Created SC2179 (markdown)

SC2179.md

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+## Use array+=("item") to append items to an array.
+
+### Problematic code:
+
+```sh
+var=(one two)
+var+=three
+```
+
+### Correct code:
+
+```sh
+var=(one two)
+var+=( three )
+```
+### Rationale:
+
+It looks like you are trying to append a string to an array with `var+=string`. This instead appends to the first element of the array (equivalent to `var[0]+=three`).
+
+In the problematic code, the array will therefore contain `onethree` `two`.
+
+Instead, append an array to the array with `var+=( elements )`. This will append the new items to the array.
+
+In the correct code, it will contain `one` `two` `three` as expected.
+
+### Exceptions:
+
+If ShellCheck mistakenly thinks the variable is an array when it's not (e.g. because the same name was used in a different context), you can ignore this error.