How is the /tmp directory cleaned up?
Solution 1:
Note! This answer is outdated since at least ubuntu 14.04. See other answers for current situation and if they prove correct then upvote them furiously. Also post comment so I can put link here to current correct answer.
For 14.04 see https://askubuntu.com/a/759048/1366
For 16.10 see https://askubuntu.com/a/857154/453746
Old answer from 2011:
The cleaning of /tmp is done by the upstart script /etc/init/mounted-tmp.conf. The script is run by upstart everytime /tmp is mounted. Practically that means at every boot.
The script does roughly the following: if a file in /tmp is older than $TMPTIME days it will be deleted.
The default value of $TMPTIME is 0, which means every file and directory in /tmp gets deleted. $TMPTIME is an environment variable defined in /etc/default/rcS.
Solution 2:
The directory is cleared by default at every boot, because TMPTIME is 0 by default.
Here you can change the time in the following file:
/etc/default/rcS
TMPTIME says how frequent the tmp dir sould be cleared in days
Solution 3:
While the /tmp folder is not a place to store files long-term, occasionally you want to keep things a little longer than the next time you reboot, which is the default on Ubuntu systems. I know a time or two I’ve downloaded something to /tmp during testing, rebooted after making changes and then lost the original data again. This can be changed if you’d like to keep your /tmp files a little bit longer.
Changing the /tmp Cleanup Frequency
The default setting that tells your system to clear /tmp at reboot is held in the /etc/default/rcS file. The value we’ll look at is TMPTIME.
The current value of TMPTIME=0 says delete files at reboot despite the age of the file. Changing this value to a different (positive) number will change the number of days a file can survive in /tmp.
TMPTIME=7
This setting would allow files to stay in /tmp until they are a week old, and then delete them on the next reboot. A negative number (TMPTIME=-1) tells the system to never delete anything in /tmp. This is probably not something you want, but is available.