Updated SC2213 (markdown)

SC2213.md

@@ -34,4 +34,26 @@ Either add a case to handle the flag, or remove it from the `getopts` option str
 
 ### Exceptions:
 
-None.
+ShellCheck may not correctly recognize less canonical uses of `while getopts ..; do case ..;`, such as when modifying the variable before using it:
+
+```
+while getopts "rf-:" OPT; do
+  if [ "$OPT" = "-" ]; then   # long option: reformulate OPT and OPTARG
+    OPT="${OPTARG%%=*}"       # extract long option name
+    OPTARG="${OPTARG#$OPT}"   # extract long option argument (may be empty)
+    OPTARG="${OPTARG#=}"      # if long option argument, remove assigning `=`
+  fi
+
+  case "$OPT" in
+    r) ... ;;
+    f) ... ;;
+    my-long-option) ... ;;
+  esac
+done
+```
+
+In such cases you can do one of:
+
+* [[Ignore]] the warning.
+* Use the external tool `getopt` (no "s") which supports long options natively.
+* Rewrite to not modify the variable first, in this case by instead doing it in a `-)` branch.