Created SC3017 (markdown)

SC3017.md

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+## In POSIX sh, unary `-a` in place of `-e` is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+if [ -a file.txt ]
+then
+  echo "file.txt exists"
+fi
+```
+
+### Correct code:
+
+```sh
+#!/bin/sh
+if [ -e file.txt ]
+then
+  echo "file.txt exists"
+fi
+```
+
+### Rationale:
+
+`-a` was used in some shells to check for the existence of a file, but the POSIX standard and better supported equivalent is `-e`.
+
+### 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!