MacOS – is it safe to delete /private/tmp vs /private/var/tmp

It's a simple way. You can use the getconf DARWIN_USER_CACHE_DIR command to get the per-user temporary files and caches directory. and remove it. edit the '/etc/profile' file, add the following codes in it(not test yet):

userTempDir=`/usr/bin/getconf DARWIN_USER_CACHE_DIR`
rm -fr $userTempDir*

Maybe resetting the permissions on a user's home directory helps: This is easily accomplished by resetting ACLs on the desired home directly by using the Reset Password Utility in the Recovery Partition: 1 Restart your computer from the recovery partition (restart while holding CMD+R). 2 Open Disk Utility and run a permissions repair on your startup volume. After this is complete, close Disk Utility (doing this just for good measure). 3 Open Terminal from the Utilities menu. Type in resetpassword and select your user account (NOT System Administrator/root) from the drop down menu. 4 Click the Reset Button at the bottom of the window in the Reset home folder permissions and ACLs section. 5 Quit the Password Utility and go back to the main recovery screen. 6 On your keyboard, hit CMD+Q and restart your computer. It's very important that you don't hold down the power button to exit the recovery session, or the ACL reset won't occur.