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. $TMPTIME is an environment variable defined in /etc/default/rcS.
Notice that we can create temporary files with mktemp, for example:
and write to it with:
echo "ciaooo" > "$my_tmp_file"
remove it with:
we can even create a temporary directory: