уведомлять сервисного менеджера о завершении запуска и других изменениях статуса сервиса (notify service manager about start-up completion and other service status changes)
Примечание (Note)
These APIs are implemented as a shared library, which can be
compiled and linked to with the libsystemd pkg-config
(1) file.
These functions send a single datagram with the state string as
payload to the AF_UNIX
socket referenced in the $NOTIFY_SOCKET
environment variable. If the first character of $NOTIFY_SOCKET is
"@", the string is understood as Linux abstract namespace socket.
The datagram is accompanied by the process credentials of the
sending service, using SCM_CREDENTIALS.