mount 命令 device 指定 none

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?