crond man page on aLinux

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CROND(8)							      CROND(8)

       crond - cron daemon (Dillon's Cron)

       crond [-l#] [-d[#]] [-f] [-b] [-c directory]

       crond  is  a background daemon that parses individual crontab files and
       executes commands on behalf of the users in question.

	    set logging level, default is 8.

	    set debugging level, default is 0, if no level specified  with  -d
	    option default is 1.  This option also sets the logging level to 0
	    and causes crond to run in the foreground.

       -f   run crond in the foreground.

       -b   run crond in the background (default unless -d specified).

       -c directory
	    specify directory containing crontab files.

       -s directory
	    specify  directory	containing  system-wide	 crontab  files.    By
	    default, /etc/cron.d is checked.

       crond  is  responsible for scanning the crontab files and running their
       commands at the appropriate time.   The	crontab	 program  communicates
       with  crond  through  the  "cron.update" file which resides in crontabs
       directory, usually /var/spool/cron/crontabs.  This is  accomplished  by
       appending  the  filename	 of  the  modified  or deleted crontab file to
       "cron.update" which crond then picks up to resynchronize or remove  its
       internal representation of the file.

       Crond  has  a number of built in limitations to reduce the chance of it
       being ill-used.	Potentially infinite loops during  parsing  are	 dealt
       with via a failsafe counter, and user crontabs are generally limited to
       256 crontab entries.  crontab lines may not be longer than 1024 charac‐
       ters, including the newline.

       Whenever	 crond	must run a job, it first creates a daemon-owned tempo‐
       rary file O_EXCL and O_APPEND to store any  output,  then  fork()s  and
       changes	its  user  and group permissions to match that of the user the
       job is being run for, then exec's /bin/sh -c to run the job.  The  tem‐
       porary  file  remains  under the ownership of the daemon to prevent the
       user from tampering with it.  Upon job completion, crond	 verifies  the
       secureness  of  the mail file and, if it has been appended to, mails to
       the file to user.  The sendmail program is run under the user's uid  to
       prevent	mail  related security holes.  Unlike crontab , the crond pro‐
       gram does not leave an open descriptor to the file for the duration  of
       the  job's  execution  as this might cause crond to run out of descrip‐
       tors.  When crontab program allows a  user  to  edit  his  crontab,  it
       copies the crontab to a user owned file before running the user's pref‐
       ered editor.  The suid crontab programs keeps an open descriptor to the
       file  which  it	later uses to copy the file back, thereby ensuring the
       user has not tampered with the file type.

       Crond always synchronizes to the top of the minute, checking  the  cur‐
       rent  time  against the list of possible jobs.  The list is stored such
       that the scan goes very quickly, and crond can deal with several	 thou‐
       sand entries without taking any noticable amount of cpu.

       Matthew Dillon (

				  1 May 1994			      CROND(8)

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