Updated SC2157 (markdown)

SC2157.md

@@ -1,11 +1,12 @@
 ## Argument to implicit -n is always true due to literal strings.
+(Or: Argument to -z is always false due to literal strings. )
 
 ### Problematic code:
 
 ```sh
 if [ "$foo " ]
 then
-  echo "this is always true"
+  echo "this is always true because of the trailing space"
 fi
 ```
 
@@ -20,9 +21,16 @@ fi
 
 ### Rationale:
 
-Since `[ str ]` checks that the string is non-empty, the space inside the quotes in the problematic code causes the test to always be true, since a string with a space can not be empty.
+Since `[ str ]` and `[ -n str ]` check that the string is non-empty, any literal characters in the string -- including a space character like in the example -- will cause the test to always be true.
+
+Equivalently, since `[ -z str ]` checks that the string is empty, any literal character in the string will cause the test to always be false.
+
+Double check the string: you may have added trailing characters, or bad quotes or syntax. Some examples include:
+
+* `[ "$foo " ]` like in the example, where the space becomes part of the string
+* `[ "{$foo}" ]` instead of `[ "${foo}" ]`, where the `{` becomes part of the string
+* `[ "$foo -gt 0" ]` instead of `[ "$foo" -gt "0" ]`, where the `-gt` becomes part of the string
 
-Sometimes this is also caused by overquoting an example, e.g. `[ "$foo -gt 0" ]`, which is always true for the same reason. The intention here was `[ "$foo" -gt 0 ]`.
 
 ### Exceptions: