Updated SC2016 (markdown)

SC2016.md

@@ -4,33 +4,44 @@
 
 ```sh
 name=World
-echo 'Hello $name'
+echo 'Hello $name'   # Outputs Hello $name
 ```
 
 ### Correct code:
 
 ```sh
 name=World
-echo "Hello $name"
+echo "Hello $name"   # Outputs Hello World
 ```
 
 ### Rationale:
 
-Single quotes prevent expansion of everything, including variables and command substitution.
+ShellCheck found an expansion like `$var`, `$(cmd)`, or `` `cmd` `` in single quotes.
 
-If you want to use the values of variables and such, use double quotes instead.
+Single quotes express all such expansions. If you want the expression to expand, use double quotes instead. 
 
-Note that if you have other items that needs single quoting, you can use both in a single word:
+If switching to double quotes would require excessive escaping of other metacharacters, note that you can mix and match quotes in the same shell word:
 
 ```sh
-echo '$1 USD is '"$rate GBP"
+dialog --msgbox "Filename $file may not contain any of: "'`&;"\#%$' 10 70
 ```
 
 ### Exceptions
 
-If you want `$stuff` to be a literal dollar sign followed by the characters "stuff", you can [[ignore]] this message.
+If you know that you want the expression literally without expansion, you can [[ignore]] this message:
 
-ShellCheck tries to be smart about it, and won't warn when this is used with awk, perl and similar, but there are some inherent ambiguities like `'I have $1 in my wallet'`, which could be "one dollar" or "whatever's in the first parameter".
+```
+# We want this to output $PATH without expansion
+# shellcheck disable=SC2016
+echo 'PATH=$PATH:/usr/local/bin' >> ~/.bashrc
+```
 
-In the particular case of `sed`, ShellCheck uses additional heuristics to try to separate cases like `'s/$foo/bar/'` (failing to replace the variable `$foo`) with from the false positives like `'$d'` (delete last line). If you're still triggering these, consider being more generous with your spaces: use `$ { s/foo/bar; }` instead of `${s/foo/bar/;}`
+ShellCheck also does not warn about escaped expansions in double quotes:
 
+```
+echo "PATH=\$PATH:/usr/local/bin" >> ~/.bashrc
+```
+
+### Related resources:
+
+* StackOverflow: [How do I use variables in single quoted strings?](https://stackoverflow.com/questions/21192420/how-do-i-use-variables-in-single-quoted-strings)