How to add cron job on Asuswrt Merlin Wifi Router?

To list, delete, or add new cron jobs, you need to use the command:

cru

Add a new cron job:

cru a {id} min hour day month week command

List cron jobs:

cru l

Delete cron job:

cru d {id}

How to scheduled cron jobs after rebooting the router

Because of a small volume of storage on a router, you require to store your cron job as a script using "cru" command.

Create a script named "service-start"

Type the following  command:

# cd /jffs/scripts

Edit/update a file named service-start file i.e. /jffs/scripts/service-start

#!/bin/sh
# This cript get called after all other system services 
# have been started at boot on router
# ---------------------------------------------------------
# Sample cron job to install after reboot
cru a pinggw "0 * * * * /bin/ping -c 10 -q 142.108.1.123"
# Run my usb backup script every day at midnight 
cru a backup "0 0 * * * /jffs/scripts/usb-backup-script.sh
# ---------------------------------------------------------

Sample /jffs/scripts/usb-backup-script.sh file displayed using the "cat" command:

cat /jffs/scripts/usb-backup-script.sh

Sample outputs:

# ---------------------------------------------------------
DEST="/tmp/mnt/sda1"
nUSER="Ashu"
nSERVER="162.154.1.153"
SRC="/backups"
# ---------------------------------------------------------

Note: 

1) Set executable permission using the "chmod" command:

chmod a+rx /jffs/scripts/service-start
chmod a+rx /jffs/scripts/usb-backup-script.sh

2) Enable boot time support for /jffs/scripts/

Must be enabled, under Administration > System on the webui:

 DONE!!