The s6-svwait program

s6-svwait blocks until a collection of supervised services goes up, or down.

s6-svwait only waits for notifications; it never polls.

Interface

     s6-svwait [ -U | -u | -d ] [ -a | -o ] [ -t timeout ] servicedir...

s6-svwait monitors one or more service directories given as its arguments, waiting for a state (ready, up or down) to happen. It exits 0 when the wanted condition becomes true.

Options

-u : up. s6-svwait will wait until the services are up, as reported by s6-supervise. This is the default; it is not reliable, but it does not depend on specific support in the service programs. See this page for details.
-U : really up. s6-svwait will wait until the services are up and ready as reported by the services themselves. This requires specific support in the service programs, and the use of the notification-fd file in the service directory. See the explanation on this page.
-d : down. s6-svwait will wait until the services are down.
-o : or. s6-svwait will wait until one of the given services comes up or down.
-a : and. s6-svwait will wait until all of the given services come up or down. This is the default.
-t timeout : if the requested events have not happened after timeout milliseconds, s6-svwait will print a message to stderr and exit 1. By default, timeout is 0, which means no time limit.

Notes

s6-svwait should be given one or more service directories as arguments, not a scan directory. If you need to wait for a whole scan directory, give all its contents as arguments to s6-svwait.
s6-svwait will only work on service directories that are already active, i.e. have a s6-supervise process running on them. It will not work on a service directory where s6-supervise has not been started yet.

Internals

s6-svwait spawns a s6-ftrigrd child to listen to notifications sent by s6-supervise. It also checks supervise/status files, as well as the supervise/ready files if necessary, to get the current service states, so it is immune to race conditions.