Today I noticed that there were two cron related files in /run and was wondering about the /run/crond.reboot file found in there.

The cron system allows you to run jobs each time your reboot your machine. These are defined with the @reboot special pattern. Only, cron wants to run those only once. That is, if you restart cron later on, which could happen because of an upgrade, for example, then the @reboot jobs need to be ignored.

In order to avoid running those jobs again, the cron daemon creates that /run/crond.reboot file. In other words, if the file already exists, then cron already tried to run the @reboot jobs and it won't try again.

Note that means it tried to run the jobs, not that the jobs were successful. The order of things is:

1. Check whether the file exist, if so, do nothing more.
2. Create the file, if that fails, bail out (the daemon dies right there!) since running those @reboot jobs once cannot be guaranteed.
3. Run the @reboot jobs now.

To know whether the @reboot jobs worked, you'll have to check your syslog (or, if you moved the cron logs to another file, that other file). Obviously, that needs to be done a little after you reboot or within a few days as logs tend to get deleted after a little while (funnily enough, in general that happens by a running the logrotate cron job).