Additionally,  cron  reads  the files in /etc/cron.d: it treats the files in /etc/cron.d as in the same way as the /etc/crontab file … cron then wakes up every minute, examining all stored crontabs, checking each com‐ mand to see if it should be run in the current minute. When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists). The children copies of cron running these processes have their name coerced to uppercase, as will be seen in the syslog and ps output. Additionally, cron checks each minute to see if its spool directory's modtime (or the modtime on /etc/crontab) has changed, and if it has, cron will then examine the modtime on all crontabs and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modified. Note that the crontab(1) command updates the modtime of the spool directory whenever it changes a crontab. 

另请注意,cron不会“将cron.d的内容插入到全局crontab中” – 这些是独立的:

  However, they are independent of /etc/crontab: they do not, for example, inherit environment variable settings from it.