Created SC3045 (markdown)

SC3045.md

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+## In POSIX sh, some-command-with-flag is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+ShellCheck has noticed you're using flags for commands that don't necessarily support them. Some examples:
+
+```sh
+#!/bin/sh
+read -e     # Using libreadline
+export -f   # Exporting functions
+ulimit -v   # Setting vspace limits
+wait   -n   # Waiting for a single process
+```
+
+### Correct code:
+
+There are generally no simple rewrites. The easiest solution is instead to change the shebang and switch to a shell that *does* support the features you want, such as `bash`.
+
+Alternatively, look up how to do what you want to do in POSIX sh.
+
+### Rationale:
+
+External commands (`grep`, `ls`, `find`) invoke a binary on the system and therefore accept the same flags from all shells.
+
+However, some commands are instead built into the shell and therefore accept different flags depending on which shell is running them.
+
+You have specified `sh` or `dash` in the shebang, but the flags you are using only works when it's executed in e.g. `bash`. You should either explicitly declare that the script requires `bash` to run, or you should only use features that work on all shells. 
+
+### Exceptions:
+
+If the code is gated on a check of the current shell, you can [[ignore]] this warning.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!