The Linux Crontab & Systemd

Crontab Introduction

Crontab Basics

• Super simple to set up
• Reliable (unlike some operating systems ¹)
• Allows you to consistently run tasks
  • Get status updates via email (or not)

Cronjob Examples

We call tasks that run on the crontab “cronjobs”

Layout: minute hour dom month dow task [arguments]

• */1 * * * * /home/jacob/notification-checker.py
  • Run a script to check if we need to send a notification every minute.
• 0 2 * * mon /root/pull-remote-backups.sh
  • Runs every Monday at 2:00 a.m., pulling my server backups
• @reboot echo 'Message Text' | mail -s 'Server Rebooted!' email_address@domain.tld
  • Send yourself an email every time your machine reboots

Editing the Crontab

How do I edit the Crontab?

Run this command, that’s all: crontab -e

To change your editor, you can run select-editor, or just change your $EDITOR environment variable.

Configuring Emails

If you already have emailing setup on your Linux server (or home computer!), you can simply write, somewhere in your crontab

MAILTO=email_address@domain.tld

Systemd Introduction

What is systemd? systemd is an array of software tools for many different Linux
operating systems.

Its main goal is to serve as a “system and service manager” across Linux distributions.

The crontab actually runs on top of systemd on many systems!

Systemd Services

According to a helpful man systemd.service command, a systemd service is a “process controlled and supervised by systemd”.

Below is a systemd service I have for a Rust Discord bot I wrote.

[Unit]
Description=Rust Discord Bot
After=network.target

[Service]
Type=simple
EnvironmentFile=/root/Rust-Discord-Bot-Env
ExecStart=/sbin/rust-discord-bot

[Install]
WantedBy=multi-user.target

Working with Systemd Services

To configure a service to start on boot, we must first add it (as a .service file) to the systemd system directory (usually /etc/systemd/system/).

Then, we use the systemctl command to configure the service (sudo here only necessary our service is a system service).

sudo systemctl enable service-name.service # Makes the service start on boot

sudo systemctl start service-name.service # Starts the service now (so no reboot)

sudo systemctl status service-name.service # Checks the current status of the service

journalctl -u service-name.service # Shows service logs

Systemd User Services?

You can also have systemd services run on your user accounts (no root needed)!

This is just as simple as the system services, with three differences to keep in mind:

1. You place the service file in ~/.config/systemd/user/ instead of /etc/systemd/system/

2. You add a --user flag to all systemctl commands

3. Your service can’t start on boot anymore, now, since it must start within the context of your user account.

Original Presentation Slides Available Here

April 2024