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    this exec 3>P cause hang in bash, why? Commented Dec 4, 2017 at 16:25
  • @Wang It shouldn't. If it does then you're probably not doing the same thing as the as the POC code in the question. The only reason I can think that it would block is if you're instead doing something like exec 2>P, and you have trace mode turned on (set -x), in which bash is going to write to the pipe, but there's no reader so it blocks waiting for something to read. Commented Feb 19, 2018 at 13:45
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    @Wang @Patrick Indeed exec 3>P hangs in bash on my machine too. This is because there is no process reading from P. Thus, the solution was to swap lines exec 3>P and program < P & (adding the ampersand so that program runs in background). Commented Oct 17, 2018 at 9:04
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    @macieksk How about exec 3<>P? It counts as a writer and as a reader. Useful in case one wants program in the foreground. Commented Feb 14, 2021 at 3:39
  • fails when the reader is python interpreter: it does not execute the input commands! Commented Dec 31, 2022 at 10:02