Periodic Command Scheduler. It executes scheduled commands according to the /etc/crontab file. It is one of an essential part of Linux
A task scheduling tool. It will be necessary, only if you are going to use scheduled tasks.
Now lets see the manual of crond service.Manual crond:NAME
cron - daemon to execute scheduled commands (ISC Cron V4.1)SYNOPSIS
cron [-l load_avg] [-n] [-p]DESCRIPTION
Cron should be started from /etc/rc or /etc/rc.local. It will return immediately, so you don't need to start it
with '&'. The -n option changes this default behavior causing it to run in the foreground. This can be useful
when starting it out of init.
Cron searches /var/spool/cron for crontab files which are named after accounts in /etc/passwd; crontabs found are
loaded into memory. Cron also searches for /etc/crontab and the files in the /etc/cron.d directory, which are in a
different format (see crontab(5)). Cron then wakes up every minute, examining all stored crontabs, checking each
command 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).
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.
Daylight Saving Time and other time changes
Local time changes of less than three hours, such as those caused by the start or end of Daylight Saving Time, are
handled specially. This only applies to jobs that run at a specific time and jobs that are run with a granularity
greater than one hour. Jobs that run more frequently are scheduled normally.
If time has moved forward, those jobs that would have run in the interval that has been skipped will be run immediately.
Conversely, if time has moved backward, care is taken to avoid running jobs twice.
Time changes of more than 3 hours are considered to be corrections to the clock or timezone, and the new time is
PAM Access Control
On Red Hat systems, crond now supports access control with PAM - see pam(8). A PAM configuration file for crond is
installed in /etc/pam.d/crond . crond loads the PAM environment from the pam_env module, but these can be overriden
by settings in the crontab file.SIGNALS
On receipt of a SIGHUP, the cron daemon will close and reopen its log file. This is useful in scripts which rotate
and age log files. Naturally this is not relevant if cron was built to use syslog(3).CAVEATS
In this version of cron , without the -p option, /etc/crontab must not be writable by any user other than root, no
crontab files may be links, or linked to by any other file, and no crontab files may be executable, or be writable
by any user other than their owner.SEE ALSO
crontab(1), crontab(5), pam(8)AUTHOR
Paul Vixie <firstname.lastname@example.org>