Systemd

Replaces initd. It should be more efficient. Honestly I don't see the benefits.

It also provides timed execution like cron does equally well.

The service command invokes a script in /etc/init.d.

Service commands

service --status-all

Run all scripts with the 'status' option. Tells you which services are up.

service <service>

Show all options for service, usually 'start|stop|restart|reload|status'

service <service> status

Show detailed status for <service>

systemctl enable <service>

Make the service start at boot-time

systemctl list-units

Show all known units. Alternative for service --status-all that does not work on modern RedHat versions.

systemctl list-timers

Show all active timers

E.g for certbot (letsencrypt)

$ systemctl list-timers
NEXT                        LEFT         LAST                        PASSED UNIT                         ACTIVATES
do 2018-11-29 00:00:00 CET  2h 5min left wo 2018-11-28 12:00:01 CET  9h ago certbot.timer                certbot.service

2 timers listed.
Pass --all to see loaded but inactive timers, too.

$ cat /lib/systemd/system/certbot.timer
[Unit]
Description=Run certbot twice daily

[Timer]
OnCalendar=*-*-* 00,12:00:00
Persistent=true

[Install]
WantedBy=timers.target

$ cat /lib/systemd/system/certbot.service 
[Unit]
Description=Certbot
Documentation=file:///usr/share/doc/python-certbot-doc/html/index.html
Documentation=https://letsencrypt.readthedocs.io/en/latest/
[Service]
Type=oneshot
ExecStartPre=/usr/bin/perl -e 'sleep int(rand(3600))'
ExecStart=/usr/bin/certbot -q renew
PrivateTmp=true

systemctl daemon-reload

After changing a configuration file reload it

systemctl mask <UNIT>

Link the timer to /dev/null effectively disabling it

systemd-analyze [blame|critical-chain]

Utility that shows what is holding up the boot process

Create a service

/etc/systemd/system/daemontest.service

[Unit]
Description=Test daemon
After=mnt-www.mount             # Start after /mnt/www is mounted

[Service]
Type=simple
User=<username>                 # User dat will run the program
ExecStart=<path to>/daemontest.py
Restart=on-failure

[Install]
WantedBy=default.target

systemctl daemon-reload
systemctl enable daemontest.service

#!/usr/bin/env python3

import daemon
from time import sleep
from datetime import datetime

outfile = '/tmp/testdaemon_{}.out'

def main():
	count = 5
	while count:
		with open(outfile.format(count),'w') as fh:
			count -= 1
			count2 = 3
			while count2:
				count2 -= 1
				fh.write(f"{datetime.now()} Dit is een test\n")
				sleep(10)


with daemon.DaemonContext():
    main()

System messages

The command journalctl shows system services messages

journalctl

Show all from old to new

journalctl -u <servicename>

Show all for <servicename>

journalctl -S <YYYY-mm-dd HH:MM:SS> -U <YYYY-mm-dd HH:MM:SS>

journalctl -S yesterday

Show messages since -S-date/time until -U-date/time