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+## In POSIX sh, /dev/{tcp,udp} is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+echo foo > /dev/tcp/myhost/1234
+```
+
+### Correct code:
+
+Rewrite using a tool like netcat (`nc`):
+
+```sh
+echo foo | nc myhost 1234
+```
+
+### Rationale:
+
+`/dev/tcp/$host/$port` and `/dev/udp/$host/$port` are recognized in redirections by bash and ksh, and a socket connection is made instead of opening a file. They do not physically exist in `/dev`.
+
+POSIX sh and dash do not support this, so any such code should be rewritten to use a socket tool explicitly. The obvious candidate is netcat aka `nc`.  
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!