tells the print command to append each username one after another to a file only.

告诉打印命令仅将每个用户名依次追加到一个文件中。

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A file with the 'a' attribute set can only be open in append mode for writing. Only the superuser or a process possessing the CAP_LINUX_IMMUTABLE capability can set or clear this attribute.

如果文件设置了‘a’属性，数据只能采用追加模式，仅仅超级用户或者拥有CAP _ LINUX _ IMMUTABLE能力的进程可以设置和删除该属性。

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