2.2 KiB
| title | tags | ||
|---|---|---|---|
| cron |
|
Cronie
The cronie program is also known as crond.
Install
sudo apt search -n ^cron
Once installed, search for the service name, and start it.
sudo systemctl list-unit-files | grep cron
sudo systemctl enable --now $NAME
Usage
Show your current crontab:
crontab -l
You can put this in a file and edit it:
crontab -l > $filename
echo '39 3 */3 * * /bin/tar czf /tmp/etc_backup.tgz /etc/' >> $filename
Then apply that crontab:
crontab $filename
rm $filename
The cron program will check your syntax before adding the tab.
Your crontab file sits somewhere in /var/spool/.
Probably in /var/spool/cron.
Syntax
* * * * *
These five points refer to:
minute hour day month weekday
So '3pm every Sunday' would be:
0 15 * * 7
Here 'Sunday' is indicated by "7", and '3pm' is 'the 15th hour'. The minute is '0' (i.e. '0 minutes past three pm').
Doing the same thing, but only in February, would be:
0 15 * 2 7
Variables
cronie doesn't know where you live, so to put something in your $HOME directory, you have to tell it:
echo "HOME=$HOME" > $filename
crontab -l >> $filename
crontab $filename
cronie doesn't know where anything lives, including programs.
You can give it your usual $PATH variable like this:
echo $PATH > $filename
crontab -l >> $filename
crontab $filename
Now instead of doing this
40 */3 * * * /usr/bin/du -sh $HOME/* | sort -h > $HOME/sum.txt
You can simply do this:
40 */3 * * * du -sh $HOME/* | sort -h > $HOME/sum.txt
Run as Root
You can execute a script as root by putting it into a directory, instead of in the tab. Look at the available cron directories:
ls -d /etc/cron.*
Make a script which runs daily:
f=apt_update.sh
echo '#!/bin/bash' > $f
echo 'apt update --yes' >> $f
chmod +x $f
sudo mv $f /etc/cron.daily/
Testing with runparts
Run-parts runs all executable scripts in a directory.
run-parts /etc/cron.hourly
Troubleshooting
date Commands
Cron doesn't understand the % sign, so if you want to use date +%R, then it should be escaped with a backslash: date +\%R.