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CROND
Section: Maintenance Commands (8)
Updated: 1 May 1994
Index
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NAME
crond - cron daemon (Dillon's Cron)
SYNOPSIS
crond [-l#] [-d[#]] [-f] [-b] [-c directory]
OPTIONS
crond
is a background daemon that parses individual crontab files and
executes commands on behalf of the users in question.
- -l<loglevel>
-
set logging level, default is 8.
- -d[<debuglevel>]
-
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.
DESCRIPTION
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
characters, including the newline.
Whenever crond must run a job, it first creates a daemon-owned temporary
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 temporary 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 program 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 descriptors. When
crontab
program allows a user to edit his crontab, it copies the crontab to a user
owned file before running the user's prefered 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 current time
against the list of possible jobs. The list is stored such that the
scan goes very quickly, and crond can deal with several thousand entries
without taking any noticable amount of cpu.
AUTHOR
Matthew Dillon (dillon@apollo.backplane.com)
Index
- NAME
-
- SYNOPSIS
-
- OPTIONS
-
- DESCRIPTION
-
- AUTHOR
-
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