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