Want clean the /private/var/folders/* at the OS X boot, by creating an launchd entry. (AFAIK, the OS X doesn't supports the rc.conf rc.d - everything must be done by creating an entry for the launchd)

Here is an similar question. Unfortunately the accepted answer is wrong, because here are daemons (especially deaemons what relies on the mktemp call) what could run and has still opened files - so, you couldn't delete anytime anything in the /private/var/folders/*. (To be correct, the suggested "reboot after" somewhat salvages the problem - but not entirely.)

So, the question is: is safe delete the files in the /private/var/folders/* at boot time?

(I can imagine than some installation script what needs reboot (.kext installation or such) could use temp folders for some post-reboot installation data - and don't want lock-up my system after some critical installation by deleting necessary files :) - this is pure my speculation - therefore asking).

  • Or the only safe way is deleting files what are older than X days?
  • Or another idea for safe but automatic cleaning of the temp files/dirs in the /private/var/folders/* because it's keeps growing and growing... ;(

TL;DR: /var/folders contains folders for per-user caches and temporary files. You can access your own folders, but not those of other users. Deleting files that are not currently in-use should be safe, but deleting files that are in-use will probably cause problems. If you want to purge them, you should reboot afterward, to minimize the likelihood of problems.

If you do nothing, system processes will periodically clean these folders of files that have not been accessed for three days, so you don't have to do anything, unless you are running very low on storage.

The details

/var/folders mostly contains temporary files and cache files, stored on a per-user basis. If you look at the root of /var/folders, you won't see much:

$ ls -l /var/folders
total 0
drwxr-xr-x   3 root  wheel   102 Mar  2  2014 43/
drwxr-xr-x   3 root  wheel   102 Mar  2  2014 4c/
drwxr-xr-x   3 root  wheel   102 Mar  2  2014 c3/

But if you look one level lower, you will see that these contain per-user folders. Typically, each top-level (two character) folder contains one user folder, but the "zz" folder contains many system-users. For example (user-names changed for privacy purposes):

$ ls -l /var/folders/*
total 0
drwxr-xr-x  6 user1  staff  204 Oct  3  2016 3lsw0k6d7vqbywvsf226nkqc0000gs/

total 0
drwxr-xr-x  6 user2  staff  204 Jul 15  2016 yc9pgt3j5y18vr29jcgbbyb4000101/

total 0
drwxr-xr-x  6 user3  staff  204 Aug 16  2016 wv6q5xlx1flgtwcs0rjlkbd4000102/

total 0
drwxr-xr-x  4 admin  staff  136 Jul  3  2016 qjg8tsk964scvw2510_n6yz00000gt/

total 0
drwxr-xr-x  4 macports  macports  136 Jul  3  2016 m3d21nm91wnd4c0k2yjk57200000gn/

total 0
drwxr-xr-x  6 root       wheel      204 Jul  3  2016 zyxvpxvq6csfxvn_n0000000000000/
drwxr-xr-x  3 _networkd  _networkd  102 Jul  3  2016 zyxvpxvq6csfxvn_n000003000000r/
drwxr-xr-x  3 _lp        _lp        102 Jul  3  2016 zyxvpxvq6csfxvn_n000003800000t/

Within each one of these, you will find a few magic folders:

$ ls -l /var/folders/4c/yc9pgt3j5y18vr29jcgbbyb4000101/
total 0
drwxr-xr-x   6 user2  staff   204 Sep  1 10:31 0/
drwx------  53 user2  staff  1802 Jul 28 18:44 C/
drwxr-xr-x   2 user2  staff    68 Jul  3  2016 Cleanup At Startup/
drwx------  83 user2  staff  2822 Sep  1 10:53 T/

The "C" folder is that user's caches - containing what (prior to MacOS X 10.5) would have gone in /Library/Caches. The "T" folder is that user's temporary files - taking the place of /tmp or maybe /var/tmp.

I'm not sure about the other two folders. Cleanup At Startup is empty. I suspect its purpose is to store files that are needed up to the moment of a reboot, but should be purged afterward (e.g. temporary files used by system updaters), and will therefore be erased after a reboot (or maybe even a logout/login).

The "0" folder contains a few desktop-related databases and is probably intended to act as a cache for these services:

$ ls -l /var/folders/4c/yc9pgt3j5y18vr29jcgbbyb4000101/0
total 14104
-rw-------  1 user2  staff  7221248 Sep  1 10:31 com.apple.LaunchServices-1341025.csstore
drwx------  3 user2  staff      102 Jul  3  2016 com.apple.dock.launchpad/
drwx------  3 user2  staff      102 Jul  3  2016 com.apple.notificationcenter/
drwx------  3 user2  staff      102 Sep  1 10:05 com.apple.pluginkit/

So, to summarize, every user and virtual user has his own directory under /var/folders. You will have access to your own, but not to other users (unless you use sudo to access them as root). They mostly contain caches and temporary files. As such, deleting the contents may or may not be safe, depending on whether the files are in use or not.

Deleting files where the corresponding process is still running may cause that process to misbehave or crash. Deleting files where the corresponding process is not running should be safe, but may lead to performance problems when the process re-launches (since it will likely re-generate its caches and temporary files.)

It appears that a background process runs every night at 3:35am to clean up these folders. I don't know the specific nature of the cleanup (it doesn't appear to be well documented) but what I have read seems to indicate that it will delete any file that has not been accessed for three days.

  • Is there anyway to get it to purge sooner than waiting three days? I’ve got 38 gigs in /private/bar/folder. The data was originally iPhone backups in iTunes that I just “deleted” from the iTunes preferences -> devices. It had been under ~/Library/Application Support... until I “deleted” it. I want to upgrade my OS to Mojave, but I don’t have enough disk space free. – hepcat72 Jan 3 at 0:04

Normally /var/folders should be properly purged and regulated by the system when necessary. Removing some things from there without knowing what's removed can likely cause some unexpected and highly undesirable results.

If you're talking about cleaning this folder manually at reboot, I think it should be safe as it's not likely to be used by some running programs (just some leftovers from the last time). However you've right, it's always safer to remove files which are 1 day older just in case.

I think it's good to find how OS X cleans that folder and how to invoke that manually, so it would be a better solution.

See also:

  • 1
    "Normally /var/folders should be properly purged and regulated by the system" -- should being the operative word, but in practice the system fails horribly. I've got gigabytes of files, some that are 3 years and 2 OS upgrades old in /var/folders. – CXJ Nov 28 '16 at 19:48

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