Re: [Help-bash] When pipes fail (and when not)

[Paul Wagner]

On 28.11.2018 13:54, Eric Blake wrote:
> > I still don't understand why setting the trap on a system that has 
> > SIGPIPE vital doesn't work, so I'd be very greatful for any hints.
>
> It's a lame restriction from POSIX based on historical practice (that
> I _really_ wish we weren't stuck with):
> http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#trap
>
> "Signals that were ignored on entry to a non-interactive shell cannot
> be trapped or reset, although no error need be reported when
> attempting to do so. An interactive shell may reset or catch signals
> ignored on entry. Traps shall remain in place for a given shell until
> explicitly changed with another trap command."

Maybe I don't get your point, but I think there might be a 
missunderstanding:  In my previous post I was referring to my example on 
Linux, where SIGPIPE is not ignored by default:

for i in {0..9}; do echo $i; sleep 1; done | dd bs=1 count=10

ends as expected after 5 iterations, so SIGPIPE is not ignored in the 
current shell;

trap '' pipe
for i in {0..9}; do echo $i; sleep 1; done | dd bs=1 count=10

reports a write error five times after the fifth iteration, as expected; 
but installing another trap like

trap 'echo foo >&2' pipe
for i in {0..9}; do echo $i; sleep 1; done | dd bs=1 count=10

ends after 5 iterations (so SIGPIPE is not trapped, but handled again), 
but does not write 'foo' to stderr (the terminal), and I don't 
understand why.

Thanks for your patience,

Paul