[TUHS] UNIX of choice these days?

Chet Ramey chet.ramey at case.edu
Wed Sep 27 23:50:10 AEST 2017

On 9/26/17 10:02 PM, Larry McVoy wrote:

>>> Yeah, but still doesn't answer the question, application-wise.?? I'm gonna
>>> guess (without digging through the source) that it's a "dup2(0, 255)*" or
>>> something, to "save" a copy of stdin/out/err for some obscure reason.
>> I already answered this.
> OK, I'm gonna be that guy because I've learned when I ask, I learn.  
> You answered but I didn't get any insight.  Why have an extra fd talking
> to the tty?  bash has 0, 1, 2 talking to it.   If it were redirected 
> would it have 255 pointing to some random tty?  I don't get the reason
> for the extra fd.

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.

``The lyf so short, the craft so long to lerne.'' - Chaucer
		 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet at case.edu    http://cnswww.cns.cwru.edu/~chet/

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