Created SC2218 (markdown)

SC2218.md

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+## This function is only defined later. Move the definition up.
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+myfunction
+
+myfunction() {
+  echo "Hello World"
+}
+```
+
+### Correct code:
+
+```sh
+#!/bin/sh
+myfunction() {
+  echo "Hello World"
+}
+myfunction
+```
+
+### Rationale:
+
+You are calling a function that you are defining later in the file. The function definition must come first.
+
+Function definitions are much like variable assignments, and define a name at the point the definition is "executed". This is why they must happen before their first use.
+
+This is especially apparent when defining functions conditionally:
+
+```
+case "$(uname -s)" in
+  Linux) hi() { echo "Hello from Linux"; } ;;
+  Darwin) hi() { echo "Hello from macOS"; } ;;
+  *) hi() { echo "Hello from something else"; } ;;
+esac
+
+hi
+```
+
+
+### Exceptions:
+
+None.