linux-systemd.md

Basics

Unit : entity managed by systemd.

• /lib/systemd/system : systemd standard default units
• /usr/lib/systemd/system : maintainer, locally installed packages
• /run/systemd/system : transient unit files used at runtime
• /run/systemd/generator : place where systemd generates units automagically (eg. mount units based on /etc/fstab, cf man systemd.mount)
• /etc/systemd/system : admin custom unit files

https://unix.stackexchange.com/questions/206315/whats-the-difference-between-usr-lib-systemd-system-and-etc-systemd-system

Nice place to put service startup script ? /usr/local/sbin

systemd-delta : identify and compare overriding unit files

Helpful man pages : systemd.exec, systemd.mount, systemd.service

List available unit types

$ systemctl -t help
Available unit types:
service
socket
busname
target
snapshot
device
mount
automount
swap
timer
path
slice
scope

List loaded units

$ systemctl [list-units]
  UNIT                                                            LOAD   ACTIVE SUB       DESCRIPTION
  proc-sys-fs-binfmt_misc.automount                               loaded active running   Arbitrary Executable File Formats File System Automount Point
  sys-devices-pci0000:00-0000:00:07.1-ata1-host1-target1:0:0-1:0:0:0-block-sr0.device loaded active plugged   VMware_Virtual_IDE_CDROM_Drive
  sys-devices-pci0000:00-0000:00:15.0-0000:03:00.0-host0-target0:0:0-0:0:0:0-block-sda-sda1.device loaded active plugged   Virtual_disk 1
  sys-devices-pci0000:00-0000:00:15.0-0000:03:00.0-host0-target0:0:0-0:0:0:0-block-sda-sda2.device loaded active plugged   Virtual_disk 2
  ...
    boot.mount                                                      loaded active mounted   /boot
  dev-hugepages.mount                                             loaded active mounted   Huge Pages File System
  dev-mqueue.mount                                                loaded active mounted   POSIX Message Queue File System
  etc-apache2.mount                                               loaded active mounted   /etc/apache2
  exploit.mount                                                   loaded active mounted   /exploit
  home.mount                                                      loaded active mounted   /home
  opt-forge.mount                                                 loaded active mounted   /opt/forge
  opt.mount                                                       loaded active mounted   /opt
  proc-sys-fs-binfmt_misc.mount                                   loaded active mounted   Arbitrary Executable File Formats File System
  run-user-0.mount                                                loaded active mounted   /run/user/0
  run-user-491.mount                                              loaded active mounted   /run/user/491
  run-user-6010.mount                                             loaded active mounted   /run/user/6010
  run-user-61119.mount                                            loaded active mounted   /run/user/61119
  run-user-61282.mount                                            loaded active mounted   /run/user/61282

# Filtering 
$ systemctl --type=service
$ systemctl -t service
$ systemctl list-units --type=service --all

List installed units (similar to chkconfig --list)

$ systemctl list-unit-files -t service

List services in failed state

$ systemctl --failed

List a tree view of all process started by systemd

$ systemctl status
● my-server
    State: degraded
     Jobs: 0 queued
   Failed: 1 units
    Since: Sat 2022-04-30 20:45:29 CEST; 1 months 0 days ago
   CGroup: /
           ├─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 22
           ├─user.slice
           │ ├─user-491.slice
           │ │ ├─session-9907.scope
           │ │ │ ├─ 7151 systemctl status
           │ │ │ ├─ 7152 less

Start and enable services in one command

$ systemctl enable --now httpd mariadb

Control remote host

systemctl -H [hostname] restart httpd

Get last unit loaded by systemd

$ systemctl get-default

Customizing units : viewing

# Show default and overrides aggregate
$ systemctl cat httpd

Customizing units : available options

# List unit's properties
$ systemctl show --all httpd

# Query single property
$ systemctl show -p Restart httpd

Customizing units : Drop-ins (manual)

# Create directory
$ mkdir /etc/systemd/system/[name.type.d]
$ Create the drop-in
$ vi /etc/systemd/system/httpd.service.d/50-httpd.conf
[Service]
Restart=always
# Notify systemd of the changes
$ systemctl daemon-reload

Customizing units : Drop-ins (via systemctl)

# Create the drop-in
$ systemctl edit httpd
[Service]
Restart=always
# Changes take effect upon writing the file
$ systemctl show -p Restart httpd
Restart=always

Tip : pass --full to create a copy of the original unit file

Enable a unit

$ sudo systemctl enable my-startup.service
Created symlink /etc/systemd/system/multi-user.target.wants/my-startup.service -> /etc/systemd/system/my-startup.service

Simple custom unit

$ vi /etc/systemd/system/my-service.service
[Unit]
Description=A simple service
After=network.target

[Service]
ExecStart=/usr/local/bin/my-service

[Install]
WantedBy=multi-user.target

Check faulty configuration

$ systemd-analyze verify service_name