Linux Services

Folders #

Note: “runtime units”: the ability to make modifications to a process (unit) during the current boot without that change/modification persisting across a reboot.

Tip: check which package a file belongs to:

• for RPM (Cent/Fedora/RHEL): rpm -qf /usr/lib/systemd/system/* |sort -u | head
• for Debian/Ubuntu: dpkg -S /lib/systemd/system/* |sort -u | head
  • -S == --search
  • This command will not list extra files created by maintainer scripts, nor will it list alternatives.
  • or: dpkg-query -S ...

Ref’s: man systemd.unit, man 7 file-hierarchy, stackoverflow.

Filenames #

<service_unit_name>[@<instance>].<type> [email protected] [email protected] [email protected]

ref: https://unix.stackexchange.com/questions/588629/why-do-some-unit-filenames-end-with

Content #

[Unit]
Description=<some description>
After=network.target

[Service]
Type=simple
[User=optional__user_name]
Restart=on-abort
ExecStart=/cmd/starts/with/abs/path/to/executable/binary/or/script.sh param1 param2

[Install]
WantedBy=multi-user.target

Commands #

systemctl daemon-reload # whenever service-related files change
systemctl enable my_service # or: systemctl enable my_service@$USER
systemctl start my_service

Example: Add a Service (OpenVPN Client Example) #

# create the service file
sudo touch /etc/systemd/system/openvpn-client.service && \
sudo chmod 777 /etc/systemd/system/openvpn-client.service && \
ll /etc/systemd/system/*.service* && \
sudo vim /etc/systemd/system/openvpn-client.service

Tip 1: “set paste” in vim. Tip 2: which openvpn for abs./full path.

[Unit]
Description=openvpn-client
After=network.target

[Service]
Type=notify
Restart=on-failure
RestartSec=2
ExecStart=/usr/sbin/openvpn --config <path/to/client-config/e.g./etc/openvpn/client/client_hostname.ovpn>

[Install]
WantedBy=multi-user.target

# enable & run
sudo systemctl daemon-reload && \
sudo systemctl enable openvpn-client && \
sudo systemctl start openvpn-client

# check status
systemctl status openvpn-client.service

# check journal for errors' details if any
sudo journalctl -xeu openvpn-client.service

service (systemd unit) auto-restart when network change (clients may need) #

It depends on which network-related dispatcher is available from the OS. ref

Situation 1: systemd-networkd dispatcher. Check: systemctl status networkd-dispatcher.service ( or ps auxf |grep networkd ) If any error: journalctl -b -u networkd-dispatcher

Subdir meanings:

• /etc/networkd-dispatcher 下的脚本目录及含义如下：
• routable.d/：当网络接口处于可路由状态时(网络已正常，用networkctl status可见)，执行该目录下的脚本。
• dormant.d/：网络接口处于休眠状态时，执行该目录下的脚本。
• no-carrier.d/：网络接口没有载波时(例如有网口插上网线)，执行该目录下的脚本。
• off.d/：网络接口关闭时，执行该目录下的脚本。
• carrier.d/：网络接口有载波时(例如有网口拔下网线)，执行该目录下的脚本。
• degraded.d/：网络接口处于降级状态时，执行该目录下的脚本。
• configured.d/：网络接口配置完成时，执行该目录下的脚本。
• configuring.d/：网络接口正在配置时，执行该目录下的脚本。

执行时，按照此子文件夹中所有文件名称排序后执行。如果需要根据具体端口或者其他情况判断，需要自行在脚本中使用 if else。

Tip: owner shoud be root, permission shoud be set 755: chmod -R a+x /etc/networkd-dispatcher/*

ref: systemd-networkd gitlab

Situation 2: NetworkManager (with NetworkManager-dispatcher). Check: systemctl status NetworkManager-dispatcher.service If available: see ref-stackExchange