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Re: Bug in select-command


From: Chet Ramey
Subject: Re: Bug in select-command
Date: Sun, 25 Jun 2017 13:01:28 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.1.1

On 6/24/17 10:46 AM, Eduardo A. Bustamante López wrote:

> I think this is an easier way to reproduce the problem. I have a terminal
> window with the following dimensions:
> 
>   address@hidden:~$ declare -p COLUMNS LINES
>   declare -- COLUMNS="191"
>   declare -- LINES="49"
> 
>   address@hidden:~$ bash --version | head -n1
>   GNU bash, version 4.4.12(1)-release (x86_64-pc-linux-gnu)
> 
> Running the script in `interactive' mode (which triggers calls to
> `get_new_window_size' through `get_tty_state' when checkwinsize is
> enabled), lays out the options on seven columns:

Yes, bash is actually doing more work here than it should. The reason for
the calls to get_tty_state is if a program terminates prematurely and
leaves the terminal in a state where interactive input is inconvenient
(noecho mode, for example). If bash isn't reading input from the terminal,
this isn't strictly necessary. But for now, it works.


> Whereas running the same script in `non-interactive' mode results in three
> columns.

Yes, there's no reason to worry about tty settings, so the shell relies on
the environment for the value of COLUMNS.  If it's not there, or doesn't
have a sane value, select defaults to 80.

> So the trick is to run an external command in foreground. To see why this
> works, read:
> http://git.savannah.gnu.org/cgit/bash.git/tree/jobs.c?h=devel#n3390

Yes, since the point is to protect interactive shell input from badly-
behaved external programs that can modify the terminal settings.  The
shell knows what it does to the terminal and how to undo it.

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



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