cron in linux

On linux systems we can schedule jobs using cron job (crond)

Syntax:

# Example of job definition:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .----- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * user-name command to be executed

Useful commands:

# Check crond status
systemctl status crond

# Tail cron log
tail -f /var/log/cron

# List crontab for user
crontab -u user -l

# Edit crontab for user
crontab -e user

# Crontab files location
/var/spool/cron/

cron’s @reboot is only run after an actual reboot. It won’t be run after a cold boot (e.g. shutdown, crash, power loss).

Example cron jobs:

# this is for automation on set up. This can be removed after the first boot
@reboot /etc/rc.local
# end
0 6 * * * /usr/local/cpanel/scripts/exim_tidydb > /dev/null 2>&1
30 5 * * * /usr/local/cpanel/scripts/optimize_eximstats > /dev/null 2>&1
26 3 * * * /usr/local/cpanel/scripts/upcp --cron
0 1 * * * /usr/local/cpanel/scripts/cpbackup
0 2 * * * /usr/local/cpanel/bin/backup
35 * * * * /usr/bin/test -x /usr/local/cpanel/bin/tail-check && /usr/local/cpanel/bin/tail-check
45 */4 * * * /usr/bin/test -x /usr/local/cpanel/scripts/update_mailman_cache && /usr/local/cpanel/scripts/update_mailman_cache
30 */4 * * * /usr/bin/test -x /usr/local/cpanel/scripts/update_db_cache && /usr/local/cpanel/scripts/update_db_cache
30 */2 * * * /usr/local/cpanel/bin/mysqluserstore >/dev/null 2>&1
15 */2 * * * /usr/local/cpanel/bin/dbindex >/dev/null 2>&1
15 */6 * * * /usr/local/cpanel/scripts/autorepair recoverymgmt >/dev/null 2>&1
*/5 * * * * /usr/local/cpanel/scripts/dcpumon-wrapper >/dev/null 2>&1
46 3 * * * /usr/local/cpanel/whostmgr/docroot/cgi/cpaddons_report.pl --notify
11,26,41,56 * * * * /usr/local/cpanel/whostmgr/bin/dnsqueue > /dev/null 2>&1
09,39 * * * * /usr/local/cpanel/scripts/clean_user_php_sessions > /dev/null 2>&1