CROND(8)CROND(8)NAMEcrond - cron daemon (Dillon's Cron)
SYNOPSIScrond [-l#] [-d[#]] [-f] [-b] [-c directory]
OPTIONScrond 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).
specify directory containing crontab files.
specify directory containing system-wide crontab files. By
default, /etc/cron.d is checked.
DESCRIPTIONcrond 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 (firstname.lastname@example.org)
1 May 1994 CROND(8)