Re: [TUHS] UNIX of choice these days?

On 9/26/17 10:02 PM, Larry McVoy wrote: OK. The only way to guarantee you have a descriptor open to your controlling terminal is to open /dev/tty yourself. You can run `bash -im' and have yourself an interactive shell with job control enabled no matter where stdin/stdout/stderr point, and they can be redirected at any point during execution, so you can't count on them. But why do you have to have the fd in the first place? My initial message said you need it for "job control and other terminal operations." Here's what that means. You initially need a handle to your controlling terminal to determine whether or not you have one and whether you're in the background. If you don't have a controlling terminal, job control is impossible. You need the terminal's process group to determine whether or not you're in the foreground, and to get that you need a handle to the controlling tty. Every shell does this. Once you've determined the original tty pgrp, you need to become a process group leader if you're not one already and set the terminal's process group to your own so you can stay in the foreground. If you don't, you can't read from the terminal. If you can't change the terminal's process group, you can't enable job control. You need the handle to call tcsetpgrp(). Again, this is something every shell does. Now that you've set up job control, and you have a handle to your controlling terminal, you might as well keep it around -- your controlling terminal is not going to change throughout the life of the process. You'll need that handle every time you want to set the terminal's process group, and that's the essence of job control: setting the terminal's process group to the process group ID of whatever you've determined is the foreground job (which, most of the time, is yourself). So bash saves it to an fd that is unlikely to collide with anything else, which is essential because bash allows the file descriptor numbers used in redirections to consist of multiple digits. (I explained the algorithm bash uses; other shells do something different. dash and zsh use fd 10. mksh uses 24 (?). ksh93 uses fd 2 no matter what, so you don't get job control if you redirect stderr away from the terminal.) There is another "terminal operation" for which the controlling terminal is required: saving and restoring terminal state. Bash fetches the terminal attributes before running an external command, and restores the attributes if a process exits with a non-zero status or is stopped. The former is a useful convenience; the latter is required. You need the handle for the calls to tcgetattr()/tcsetattr(). You can get away without all of this stuff, of course. But that makes job control harder and less efficient. Chet -- ``The lyf so short, the craft so long to lerne.'' - Chaucer ``Ars longa, vita brevis'' - Hippocrates Chet Ramey, UTech, CWRU chet(a)case.edu http://cnswww.cns.cwru.edu/~chet/