1CRON(8) System Administration CRON(8)
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6 crond - daemon to execute scheduled commands
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9 crond [-c | -h | -i | -n | -p | -P | -s | -m<mailcommand>]
10 crond -x [ext,sch,proc,pars,load,misc,test,bit]
11 crond -V
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14 Cron is started from /etc/rc.d/init.d or /etc/init.d when classical
15 sysvinit scripts are used. In case systemd is enabled, then unit file
16 is installed into /lib/systemd/system/crond.service and daemon is
17 started by systemctl start crond.service command. It returns immedi‐
18 ately, thus, there is no need to need to start it with the '&' parame‐
19 ter.
20 Cron searches /var/spool/cron for crontab files which are named after
22 accounts in /etc/passwd; The found crontabs are loaded into the memory.
23 Cron also searches for /etc/anacrontab and any files in the /etc/cron.d
24 directory, which have a different format (see crontab(5)). Cron exam‐
25 ines all stored crontabs and checks each job to see if it needs to be
26 run in the current minute. When executing commands, any output is
27 mailed to the owner of the crontab (or to the user specified in the
28 MAILTO environment variable in the crontab, if such exists). Any job
29 output can also be sent to syslog by using the -s option.
30 There are two ways how changes in crontables are checked. The first
32 method is checking the modtime of a file. The second method is using
33 the inotify support. Using of inotify is logged in the /var/log/cron
34 log after the daemon is started. The inotify support checks for
35 changes in all crontables and accesses the hard disk only when a change
36 is detected.
37 When using the modtime option, Cron checks its crontables' modtimes
39 every minute to check for any changes and reloads the crontables which
40 have changed. There is no need to restart Cron after some of the
41 crontables were modified. The modtime option is also used when inotify
42 can not be initialized.
43 Cron checks these files and directories:
45 /etc/crontab
47 system crontab. Nowadays the file is empty by default. Origi‐
48 nally it was usually used to run daily, weekly, monthly jobs.
49 By default these jobs are now run through anacron which reads
50 /etc/anacrontab configuration file. See anacrontab(5) for more
51 details.
52 /etc/cron.d/
54 directory that contains system cronjobs stored for different
55 users.
56 /var/spool/cron
58 directory that contains user crontables created by the crontab
59 command.
60 Note that the crontab(1) command updates the modtime of the spool
62 directory whenever it changes a crontab.
63 Daylight Saving Time and other time changes
65 Local time changes of less than three hours,