在curl命令中，文件名前的@（@）符号是什么意思？

我正在看一个curl命令示例，该命令在文件名之前使用@符号将数据发布到服务器。

curl http://localhost/ --data-binary @file.txt

这是什么意思？是否将文件内容插入命令？是特定于curl？

它特定于卷曲。来自man curl：

--data-binary <data>
  (HTTP) This posts data exactly as specified with no extra processing whatsoever.
  If you start the data with the letter @, the rest should be a filename.  Data is
  posted in a similar manner as --data-ascii does, except that newlines are preserved
  and conversions are never done.

  If this option is used several times, the ones following the first will append data
  as described in -d, --data.