Moxa W325A-LX [34/74] Bin sh ntpdate time nist gov hwclock systohc exit 0

W315A/325A Linux User's Manual Managing Embedded Linux

3-9

Cron—Daemon for Executing Scheduled

Commands

Cron is a scheduling service in Linux. Cron wakes up every minute, and checks the configuration file named

crontab to see if any scheduled commands should be run at the current moment.

Crontab is located in the /etc/cron.d directory. Modify the file /etc/cron.d/crontab to set up your

scheduled applications. Crontab has the following format:

mm h dom mon dow user command

min hour date month week user command

0-59 0-23 1-31 1-12 0-6 (0 is Sunday)

The following example demonstrates how to use Cron.

How to use cron to update the system time and RTC time every day at 8:00.

STEP 1: Write a shell script named fixtime.sh and save it to /home/.

#!/bin/sh

ntpdate time.nist.gov

hwclock –-systohc

exit 0

STEP 2: Change mode of fixtime.sh

#chmod 755 fixtime.sh

STEP 3: Modify /etc/cron.d/crontab file to run fixtime.sh at 8:00 every day.

Add the following line to the end of crontab:

* 8 * * * root /home/fixtime.sh

STEP 4: Enable the cron daemon manually.

#/etc/init.d/cron start

STEP 5: Enable cron when the system boots up.

By default, cron service is disabled on boot. To enable cron service, refer to the section “Enabling and Disabling

Daemons” in this chapter