Откуда взялася $MAINPID?

https://www.freedesktop.org/software/systemd/man/systemd.service.html, Ctrl-F, «mainpid», Enter, Enter.

Ctrl-F, «mainpid», Enter, Enter.

Это прекрасно.

Очень наглядно, спасибо)

Ссылки имеют свойство внезапно меняться, так что пусть здесь будет копия:

ExecReload=

Commands to execute to trigger a configuration reload in the service. This argument takes multiple command lines, following the same scheme as described for ExecStart= above. Use of this setting is optional. Specifier and environment variable substitution is supported here following the same scheme as for ExecStart=.

One additional, special environment variable is set: if known, $MAINPID is set to the main process of the daemon, and may be used for command lines like the following:

    /bin/kill -HUP $MAINPID

Note however that reloading a daemon by sending a signal (as with the example line above) is usually not a good choice, because this is an asynchronous operation and hence not suitable to order reloads of multiple services against each other. It is strongly recommended to set ExecReload= to a command that not only triggers a configuration reload of the daemon, but also synchronously waits for it to complete.