readme或xidel --usage：

Variable assignment:                                         $var := value

  adds $var to a set of global variables, which can be created and accessed everywhere.
  (Xidel prints the value of all variables to stdout, unless you use the --extract-exclude option)

例如：

$ xidel -se 'var:="bar"' -e '"foo"||$var'
var := bar
foobar

$ xidel -s --extract-exclude=var -e 'var:="bar"' -e '"foo"||$var'
foobar

当然也可以使用XQuery Let子句（非全局）：

$ xidel -se 'let $var:="bar" return "foo"||$var'
foobar