Bash variable assigment modifiers

I always seem to forget this about bash variable assignment shortcuts and always have a hard time finding it.

Rather than:

if [ "" = "$var" ]; then
    $var=hello
fi

Use

var=${var:-hello}

Conversely, rather than:

if [ "" != "$var" ]; then
    $var=hello
fi

Use

var=${var:+hello}

There's also:

if [ "" = "$var" ]; then
    echo "Error, variable var must be set"
else
    b=$var
fi

And:

b=${var:?Error, variable var must be set}

There are others too, but I find these ones most useful. Check bash man page for full set.