命令：
history
n 列出最近n条命令行表
-c 将目前的shell中的所有history内容全部清除
-a 将目前新增的history命令新增入histfiles中，若没有加histfiles，则默认写入~/.bash history.
-r 将histfiles的内容读取到目前这个shell的history记录中
-w 将目前的history记录内容写入histfiles中
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