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Re: Protect Loop Execution with Traps

From: Roger
Subject: Re: Protect Loop Execution with Traps
Date: Tue, 28 Jan 2020 16:25:51 -0500
User-agent: Mutt/1.10.1 (2018-07-13)

>Here's a simple fix, that involves setting up ONE trap within the
>shell script, to override the shell's default SIGINT handling heuristic.
>trap exit INT
>while true; do
>  ping -c 3
>There.  Now, when I hit Ctrl-C, the whole script exits, not just one
>instance of ping.
>(Switching from -c 1 to -c 3 made it a *lot* less spammy, and also much
>harder to kill using the "press Ctrl-C twice really fast" approach.
>The INT trap works around it beautifully for me, though.)
>Whether this same stuff applies to ffmpeg, I have no idea.  I also
>don't know why you're Ctrl-C'ing out of an ffmpeg loop often enough
>that this has become a concern.
>But, the "simple fix" that I used here has the same issue that ping
>itself has -- we're catching SIGINT and handling it and exiting,
>instead of letting SIGINT kill us.  If something runs *us* in a loop,
>it'll have the same problem that *we* had when we tried to run ping
>in a loop.

Yes. That's not desirable then, as sweet and short as it is.

>So, in the interest of not causing problems for other programs, here's
>the more correct fix:
>trap 'trap INT; kill -INT $$' INT
>while true; do
>  ping -c 3

Wow, " trap 'trap INT; kill -INT $$' INT "  not easily readable for me.

A trap calling kill inside of a trap.

I'm thinking, put "trap INT; kill -INT $$" inside of it's own function, named 
something like exit_recurs_loop, so the first line reads "trap exit_recurs_loop 

I'll put these additional trap ideas into practice here whenever I'm doing 
loops and see what happens.

This trap stuff has me pulling out my hair, as I likely do not know the details 
of how programs use signals and how Bash interacts with such activity.

Thanks for your time!


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