How are files under /etc/cron.d used?
From https://www.cyberciti.biz/faq/how-do-i-add-jobs-to-cron-under-linux-or-unix-oses/
cron reads the files in /etc/cron.d/ directory. Usually system daemon
such as sa-update or sysstat places their cronjob here. As a root user
or superuser you can use following directories to configure cron jobs.
You can directly drop your scripts here. The run-parts command run
scripts or programs in a directory via /etc/crontab file:/etc/cron.d/ Put all scripts here and call them from /etc/crontab
file.
On Lubuntu 18.04, the files under /etc/cron.d seem to be crontab files not shell scripts (which was mentioned in the above link):
$ cat /etc/cron.d/anacron
# /etc/cron.d/anacron: crontab entries for the anacron package
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
30 7 * * * root [ -x /etc/init.d/anacron ] && if [ ! -d /run/systemd/system ]; then /usr/sbin/invoke-rc.d anacron start >/dev/null; fi
My /etc/crontab file never refers to files under/etc/cron.d, contrary to what the link says:
$ cat /etc/crontab
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.
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 )
Could you explain how the files under /etc/cron.d are used? Thanks.
Best Answer
In Debian derivatives, including Lubuntu, the files in
/etc/cron.d
are effectively/etc/crontab
snippets, with the same format. Quoting thecron
manpage:The Debian-specific section hints at the reason system administrators shouldn’t use
/etc/cron.d
:It’s designed to allow packages to install crontab snippets without having to modify
/etc/crontab
.