Created SC2309 (markdown)

SC2309.md

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+## -eq treats this as a variable. Use = to compare as string (or expand explicitly with $var)
+
+### Problematic code:
+
+```sh
+read -p "Continue? [y/n] " var
+[ "$var" -eq "n" ​] && exit 1
+```
+
+### Correct code:
+
+```sh
+#read -p "Continue? [y/n] " var
+[ "$var" = "n" ​] && exit 1
+```
+
+### Rationale:
+
+ShellCheck found a string used as an argument to a numerical operator like `-eq`, `-ne`, `-lt`, `-ge`. Such strings will be treated as arithmetic expressions, meaning `n` will refer to a variable `$n`, and `24/12` will be evaluated into `2`. 
+
+In the problematic example, the intention was instead to compare `"n"` as a string, so it should use the equivalent string operator instead, in this case `=`.
+
+### Exceptions:
+
+It is perfectly valid to use variables as operands. ShellCheck will not flag any value that is an unquoted variable name assigned in the script:
+
+    a=42; [[ "a" -eq 0 ]]  # Flagged due to quotes
+          [[ b -eq 0 ]]    # Flagged due to not being assigned
+    c=42; [[ c -eq 0 ]]    # Not flagged
+
+However, ShellCheck does not know whether you intended `foo/bar` to be division or a file path. 
+
+If you intended to divide `$foo` and `$bar`, you can either make it explicit with `[[ $((foo/bar)) -ge 0 ]]`, or simply [[ignore]] the warning.
+ 
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc