(or "In dash, ... is not supported." when using dash)

var="Hello "
var+="World"

var="Hello "
var="${var}World"

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})

None.

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