Created SC2304 (markdown)

SC2304.md

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+## `*` must be escaped to multiply: `\*`. Modern `$((x * y))` avoids this issue.
+
+### Problematic code:
+
+```sh
+result=$(expr 2 * 3)
+```
+
+### Correct code:
+
+```sh
+# Modern, efficient, POSIX standard approach
+result=$(( 2 * 3 ))
+
+# Older, slower approach
+result=$(expr 2 \* 3)
+```
+### Rationale:
+
+When using `expr`, each argument is expanded the same way as for any other command. This means that `expr 2 * 3` will turn into `expr 2 Desktop Documents Downloads Music Pictures 3` depending on the files in the current directory, causing an error like `expr: syntax error: unexpected argument ‘Desktop’`
+
+The best way to avoid this is to avoid `expr` and instead use `$((..))` instead. If you for any reason prefer the 200x slower, heavyweight process of forking a new process, you can escape the `*`. Both ways are demonstrated in the correct example.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!