Created SC3001 (markdown)

SC3001.md

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+## In POSIX sh, process substitution is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+while IFS= read -r n
+do
+  sum=$((sum+n))
+done < <(program)
+```
+
+### Correct code:
+
+The easiest fix is to switch to a shell that does support process substitution, by changing the shebang to `#!/bin/bash` or `ksh`.
+
+```sh
+#!/bin/bash
+while IFS= read -r n
+do
+  sum=$((sum+n))
+done < <(program)
+```
+
+Alternatively, process substitution can often be replaced with temporary files:
+
+```sh
+#!/bin/sh
+tmp="$(mktemp)"
+program > "$tmp"
+while IFS= read -r n
+do
+  sum=$((sum+n))
+done < "$tmp"
+rm "$tmp"
+```
+
+If streaming is important, the temporary file can be a named pipe, and the producer or consumer can be run as a background job.
+
+### Rationale:
+
+Process substitution is a ksh and bash extension. It does not work in sh or dash scripts.
+
+### Exceptions:
+
+If you only intend to target shells that supports this feature, you can change
+the shebang to a shell that guarantees support, or [[ignore]] this warning.
+
+You can use `# shellcheck disable=SC3000-SC4000` to ignore all such compatibility
+warnings.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!