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Re: Protect Loop Execution with Traps

From: Greg Wooledge
Subject: Re: Protect Loop Execution with Traps
Date: Tue, 28 Jan 2020 08:49:35 -0500
User-agent: Mutt/1.10.1 (2018-07-13)

On Mon, Jan 27, 2020 at 09:03:22PM -0500, Roger wrote:
> I've always had a problem with Bash script (eg. for/while) loops creating 
> havoc 
> upon a ctrl-c keypress.

What's *in* the loop?  It matters.

Consider the following two scripts:

while true; do
  echo "Zzz"
  sleep 1

while true; do
  # Assuming Linux
  ping -c 1

If you run these, and try to kill them with Ctrl-C, you may find that
the first one behaves perfectly (stops when you ask), and the second
one does not.  It may take several tries to kill the second one.  You
might have better luck suspending it with Ctrl-Z first, then killing
the shell only, then fg'ing the job.

See <https://www.cons.org/cracauer/sigint.html> for details.

In a nutshell, ping(8) is a misbehaving program.  It sets a SIGINT
trap (so that it can report statistics when you interrupt it), but
it doesn't exit as a result of the signal.  The shell sees that the
program didn't exit due to SIGINT, and decides that it (the shell)
should not exit either.

Whatever program you're using inside your loop may have the same
issues.  It's extremely common.

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