added example of grep

SC2081.md

@@ -25,6 +25,12 @@ In bash/ksh, you can instead use `[[ .. ]]` which supports this behavior.
 
 In sh, you can rewrite to use `grep`. 
 
+```sh
+if echo $var | grep -q '^[0-9]*$'; then
+  echo "$var is numeric"
+fi
+```
+
 ### Exceptions:
 
 None. If you are not trying to match a glob, quote the argument (e.g. `[ $var == '*' ]` to match literal asterisk.