file join
file join a b /foo bar
/foo/bar
=>file join /foo bar
/foo/bar
=>file join aaa bbb /foo bar
/foo/bar
=>file join aaa bbb foo bar
aaa/bbb/foo/bar
=>file join aaa bbb /foo bar
/foo/bar
=>file join aaa bbb /foo /bar
/bar
/foo/bar
=>file join /foo bar
/foo/bar
=>file join aaa bbb /foo bar
/foo/bar
=>file join aaa bbb foo bar
aaa/bbb/foo/bar
=>file join aaa bbb /foo bar
/foo/bar
=>file join aaa bbb /foo /bar
/bar