Use crontab to notify when a piece of software has been released

Sometimes I eagerly await the release of a distro version or a new version of a piece of software. But I don't want to remember to check constantly, so I wrote a script that can be run from cron to let me know.

#!/bin/sh
case $# in
0|1)
       echo Usage: $0 url message
       exit
       ;;
esac
url="$1"
shift
wget -q --spider "$url" && echo "$@"

As you can see, this runs a wget on a specified URL which does not produce any output, but if successful, will print the message. Put this in crontab and the message will be mailed to you.

This script depends on knowing a URL that will exist when the release happens. Often you can guess the URL from previous releases. Here are a couple of examples, each is all on a single line:

1. Check at 0808 every day to see if AntiX 17 has been released:

8 8 * * * watchurl https://sourceforge.net/projects/antix-linux/files/Final/antiX-17/ AntiX 17 released

2. Check at 0809 every day to see if VirtualBox 5.1.30 has been released:

9 8 * * * watchurl https://www.virtualbox.org/download/hashes/5.1.30/MD5SUMS VirtualBox 5.1.30 has been released

How to rerun @reboot crontab entries without a reboot

Vixie cron and its descendants have a feature where an entry with the special time specification @reboot in place of the first five date and time fields indicates a one-shot action to be run when the machine is first booted.

But how do you test such entries without actually having to reboot the machine? My thinking was that somewhere crond must note the information that it already has been run once at boot.

Indeed a quick search of the filesystem found the zero length file /var/run/cron.reboot used as a flag that @reboot jobs have already been done.

So, to rerun @reboot jobs:

rm -f /var/run/cron.reboot

followed by:

systemctl restart crond

for systemd systems or

/etc/init.d/crond restart

for SysVinit systems.