Master Linux Cron: Schedule Tasks Efficiently with Real-World Examples

cron is the Linux daemon for scheduling periodic tasks, similar to Windows Task Scheduler. After OS installation, the crond service starts automatically and checks every minute for jobs to run.

cron service commands

service crond start   // start service
service crond stop    // stop service
service crond restart // restart service
service crond reload  // reload configuration
service crond status  // view service status

Configuration files

Each user’s crontab is stored in /var/spool/cron/ named after the user (e.g., /var/spool/cron/tom). Typically a user has only one crontab file.

The system‑wide file /etc/crontab is maintained by the administrator and can schedule tasks for any user. Example header:

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
# Example of job definition:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12)
# | | | | .---- day of week (0 - 6, Sunday=0 or 7)
# * * * * * user-name command to be executed

The MAILTO variable defines where error output is mailed; users often change it to their own address.

Directory /etc/cron.d/

Stores additional crontab files or scripts.

Creating a cron script

1. Write a script file, e.g., crontest.cron.

15,30,45,59 * * * * echo "xgmtest....." >> xgmtest.txt

2. Install it with crontab crontest.cron.

3. Verify with crontab -l or check /var/spool/cron.

crontab command usage crontab installs, removes, or lists tables that drive the cron daemon. Each user can have a personal crontab; files under /var/spool/cron are not edited directly.

A crontab line consists of six fields: minute, hour, day‑of‑month, month, day‑of‑week, and the command. Valid ranges are 0‑59 for minutes, 0‑23 for hours, 1‑31 for day‑of‑month, 1‑12 for month, and 0‑6 for day‑of‑week (0 = Sunday).

minute hour day-of-month month day-of-week command

Special characters: * – every possible value / – step values (e.g., */5 every 5 units) - – range (e.g., 1-5) , – list (e.g., 1,3,5)

Common options:

-l   display current crontab
-r   remove current crontab
-e   edit current crontab with $EDITOR

Example entries

Every day at 6 am: 0 6 * * * echo "Good morning." >> /tmp/test.txt Every two hours: 0 */2 * * * echo "Have a break now." >> /tmp/test.txt At 23:00‑07:00 every two hours and at 08:00: 0 23-7/2,8 * * * echo "Have a good dream" >> /tmp/test.txt On the 4th of each month and on Mondays‑Wednesdays at 11 am: 0 11 4 * 1-3 command January 1st at 4 am:

0 4 1 1 * command