systemd-tmpfiles-clean.service clears /tmp before shutdown. /tmp contains temporary files that some programs need, but do not expect them to be persisted across reboots.

In some cases, /tmp is mounted as a tmpfs filesystem which is stored entirely in RAM and is automatically cleared on reboot. However, that is not the case in the CentOS 7 image so you will need to enable systemd-tmpfiles-clean.service in order to have /tmp cleared on shutdown.

--clean: If this option is passed, all files and directories with an age parameter configured will be cleaned up.

May I know what is an 'age parameter'?

How to enable systemd-tmpfiles-clean.service?

I run 'systemctl enable systemd-tmpfiles-clean.service' but with error:

The unit files have no [Install] section. They are not meant to be enabled
using systemctl...

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