Created SC2306 (markdown)

SC2306.md

@@ -0,0 +1,35 @@
+## Escape glob characters in arguments to expr to avoid pathname expansion.
+
+### Problematic code:
+
+```sh
+f=$(expr "$c" * 9 / 5 + 32)
+```
+
+### Correct code:
+
+Prefer rewriting to a modern style (see [[SC2003]]):
+
+```sh
+f=$((c * 9 / 5 + 32))
+```
+
+If you do not wish to do so, at least escape the glob characters when passing them to `expr`:
+
+```sh
+f=$(expr "$c" \* 9 / 5 + 32)
+```
+
+### Rationale:
+
+`expr` is a command so `expr 2 * 2` will consider `*` to mean "all files in the current directory". This causes the expression to fail to evaluate unless you are in an empty directory with the `failglob` and `nullglob` options turned off. 
+
+Prefer rewriting it using the modern, POSIX standard arithmetic expansion `$((..))`. If you do not wish to do so, you can escape any characters like `*` to avoid the shell performing pathname expansion on them.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!