strace

--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=5822, si_uid=0, si_status=111, si_utime=0, si_stime=0} ---
open("finish", O_RDONLY|O_NONBLOCK)     = -1 ENOENT (No such file or directory)
open("supervise/pid.new", O_WRONLY|O_CREAT|O_TRUNC|O_NONBLOCK, 0644) = 9
rename("supervise/pid.new", "supervise/pid") = 0
open("supervise/stat.new", O_WRONLY|O_CREAT|O_TRUNC|O_NONBLOCK, 0644) = 9
rename("supervise/stat.new", "supervise/stat") = 0
open("supervise/status.new", O_WRONLY|O_CREAT|O_TRUNC|O_NONBLOCK, 0644) = 9
rename("supervise/status.new", "supervise/status") = 0
stat("control/u", 0x7fff8926f320)       = -1 ENOENT (No such file or directory)
open("supervise/pid.new", O_WRONLY|O_CREAT|O_TRUNC|O_NONBLOCK, 0644) = 9
rename("supervise/pid.new", "supervise/pid") = 0
open("supervise/stat.new", O_WRONLY|O_CREAT|O_TRUNC|O_NONBLOCK, 0644) = 9
rename("supervise/stat.new", "supervise/stat") = 0
open("supervise/status.new", O_WRONLY|O_CREAT|O_TRUNC|O_NONBLOCK, 0644) = 9
rename("supervise/status.new", "supervise/status") = 0
runsv /tmp/fizz/: fatal: unable to start ./run: file does not exist

what-is-sv-doing

[root@funkyboy noah]# ls -l /etc/sv/foo
total 8
-rwxr-xr-x 1 root root   60 Feb  8 08:04 run
drwx------ 2 root root 4096 Feb  8 08:04 supervise
[root@funkyboy noah]# ls -l /etc/sv/sshd
total 4
-rwxr-xr-x 1 root root 146 Nov  3 21:32 run
lrwxrwxrwx 1 root root  25 Nov  3 21:32 supervise -> /run/runit/supervise.sshd