If the file or directory has extended attributes, the permissions
field printed by the -l option is followed by a '@' character.
Otherwise, if the file or directory has extended security
information (such as an access control list), the permissions
field printed by the -l option is followed by a '+' character.
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那什么是“extended security”呢?其实就是扩展安全信息
执行 ls -le 查看extended security: