Linux has a rich set of features for running scheduled tasks. One of the key attributes of a good sysadmin is getting the computer to do your work for you (sometimes misrepresented as laziness!) - and a well configured set of scheduled tasks is key to keeping your server running well.
Each user potentially has their own set of scheduled task which can be listed with the crontab command (list out your user crontab entry with crontab -l and then that for root with sudo crontab -l ).
However, there’s also a system-wide crontab defined in /etc/crontab - use less to look at this. Here's example, along with an explanation:
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 )
Lines beginning with "#" are comments, so # m h dom mon dow user command defines the meanings of the columns.
Although the detail is a bit complex, it's pretty clear what this does. The first line says that at 17mins after every hour, on every day, the credential for "root" will be used to run any scripts in the /etc/cron.hourly folder - and similar logic kicks off daily, weekly and monthly scripts. This is a tidy way to organise things, and many Linux distributions use this approach. It does mean we have to look in those /etc/cron.* folders to see what’s actually scheduled.
On your system type: ls /etc/cron.daily - you'll see something like this:
$ ls /etc/cron.daily
apache2 apt aptitude bsdmainutils locate logrotate man-db mlocate standard sysklog
Each of these files is a script or a shortcut to a script to do some regular task, and they're run in alphabetic order by run-parts. So in this case apache2 will run first. Use less to view some of the scripts on your system - many will look very complex and are best left well alone, but others may be just a few lines of simple commands.
Look at the articles in the resources section - you should be aware of at and anacron but are not likely to use them in a server.
Google for "logrotate", and then look at the logs in your own server to see how they've been "rotated".
All major Linux distributions now include "systemd". As well as starting and stopping services, this can also be used to run tasks at specific times via "timers". See which ones are already configured on your server with:
systemctl list-timers
Use the links in the RESOURCES section to read up about how these timers work.
- A good overview of systemd/Timers
- "How to Use Systemd Timers as a Cron Replacement"
- Automate system administration tasks by scheduling jobs
- Using cron to automate maintenance
Copyright 2012-2021 @snori74 (Steve Brorens). Can be reused under the terms of the Creative Commons Attribution 4.0 International Licence (CC BY 4.0).