Master Linux Cron: Schedule Tasks Efficiently with Crontab

Cron Service

crond is the daemon that runs scheduled tasks in Linux, similar to Windows Task Scheduler. It starts automatically after installation and checks every minute for tasks to execute.

Cron 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

Cron Configuration Files

The directory /var/spool/cron/ stores each user's crontab file, named after the user (e.g., /var/spool/cron/tom). Typically each user has only one crontab file.

/etc/crontab

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
# Example of job definition:
# * * * * * user-name command to be executed

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

/etc/cron.d/ Directory

This directory holds additional crontab files or scripts.

Permissions

Cron permissions are controlled by /var/adm/cron/cron.allow and /var/adm/cron/cron.deny. The presence or absence of these files determines which users can use the crontab command.

Creating a Cron Script

1. Write a script file, e.g., crontest.cron, with the desired schedule and command.

15,30,45,59 * * * * echo "xgmtest....." >> xgmtest.txt   # runs every 15 minutes

2. Install it with crontab crontest.cron .

3. Verify using crontab -l or by checking /var/spool/cron.

Crontab Usage

The crontab command installs, removes, or lists tables that drive the cron daemon. Each line in a crontab consists of six fields: minute, hour, day‑of‑month, month, day‑of‑week, and the command.

minute hour day-of-month month day-of-week command
Valid values: 00‑59 00‑23 01‑31 01‑12 0‑6 (0 is Sunday)

Special characters *, /, -, and , define ranges, steps, and lists.

Common options: -l list, -r remove, -e edit.

Examples

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

Other examples include specific days, months, and custom commands.