Crontab Brief

The crontab (CRON TABLE) is a list of commands that you want to run on a regular schedule.

Linux Crontab Format

MIN HOUR DOM MON DOW  command

MIN: miniute field; 0-59

HOUR: hour field; 0-23

DOM: day of month, 1-31

MON: month field; 1-12

DOW: day of week; 0-6

Crontab commands

crontab -l
crontab -e
crontab -r

Crontab example

Create a file rpairdb.sh:

#!/bin/bash
mysqlcheck -u{username}  -p{password} --auto-repair --check --all-databases

crontab -e
*    3    *    *    *    /rpairdb.sh

Every day, at 3 am, database’s repaired on regular schedule.

Crontab file location

Mac OS X /usr/lib/cron/tabs/
BSD Unix /var/cron/tabs/
Solaris, HP-UX, Debian, Ubuntu /var/spool/cron/crontabs/
AIX, Red Hat Linux, CentOS, Ferdora /var/spool/cron/

Disable Email

By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command At the end of the cron job line .

/dev/null 2>&1

Generate log file

To collect the cron execution execution log in a file :

30 18 * * * rm /home/truongdam/tmp/* > /home/truongdam/cronlogs/clean_tmp_dir.log

Ref: geeksforgeeks , adminschoice

Dam Truong