Como o cron.d notifica o sistema?

Question

Se eu colocar um novo script em /etc/cron.d/ , como o sistema sabe inseri-lo no crontab global?

Preciso reiniciar um processo para buscá-lo?

Será que init executará o crontab a cada minuto e ele será analisado novamente?

Ele usa inotify e assiste /etc/cron.d ?

Não foi possível encontrar nada muito óbvio e sempre me perguntei

    

Answer 1

Do cron (8):

   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.

Observe também que o cron não "insere o conteúdo do cron.d no crontab global" - esses são independentes:

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