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I have a server (Ubuntu 22.04) which has a tmpfs partition mounted at /tmp. The server only reboots for security updates, so I can't rely on reboots to clear out /tmp.

systemd-tmpfile is using default settings, which means that /usr/lib/tmpfiles.d/tmp.conf contains:

# Clear tmp directories separately, to make them easier to override
D /tmp 1777 root root -
#q /var/tmp 1777 root root 30d

However, this doesn't (ever) clear out /tmp. Should this be working, or should I replace the D with an r or similar? Or maybe I should just cron a daily cleanup job? If I have to cron a cleanup job, how should I over-ride the default tmp.conf to prevent it interfering? Thanks.

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In short, this has nothing to do with tmpfs, and tmp.conf doesn't do what you want it to do. The - entry for /tmp means that no automatic cleanup will be done. See the man page for tmpfiles.d:

The age field only applies to lines starting with d, D, e, v, q, Q, C, x and X. If omitted or set to "-", no automatic clean-up is done.

Copy the default tmp.conf to /etc/tmpfiles.d/tmp.conf, and replace the - with 1d or similar. This file has a higher priority than the tmp.conf at /usr/lib/tmpfiles.d/.

The D means that the contents of the directory will be removed when systemd-tmpfiles --remove is run, and your distro will do that at boot time (it probably runs systemd-tmpfiles --remove --create --boot or something similar). So, the Ubuntu default is to clean up /tmp only at boot.

Note that this doesn't mean that /tmp will be automatically emptied out every 24 hours (or whatever). /usr/lib/tmpfiles.d/ contains other entries, such as

D! /tmp/.X11-unix 1777 root root 10d

The ! means that it's unsafe to remove these files at runtime, and it's only considered if the --boot option is given. In this case, the file also has to be 10 days or older (10d) in order to be deleted.

If you can't work out why a file isn't being deleted do a manual cleanup with debug enabled:

# SYSTEMD_LOG_LEVEL=debug systemd-tmpfiles --clean

and look for output lines like:

/usr/lib/tmpfiles.d/x11.conf:12: Ignoring entry D! "/tmp/.X11-unix" because --boot is not specified.
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