2.7 systemd details

unix-and-linux-system-administration-handbook-2.7.md

systemd

• goal - to standardize the aspect of system administration
• program + daemon + library + tech + kernel config
• dependent only on Linux kernel

unit and unit file

• unit: entity managed by systemd
• cf. systemd.unit man page: https://www.freedesktop.org/software/systemd/man/systemd.unit.html
• the action of each unit defined in systemd and is configured by a unit file.
  • ex. service
    • unit file specifies the location of daemon execution file, informs systemd about how to start and stop the service, and also specifies other unit which the service is dependent on.
• unit file can be located in multiple locations.
• mainly in /usr/lib/systemd/system when installing the package.
• some systems use /lib/systemd/system as the path. -> this is the original so do not edit.
• put local unit file or edited version in /etc/systemd/system
• /run/systemd/system has unit directory and workspace for saving temporary unit.
• systemd manages looking into all the directories above so there are almost same. -> if there's the difference then /etc/systemd/system gets the highest priority.
• suffix of unit file by unit type
  • .service
  • .timer

example of ubuntu system

[Unit] # the section for all unit types
Description=fast remote file copy program daemon
ConditionPathExists=/etc/rsyncd.conf

[Service] # the section only in "service" unit type
ExecStart=/usr/bin/rsync --daemon --no-detach

[Install]
WantedBy=multi-user.target

systemctl: control of systemd

• systemctl: versatile command to assess the status of systemd or change the configuration
• systemctl subcommand option
• typically use unit name except unit type suffix(ex. cups instead of cups.service) when using subcommand for specific unit
• but the default unit type that gives specificity to a simple name varies depending on the subcommands
  • if, there are one.service and one.socket and the default unit type of stop is socket -> systemctl stop one stops one.socket

commands

• systemctl == systemctl list-units

subcommand	function
list-units [pattern]	every loaded or activated service, socket, target, mount, device at present, can choose unit type; ex. using --type=service([pattern])
list-unit-files [pattern]	installed unit file at present regardless activation status
enable unit	activate unit during booting
disable unit	stop activating unit during booting
isolate target	change operating mode to target
start unit	activate unit immediately
stop unit	deactivate unit immediately
restart unit	restart unit (or just start if it is deactivated) immediately
status unit	show status of unit and recent log items
kill pattern	send a signal to the unit that matches the pattern
reboot	reboot the computer
daemon-reload	reload unit fule and systemd configuration

unit status

• systemctl status -l unit

root@server ~ # systemctl status -l nvmefc-boot-connections
● nvmefc-boot-connections.service - Auto-connect to subsystems on FC-NVME devices found during boot
     Loaded: loaded (/lib/systemd/system/nvmefc-boot-connections.service; disabled; vendor preset: enabled)
     Active: inactive (dead)
.
.
.

• it shows if it is activated, inactive time(if it is dead), and whether it was activated during server boot(Loaded part)
• last four sentences are the recent logs
  • logs are compressed by one line by default -> with -l it shows all the logs

unit file status - systemctl list-unit-files

status	meaning
bad	systemd internal issue, usually unit file error
disabled	installed, but not start automatically
enabled	installed, and start automatically
indirect	inactive but there is a peer which can be activated in Also section
linked	unit which can be used by symbolic link(symlink)
masked	unit which is expelled logically
static	dependent unit by other unit, no installation requirements

enabled and disabled

• enabled and disabled status are located in one of the system directories of systemd(not linked by symbolic link), and applied only which has [Install] section in unit file
• enabled unit is almost installed unit
• The unit is installed. means that the command in [Install] section is already executed and ready to be activated properly
• usually enabled status activate the unit automatically after the system bootstrap

Q. - deactivated does not mean disabled status since it is on a typical active route

Q. - disabled ? deactivated unit can be activated by systemctl start

static

• there are many units without install process
  • these units cannot be enabled or disabled but only be available -> static
• static status unit file only can be activated by systemctl start or by dependency of other activated units

linked

Q.- linked unit file is produced by systemctl start

masked

• masked status means blocked for administrative purposes
• systemd is aware of masked status unit but blocks it from the activation or the influence to the environment command by systemctl mask

---

• systemctl disable for activated or linked unit
• systemctl mask for static unit
• for disabled, inactive unit
  • systemctl enable
  • systemctl start

target

dependency between units

execution order

example of unit file

local service and customization

precautions when service and engine control

systemd logging

Question

1. 뭐하는걸까?

Q. indirect, linked, masked?
Q. symbolic link로 연결 무슨 의미?
Q. - usually enabled status activate the unit automatically after the system bootstrap -> system bootstrap==서버가 켜지면?
Q. 마찬가지로 비활성화됐다는 것은 일반적인 활성 경로에 있다는 것이기 때문에 disabled 상태라는 것은 다소 부적절하다. / 활성화된(enabled) 유닛은 사실상 설치된(installed) 유닛으로 간주돼야 한다. -> deactivated does not mean disabled status since it is on a typical active route? -> disabled라는 표현이 부적절하다는걸까? 언제든 실행될 수 있기 때문에?
Q. linked는 설명을 듣고 다시 정리하자....