Does Ubuntu support "@reboot" in crontab?

Does Ubuntu support @reboot in crontab?

I find documentation suggesting it is supported, but web chatter asserts it is not.

I cannot make the @reboot section work. The "after midnight" section works fine.

Here is the test example from my /etc/crontab:

$ cat /etc/crontab
# /etc/crontab: system-wide crontab

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user  command
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
#
##

# in the beginning - 
@reboot /var/kiosk/btest.sh

# after midnight
30 0     * * *   root    /etc/cron.daily/kiosk/kioskReboot.sh

# end of crontab

shell script btest.sh

#!/bin/bash
date > /var/kiosk/STARTFLAG.txt
echo we booted >> /var/kiosk/STARTFLAG.txt
date
echo we booted

permissions

$ ls -l btest.sh
-rwxrwxrwx 1 root root 147 Aug 21 15:19 btest.sh
$ ls -ld
drwxrwxrwx 14 laptopsanytime root 4096 Aug 21 16:30 .

@reboot is supported in Ubuntu. The reason why your entry

@reboot /var/kiosk/btest.sh

doesn't work in /etc/crontab is because it's missing the user field. The correct syntax would be

@reboot root /var/kiosk/btest.sh

Ubuntu 16.04.5 LTS: As root:

crontab -e

Add the following lines:

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
@reboot /path_to_script.sh 2>&1 >> /path_to_log.log

2>&1 >> will output stdout to /path_to_log.log so you can troubleshoot this