Created SC3042 (markdown)

SC3042.md

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+## In POSIX sh, set flag `--default` is undefined
+
+(or any other option)
+
+### Problematic code:
+
+```sh
+#!/bin/ksh
+set --default
+```
+
+### Correct code:
+
+Switch to a shell that supports these options, such as Ksh:
+
+```sh
+#!/bin/ksh
+set --default
+```
+
+### 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 Ksh.
+
+Most shell specific `set` flags are hard to emulate in POSIX, so use your best judgement as to whether you can make do without them, or if you should require a shell that supports them.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!