Created SC2221 (markdown)

SC2221.md

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+## This pattern always overrides a later one.
+
+### Problematic code:
+
+```sh
+case "$1" in
+  -?) echo "Usage: $0 [-n]";;
+  -n) echo "Hello World";;
+   *) exit 1;;
+esac
+```
+
+### Correct code:
+
+```sh
+case "$1" in
+  -\?) echo "Usage: $0 [-n]";;
+  -n) echo "Hello World";;
+   *) exit 1;;
+esac
+```
+### Rationale:
+
+You have specified multiple patterns in a `case` statement, where one will always override the other.
+
+In the example, `-?` actually matches a dash followed by any character, such as `-n`. This means that the later `-n` branch will never trigger. In this case, the correct solution is to escape the `-\?` so that it doesn't match `-n`. 
+
+Another common reason for this is accidentally duplicating a branch. In this case, fix or delete the duplicate branch.
+
+### Exceptions:
+
+None. One could argue that having `-*|--*) echo "Invalid flag";` is a readability issue, even though the second pattern follows from the first. In this case, you can either rearrange the pattern from most to least specific, i.e. `--*|-*)` or ignore the error.
+
+When ignoring this error, remember that ShellCheck directives have to go in front of the `case` statement, and not in front of the branch:
+
+    # shellcheck disable=SC2221,SC2222
+    case "$1" in
+      -n) ...;;
+      # no directive here
+      -*|--*) echo "Unknown flag" ;;
+    esac