I have two programs in bash:
{ { sleep 1s; kill 0; } | { while true; do echo "foo"; done; kill 0;} }
and
{ { while true; do echo "foo"; done; kill 0; } | { sleep 1s; kill 0; } }
(just changed order).
How is it possible that the first one writes a lot of "foo" in the output and the second one writes nothing?
Connecting two processes by a pipe redirects the output from the first to the second. Thus, connecting a process which writes output to a process that does nothing that output means no output occurs.
By contrast, connecting a process which does nothing to a process that generates output, the latter will proceed to generate output as usual.
By the way, what's the purpose of the kill 0 lines? I doubt very much that they serve a useful purpose here.
The second one does output foo, you just don't see it because it is piped to your second command.
You can prove this by redirecting to a file:
$ { { while true; do echo "foo" >> /tmp/f; done; kill 0; } | { sleep 1s; kill 0; } }
$ wc -l /tmp/f
56209
that's because in the second command you are piping the output of a echo as the input of { sleep 1s; kill 0; }
