Created SC2331 (markdown)

SC2331.md

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+## For file existence, prefer standard -e over legacy -a.
+
+### Problematic code:
+
+```sh
+if [ -a ~/.bash_aliases ]
+then
+  source ~/.bash_aliases
+fi
+```
+
+### Correct code:
+
+```sh
+if [ -e ~/.bash_aliases ]
+then
+  source ~/.bash_aliases
+fi
+```
+### Rationale:
+
+The POSIX standard way to check whether a file exists is `[ -e file ]`.
+
+Bash and Ksh have historically allowed using `-a`, but this has no benefit and some potential downsides. For example, in Bash `[ -a file ]` is true when the file exists, but `[ ! -a file ]` is also true, and unconditionally so, because `-a` is treated as logical "and" between non-empty strings.
+
+Avoid the ambiguity entirely by always using the standard, portable `-e`.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!