Somewhere, there's probably a piece of code that cares.)