less error

output.txt

~/Dev/test ⌚ 14:26:44
$ tree
.
├── bar
│   └── b.less
└── foo
    └── a.less

2 directories, 2 files

~/Dev/test ⌚ 14:26:47
$ cat foo/a.less 
div {
  background-image: url(img/a.png);
}

~/Dev/test ⌚ 14:26:54
$ cat bar/b.less 
@import "foo/a.less";

span {
  background: url(img/b.png);
}

~/Dev/test ⌚ 14:26:57
$ lessc --ru bar/b.less 
div {
  background-image: url(foo/img/a.png); <-- 相对于bar/b.less，应该是../foo/img/a.png
}
span {
  background: url(img/b.png);
}