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http://www.cnblogs.com/peida/archive/2012/12/05/2803591.html
这里列出了常用的Linux命令，之后在学习中逐次添加自己的理解。
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Linux系统信息查看命令大全
http://charlee.li/linux-sysinfo-cmds.html
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free命令 两个版本详解
http://blog.csdn.net/for_tech/article/details/53044610
https://access.redhat.com/solutions/406773#
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