According to FHS-3.0, /tmp is for temporary files and /run is for run-time variable data.  Data in /run must be deleted at next boot, which is not required for /tmp, but still programs must not assume that the data in /tmp will be available at the next program start.  All this seems quite similar to me.So, what is the difference between the two?  By which criterion should a program decide whether to put temporary data into /tmp or into /run?According to the FHS:Programs may have a subdirectory of /run; this is encouraged for  programs that use more than one run-time file.This indicates that the distinction between system programs and ordinary programs is not a criterion, neither is the lifetime of the program (like, long-running vs. short-running process).Although the following rationale is not given in the FHS, /run was introduced to overcome the problem that /var was mounted too late such that dirty tricks were needed to make /var/run available early enough.  However, now with /run being introduced, and given its description in the FHS, there does not seem to be a clear reason to have both /run and /tmp.
What's the difference between /tmp and /run?
linux;files;filesystems;fhs
No reason to have both /run and /tmpI think you're right.  /tmp is essentially deprecated now we have /run.  If your program is in a position to do so (which requires that it was installed as a privileged operation), then nowadays you would use a sub-directory of /run.  This is for security reasons.E.g. the CUPS printing daemon does not run as root, but is generally installed from an OS package.  The package installs /usr/lib/tmpfiles.d/cups.conf, and systemd-tmpfiles creates a directory it can access.  Since the directory is under /run, the name cannot have been maliciously claimed by an unprivileged user, unlike /tmp which is world-writable.Unprivileged programs which can't use /run directlyThe real distinction is if your program is being run by an arbitrary unprivileged user, under their own user ID.  But you still generally don't want to use /tmp, because it can be accessed by other unprivileged users.  You would prefer to use $XDG_RUNTIME_DIR.  Typically this is implemented as /run/user/$(id -u) - so it happens to be a subdirectory of /run as well.  The location isn't guaranteed though; programs should always use the environment variable./tmp would only be useful for ad-hoc co-operation between different unprivileged users on the system.  Such ad-hoc systems are vulnerable to a malicious user refusing to co-operate and spoiling things for everyone :).  One example would be unprivileged users deciding to run a version of the talk daemon, using a unix socket.Original information  from Lennart PoetteringNote, Poettering's checklist below claimed that /tmp would be useful for small files, whereas /run should only be used for communication primitives.  I don't think this distinction is true either.  The poster-boy for /run is udev, and I'm pretty sure /run/udev includes internal databases .  Once you have a /run directory, I don't think anyone wants to follow the claimed distinction and create another directory, to clutter /tmp.  So in practice we just use /run nowadays.Usage of world-writable shared namespaces [like /tmp] for communication purposes has always been problematic, since to establish communication you need stable names, but stable names open the doors for DoS attacks. This can be corrected partially, by establishing protected per-app directories for certain services during early boot (like we do for X11), but this only fixes the problem partially, since this only works correctly if every package installation is followed by a reboot....Another Fedora feature (for Fedora 17) changed the semantics of /tmp for many system services to make them more secure, by isolating the /tmp namespaces of the various services...Because /tmp is no longer necessarily a shared namespace it is generally unsuitable as a location for communication primitives....[/run] is guaranteed to be a tmpfs and is hence automatically flushed at boots. No automatic clean-up is done beyond that....Here's a rough guide how we suggest you (a Linux application developer) pick the right directory to use:You need a place to put your socket (or other communication primitive) and your code runs privileged: use a subdirectory beneath /run. (Or beneath /var/run for extra compatibility.)You need a place to put your socket (or other communication primitive) and your code runs unprivileged: use a subdirectory beneath $XDG_RUNTIME_DIR.You need a place to put your larger downloads and downloads in progress and run unprivileged: use $XDG_DOWNLOAD_DIR.You need a place to put cache files which should be persistent and run unprivileged: use $XDG_CACHE_HOME.Nothing of the above applies and you need to place a small file that needs no persistency: use $TMPDIR with a fallback on /tmp. And use mkstemp(), and mkdtemp() and nothing homegrown.Otherwise use $TMPDIR with a fallback on /var/tmp. Also use mkstemp()/mkdtemp().Note that these rules above are only suggested by us. These rules take into account everything we know about this topic and avoid problems with current and future distributions, as far as we can see them. Please consider updating your projects to follow these rules, and keep them in mind if you write new code.One thing we'd like to stress is that /tmp and /var/tmp more often than not are actually not the right choice for your usecase. There are valid uses of these directories, but quite often another directory might actually be the better place. So, be careful, consider the other options, but if you do go for /tmp or /var/tmp then at least make sure to use mkstemp()/mkdtemp().We kind of get away with the legacy /tmp socket used by the X window system, as described above.  I misread tmpfiles.d/x11.conf.  Looks more like it relies on co-operation :).  I assume the code's been audited, such that denial of service is the worst that can happen.