Signaling a service
Earlier, in the Start dependencies section, I showed how the sv command-line tool can be used to control a service. Later on, I demonstrated how the sv start and sv down commands can be used inside run and finish scripts to communicate between services. You may have already guessed that runsv is sending POSIX signals to the run processes that it supervises when an sv command executes. But what you may not have known is that the sv tool controls its target runsv process over a named pipe. The named pipes supervise/control and optionally log/supervise/control are opened so that other processes can send commands to runsv. Signaling a service is easy with sv commands, but if you want to, you can bypass sv entirely and write control characters directly to the control pipe(s).
The runtime directory layout of a service with no dedicated logging looks like this:
# tree /etc/sv/syslogd /etc/sv/syslogd |-- run `-- supervise |-- control ...