查这些工具的manpage看不到这样的用法，但是这些工具的参数解析可能是用getopt(1)做的，getopt(1)里面有说明。

       The parameters getopt is called with can be divided into two parts: op‐
       tions which modify the way getopt will do the parsing (the options  and
       the  optstring  in  the  SYNOPSIS),  and the parameters which are to be
       parsed (parameters in the SYNOPSIS).  The second part will start at the
       first non-option parameter that is not an option argument, or after the
       first occurrence of '--'.  If no '-o' or '--options' option is found in
       the  first  part, the first parameter of the second part is used as the
       short options string.

【 在 gnwd (gnwd) 的大作中提到: 】
: 我以前写的脚本，
:     unar -- "$FILENAME"
: 我现在搞不懂这两个减号什么意思了
: ...................
--
FROM 103.90.178.*