mount 命令 device 指定 none

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本文解释了Linux系统中为何会将/run目录下的子目录作为单独的挂载点，并使用none类型进行挂载。这些子目录如/run/lock、/run/shm和/run/user等分别挂载了tmpfs文件系统，其目的是为了设置不同的权限和参数。

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From: http://unix.stackexchange.com/questions/136646/what-is-type-none-for-mount-points-and-why-are-mount-points-of-the-same-type-b

简要说明：这里的 none 就是表明非物理设备而是用于虚拟文件系统的。

问题：

In Ubuntu 14.04 when executing command mount I get following excerpt

tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
none on /run/lock type tmpfs (rw,noexec,nosuid,nodev,size=5242880)
none on /run/shm type tmpfs (rw,nosuid,nodev)
none on /run/user type tmpfs (rw,noexec,nosuid,nodev,size=104857600,mode=0755)

I do not understand why subdirectories of /run are mounted separately inside already mounted parent directory (/run) and what none filesystem means. My /etc/fstab file is empty.

回答：

The none just means that there is no physical disk partition linked to the mount point you see when issuing the mount command. It is used for virtual filesystems like shm, ramfs, proc and tmpfs. The lock, shm and user mount points in this example are created separately so they can be mounted with different permissions and settings. Notice the part inside the parentheses?