Created SC2064 (markdown)

SC2064.md

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+## Use single quotes, otherwise this expands now rather than when signalled.
+
+### Problematic code:
+
+    trap "echo \"Finished on $(date)\"" EXIT
+
+### Correct code:
+
+    trap 'echo "Finished on $(date)"' EXIT
+
+### Rationale:
+
+With double quotes, all parameter and command expansions will expand when the trap is defined rather than when it's executed.
+
+In the example, the message will contain the date on which the trap was declared, and not the date on which the script exits.
+
+Using single quotes will prevent expansion at declaration time, and save it for execution time. 
+
+### Contraindications
+
+If you don't care that the trap code is expanded early because the commands/variables won't change during execution of the script, or because you want to use the current and not the future values, then you can ignore this message.