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

[Chet Ramey]

On 11/28/18 7:02 AM, Paul Wagner wrote:

> 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.

Each element of a pipeline is executed in a subshell environment, described
in

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_12

"A subshell environment shall be created as a duplicate of the shell
environment, except that signal traps that are not being ignored shall be
set to the default action. [...] Additionally, each command of a multi-
command pipeline is in a subshell environment; as an extension, however,
any or all commands in a pipeline may be executed in the current
environment."

Subshell environments don't inherit traps from the parent shell, unless
the trap has specified that the signal is to be ignored. Bash always
executes the first element of a pipeline in a subshell environment.

If you define the trap in the same subshell environment as the for loop,
you'll get the output you want.

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