Installation

Install and enable the cron daemon.

sudo pacman -S cronie
sudo systemctl enable --now cronie.service
systemctl status cronie.service   # should be active (running)

Setup

To edit crontab, run

crontab -e

If you get an error like this,

/bin/sh: line 1: /usr/bin/vi: No such file or directory
crontab: "/usr/bin/vi" exited with status 127

pass the following commands:

set -Ux EDITOR emacs
set -Ux VISUAL emacs

This defaults to emacs across different Fish sessions.

Then with crontab -e add jobs like this.

SHELL=/usr/bin/fish
PATH=/usr/local/bin:/usr/bin:/bin

0 * * * * /usr/bin/emacs --batch -Q -l "$HOME/Code/monitor.el" >> "$HOME/Code/monitor.log" 2>&1

Fish functions/aliases in ~/.config/fish/functions/ also work.

Syntax

Here is what each * refers to.

Field Range Meaning
minute 0–59
hour 0–23
day of month 1–31
month 1–12
day of week 0–7 (0 or 7 = Sun)
command — run by the SHELL above

Examples:

Schedule Expression
every 5 min */5 * * * *
daily 09:00 0 9 * * *
weekdays 09:00 0 9 * * 1-5
1st of month 0 0 1 * *

Shortcuts include @reboot, @hourly, @daily, @midnight, @weekly, @monthly, @yearly.

For example, the above command can be rewritten as

@hourly /usr/bin/emacs --batch -Q -l "$HOME/Code/monitor.el" >> "$HOME/Code/monitor.log" 2>&1

Useful commands:

crontab -e     # edit your crontab
crontab -l     # list your crontab
crontab -r     # remove your crontab
journalctl -t CRON --since today   # debug: did cron fire the job?

Note

The crontab resided in /var/spool/cron/<your-username>. To view it in the terminal run this.

sudo cat /var/spool/cron/$(whoami)

Do not edit this file. Always use crontab -e.

Also a brief note on the terms used above:

Term What it is Analogy
cron The concept — a time-based job scheduler in Unix. “the engine”
cronie A specific implementation of the cron daemon. “one particular engine”
crontab The file (a plain-text table of schedules) and the command (crontab -e, crontab -l used to edit it. “the instruction sheet”