Created SC2016 (markdown)

SC2016.md

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+# Expressions don't expand in single quotes, use double quotes for that.
+
+### Problematic code:
+
+    name=World
+    echo 'Hello $name'
+
+### Correct code:
+
+    name=World
+    echo "Hello $name"
+
+### Rationale:
+
+Single quotes prevent expansion of everything, including variables and command substitution.
+
+If you want to use the values of variables and such, use double quotes instead.
+
+### Contraindications
+
+If you want `$stuff` to be a literal dollar sign followed by the characters "stuff", 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".