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+## In POSIX sh, coproc is undefined.
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+coproc nc shellcheck.net 80
+```
+
+### Correct code:
+
+Either switch to Bash:
+
+```sh
+#!/usr/bin/env bash
+coproc nc shellcheck.net 80
+```
+
+or try to write your script in a POSIX compatible way using named pipes.
+
+### Rationale:
+
+Your script uses a shell feature not supported by the shebang. Either rewrite the script to be portable, or change the shebang to explicitly require a shell like Bash.
+
+There is no simple substitution for a coproc, though many use cases can be rewritten via redirection to named pipes (`mkfifo`).
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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