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+## In POSIX sh, += is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+var="Hello "
+var+="World"
+```
+
+### Correct code:
+
+```sh
+var="Hello "
+var="${var}World"
+```
+
+### Rationale:
+
+Using `+=` to concatenate to an existing variable is a ksh/bash extension. For POSIX sh or dash, write out the full expression. Be careful to use braces if the text you append could be confused for a variable (as in the example, to avoid `$varWorld` being interpreted as `${varWorld}`)  
+
+### Exceptions:
+
+None.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!