This document is a WORK IN PROGRESS.
This is just a quick personal cheat sheet: treat its contents with caution!

cron¶

cron is a daemon that runs scheduled tasks based on input from the command crontab. It accomplishes this task by waking up every minute and checking to see if there are any cron jobs to run in any of the user crontabs.

Reference(s)

Install¶

There are multiple cron implementations, in this cheat sheet I will use cronie (fork of vixie-cron, because according to the Gentoo Wiki: vixie-cron is a bit outdated while still maintained by Fedora).

First, check if cronie is already installed:

emergepacmannixaptyumdnf

$ emerge -p cronie

$ pacman -Q | grep cron

$ nix-env -q cron

Warning to NixOS users

There is no cronie nix package, only cron (which is vixie-cron), so for the rest of this cheat sheet: remember to replace any cronie reference by cron. But be warned that the following sections may not correctly apply to vixie-cron.

$ apt -qq list cronie

$ yum list installed cronie

$ dnf list installed cronie

If not, install it ...

apkaptdnfemergenixpacmanyumxbpszypper

# apk add cronie

# apt install cronie

# dnf install cronie

# emerge -a sys-process/cronie

on NixOSon non-NixOS

# nix-env -iA nixos.cron

# nix-env -iA nixpkgs.cron

# pacman -S cronie

For Artix users

If using dinit:
```
# pacman -S cronie cronie-dinit
```
If using openrc:
```
# pacman -S cronie cronie-openrc
```
If using runit:
```
# pacman -S cronie cronie-runit
```
If using s6:
```
# pacman -S cronie cronie-s6
```

# yum install cronie

# xbps-install -S cronie

# zypper install cronie

Then, add it to your init system and start it:

OpenRCRunitSysVinitSystemD

# rc-update add cronie default
# rc-service cronie start

Depending on your runit implementation, either run:

# ln -s /etc/runit/sv/cronie /service

or run:

# ln -s /etc/runit/sv/cronie /var/service

or run:

# ln -s /etc/runit/sv/cronie /run/runit/service

In any case, finally run:

# sv up cronie

# service cronie start
# chkconfig cronie on

# systemctl enable cronie
# systemctl start cronie

Config¶

cronie doesn't need to be configured anymore.

Old cronie config:

# vi /etc/crontab
    > # Global variables
    > SHELL=/bin/bash
    > PATH=/sbin:/bin:/usr/sbin:/usr/bin
    > MAILTO=root
    > HOME=/
    >
    > # check scripts in cron.hourly, cron.daily, cron.weekly and cron.monthly
    > 59  *  * * *    root    rm -f /var/spool/cron/lastrun/cron.hourly
    > 9  3  * * *     root    rm -f /var/spool/cron/lastrun/cron.daily
    > 19 4  * * 6     root    rm -f /var/spool/cron/lastrun/cron.weekly
    > 29 5  1 * *     root    rm -f /var/spool/cron/lastrun/cron.monthly
    > */10  *  * * *  root    test -x /usr/sbin/run-crons && /usr/sbin/run-crons

Use¶

Edit your crontab (crontabs are located in /var/spool/cron/crontabs):

$ crontab -e # edit the crontab of the current user
$ sudo crontab -e # edit the crontab of the root user

Add a line to the wanted crontab according to this format:

* * * * *  command_or_script_to_execute
- - - - -
| | | | |
| | | | +- day of week (0 - 7) (where sunday is 0 and 7)
| | | +--- month (1 - 12)
| | +----- day (1 - 31)
| +------- hour (0 - 23)
+--------- minute (0 - 59)

E.g. execute a command every ...

*/15 * * * * /home/user/command.sh  # ... every 15 min
0 0 * * * /home/user/command.sh     # ... every midnight
5 8 * * 6 /home/user/command.sh     # ... every Saturday at 8:05 AM

Check the file syntax of your crontab:
```
$ crontab -T /var/spool/cron/username
```
Delete your crontab:
```
$ crontab -r
```
By default one can check cron executions by reading the logs in /var/log/messages and searching for cron.

`anacron`¶

anacron is not a cron daemon, it is something that usually works in conjunction with one. It executes commands at intervals specified in days and it does not assume that the system is running continuously; it will run jobs that were missed while the system was down. anacron usually relies on a cron daemon to run it each day.

By default cronie includes anacron functionalities (note to Gentoo users: cronie includes anacron functionalities when the default anacron use flag is set).

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cron¶

Table of contents¶

Install¶

Config¶

Use¶

anacron¶

`anacron`¶