4

Say we have a simple pipeline:

set -eo pipefail;
echo 'foo' | cat

how does cat know when to exit? Is it when the STDIN stream ends, or does cat receive a signal? In my case, the producer process receives a SIGINT and then exits with code 1, so how can cat (consumer process) know which exit code the producer process exited with?

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13

cat doesn't know about echo's signals. It receives end-of-file condition per man pipe(7):

If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from the pipe will see end- of-file (read(2) will return 0).

For writers to the pipe, however, there is SIGPIPE. See the same man page

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  • Is there a way to trap (or listen for) the end-of-file condition in the consumer? – Alexander Mills Sep 22 at 1:05
  • Also, is there any way for the consumer to know the exit code of the producer process? – Alexander Mills Sep 22 at 1:09
  • 1
    @AlexanderMills For the first, absolutely; if you're writing in C, you can call feof(STDIN) or whatever. For the second, no; only the process's parent knows its exit code. – Draconis Sep 22 at 1:11
  • @AlexanderMills Draconis already addressed end-of-file part, but as far as exit code goes the best answer here is probably "not with simple shell pipe". If it's a pipeline created via C or Python code, in theory you could have both producer and consumer share a resource and write exit code to it. But of course that means both producer and consumer code are under your direct control. Maybe something like this would work (echo "I'm a producer"; echo $? ) | some_consumer.py but that implies consumer has to always check last line for a numeric string and handle cases when it's not. – Sergiy Kolodyazhnyy Sep 22 at 1:33

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