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I just ran df -h a minute ago and noticed a filesystem has been added that I'm not familiar with. Does anyone know why /run exists? Is this something that's been added by the kernel? By Arch Linux?

run              10M  236K  9.8M   3% /run
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Apparently, many tools (among them udev) will soon require a /run/ directory that is mounted early (as tmpfs). Arch developers introduced /run last month to prepare for this.

The udev runtime data moved from /dev/.udev/ to /run/udev/. The /run mountpoint is supposed to be a tmpfs mounted during early boot, available and writable to for all tools at any time during bootup, it replaces /var/run/, which should become a symlink some day. [1]

There is more detail here: http://www.h-online.com/open/news/item/Linux-distributions-to-include-run-directory-1219006.html

[1] From thread on the Arch Projects ML

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    is it only for Arch or what ? /run will be add in RHEL/CentOs, Ubuntu ? – Rahul Patil Apr 7 '13 at 18:59
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    Do files get automatically deleted when the process stops running? – Ian Ringrose Dec 8 '16 at 9:49
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    @Ian, if it's a tmpfs filesystem, then on reboot it will be emptied. Your question: as I know, files would not be automatically deleted, rather process itself has to do proper cleanup. – Bulat M. Feb 5 '17 at 5:49
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The /run directory is the companion directory to /var/run. Like for example /bin is the companion of /usr/bin.

That means that daemons like systemd and udev, which are started very early in the boot process - and perhaps before /var/run is available (i.e. mounted) - have with /run a standardized file system location available where they can store runtime information.

Like /bin contains important programs, which may be needed in the boot process before /usr is available (in case it is on its own filesystem).

The /run idea is a relatively new idea/standard.

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    Arch's /var/run directory is symlinked to /run. – user26112 Apr 20 '13 at 22:24
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    Debian's /var/run is also symlinked to /run – naoko Jan 16 '16 at 14:04
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    So is Ubuntu 16.10's. – Federico Poloni Mar 27 '17 at 18:01
  • This answer is misleading. /run exists because /var/run was the wrong location, and most distros either symlink or bind mount them to each other. They are not intended to be separate directories; /run is the correct location, /var/run is kept around to support legacy software. See lwn.net/Articles/436012 for the motivation behind it. – Matthew Sharp Dec 24 '20 at 4:48
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    @maxschlepzig Yes I did. My comment was probably a bit non-specific, but it would help if you were more specific about what is misleading in my comment. – Matthew Sharp Dec 26 '20 at 19:25

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