Systemd
systemd is the init system, the first process that the kernel starts after boot that starts all other processes in user mode.
- Works with units
- There are types of units, specified by their extension (ex:
*.service)- you can omit the extension when starting and stopping services
systemctlexecutes instruction in unit files
systemctl
Great Digital Ocean tutorial.
Management tool for systemd:
# check status of system daemon
$ dpkg -s openssh-server
Package: openssh-server
Status: install ok installed
Priority: optional
...
# ------------------------------------
# Units
systemctl list-units # list all active units
systemctl # list all active units
systemctl list-units --all # list all units
systemctl list-units --all --state=inactive # filter for inactive units
systemctl list-units --all --type=service # filter by type
systemctl list-unit-files # list every available unit file
systemctl cat <service> # view currently loaded service file
systemctl list-dependencies <service> # list dependencies (other units) required to start service
systemctl list-dependencies <service> --all # include recursive dependencies (other units)
systemctl list-dependencies <service> --reverse # list dependencies that depend on the unit
systemctl list-dependencies --before <service> # dependencies that must start before service
systemctl list-dependencies --after <service> # dependencies that must start after service
systemctl edit <service> # edit a service as a snippet
systemctl edit --full <service> # edit a service directly
rm -r /etc/systemd/system/nginx.service.d # remove changes made to unit as snippet
rm /etc/systemd/system/nginx.service # remove changes made to unit
# ------------------------------------
# Unit Properties
systemctl show <service> # list unit properties
systemctl show <service> -p <prop> # show specific unit property
# ------------------------------------
# Mask a Unit - Make it impossible to start the unit
# mask the unit
sudo systemctl mask apache2.service
Created symlink /etc/systemd/system/apache2.service → /dev/null.
# verify its masked
systemctl list-unit-files | grep apache2
apache2.service masked enabled
apache2@.service disabled enabled
# try to start the service - failure
sudo systemctl start apache2.service
Failed to start apache2.service: Unit apache2.service is masked.
# unmask the service
sudo systemctl unmask apache2.service
Removed "/etc/systemd/system/apache2.service".
# verify its unmasked
systemctl list-unit-files | grep apache2
apache2.service enabled enabled
apache2@.service disabled enabled
# ------------------------------------
# Service management
systemctl start <service> # start service
systemctl is-active <service> # check service is running
systemctl stop <service> # stop system daemon
systemctl restart <service> # restart service
systemctl reload <service> # reload config without restart - only some services can do this
systemctl reload-or-restart <service> # reload config in place if possible, otherwise restart
systemctl enable <service> # autoload daemon on system startup
systemctl is-enabled <service> # check service is enabled
systemctl disable <service> # disable from starting automatically
systemctl status <service> # check status
systemctl is-failed <service> # check if service failed
Service units
Files that contain instructions for systemctl commands:
- System’s service file is in
/lib/systemd/systemor/etc/systemd/system systemdlooks for autostart files in/etc/systemd/system/<target>.target.wants.
# system service file location
la -l /lib/systemd/system
total 1528
-rw-r--r-- 1 root root 438 Mar 18 2024 apache2.service
-rw-r--r-- 1 root root 491 Mar 18 2024 apache2@.service
-rw-r--r-- 1 root root 603 Mar 18 2024 apache-htcacheclean.service
-rw-r--r-- 1 root root 612 Mar 18 2024 apache-htcacheclean@.service
...
# systemd looks for autostart files in
# Ex: apache2 system unit
systemctl cat apache2
# /usr/lib/systemd/system/apache2.service
[Unit]
Description=The Apache HTTP Server
After=network.target remote-fs.target nss-lookup.target
Documentation=https://httpd.apache.org/docs/2.4/
[Service]
Type=forking
Environment=APACHE_STARTED_BY_SYSTEMD=true
ExecStart=/usr/sbin/apachectl start
ExecStop=/usr/sbin/apachectl graceful-stop
ExecReload=/usr/sbin/apachectl graceful
KillMode=mixed
PrivateTmp=true
Restart=on-abort
OOMPolicy=continue
[Install]
WantedBy=multi-user.target
Targets
A target is a special unit file that groups together and organizes other units into a logical state (target state):
- Similar to how other systems use runlevels
- Each system has a default target that it uses during boot
- You can specify the state, instead of each unit individually
- If a unit is part of the target, it can be
WantedBy=<target>,RequiredBy=<target> - If a unit needs a target to be available, it
Wants=<target>,Requires=<target>, orAfter=<target>
# get default target
systemctl get-default
graphical.target
# set default target
sudo systemctl set-default graphical.target
# list available targets
systemctl list-unit-files --type=target
# list active targets - targets that systemd has tried to start
systemctl list-units --type=target
# ------------------------------------
# Shortcuts
sudo systemctl rescue # put system in single-user mode
sudo systemctl halt # halt the system
sudo systemctl poweroff # shutdown the system
sudo systemctl reboot # reboot the system (same as sudo reboot)
systemd timers
Create jobs run by systemd:
- create in
/etc/systemd/system
# list all timers
systemctl list-timers --all
NEXT LEFT LAST PASSED UNIT ACTIVATES
Mon 2024-11-18 01:50:00 UTC 1min 12s Mon 2024-11-18 01:40:11 UTC 8min ago sysstat-collect.timer sysstat-collect.service
Mon 2024-11-18 02:27:52 UTC 39min Mon 2024-11-18 01:01:51 UTC 46min ago fwupd-refresh.timer fwupd-refresh.service
Mon 2024-11-18 02:37:40 UTC 48min Tue 2024-11-12 13:04:02 UTC - motd-news.timer motd-news.service
Mon 2024-11-18 06:21:09 UTC 4h 32min Sun 2024-11-17 22:54:28 UTC - apt-daily-upgrade.timer apt-daily-upgrade.service
Mon 2024-11-18 07:32:37 UTC 5h 43min - - anacron.timer anacron.service
Mon 2024-11-18 10:14:24 UTC 8h Sun 2024-11-17 22:54:28 UTC - man-db.timer man-db.service
Mon 2024-11-18 16:50:23 UTC 15h Mon 2024-11-18 01:29:55 UTC 18min ago apt-daily.timer apt-daily.service
Tue 2024-11-19 00:00:00 UTC 22h Mon 2024-11-18 00:51:42 UTC 57min ago dpkg-db-backup.timer dpkg-db-backup.service
...
19 timers listed.
Check active services
systemctl list-unit-files --type=service --state=enabled
UNIT FILE STATE PRESET
anacron.service enabled enabled
apache2.service enabled enabled
apparmor.service enabled enabled
apport.service enabled enabled
blk-availability.service enabled enabled
...