默认情况下，如果不指定要列出的内容，ls就列出当前目录。
By default, if you don't specify something to list, ls lists its current directory.
目录不是常规文件，但是在列出目录内容或将其作为当前工作目录时，对其进行了访问，所以需要在搜索中使用否定和- type标志排除它们。
Directories aren't regular files but are accessed every time you list them or make them the current working directory, so exclude them in the search using a negation and the -type flag.
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