The s6-svlink program

s6-svlink creates, in a scan directory, a symlink to a service directory, and notifies s6-svscan that a new service has been registered. It waits until a s6-supervise supervisor process has been spawned to manage the new service, then exits.

The point of s6-svlink is to help integrate service directories into an existing service manager sequence and eliminate race conditions.

Interface

     s6-svlink [ -d | -D ] [ -P ] [ -f ] [ -t timeout ] scandir servicedir [ name ]

s6-svlink expects a running s6-svscan process on scandir and a fully functional, but unsupervised, service directory in servicedir.
It symlinks servicedir into scandir. The symbolic link is named name; if no name argument has been given, the name given to the symbolic link is the basename of servicedir.
It sends a command to s6-svscan to signal it that a new service is available.
It waits for an s6-supervise process to be spawned on servicedir.
It exits 0.

Exit codes

0: success
99: timeout while waiting for the supervisor to start
100: wrong usage
111: system call failed

Options

-d : down. The supervisor will be started, but the service itself will remain down. Any servicedir/down file will be deleted. By default, if neither the -d nor -D options have been given, the supervisor auto-starts the service as soon as it runs.
-D : down, and stay down. The supervisor will be started, but the service itself will remain down. A servicedir/down file will be created. By default, if neither the -d nor -D options have been given, the supervisor auto-starts the service as soon as it runs.
-P : public. If servicedir/event does not exist, it will be created as public, i.e. anyone will be able to subscribe to this fifodir. By default, it will be created as private, i.e. only processes running with the same gid as the s6-svscan process will be able to susbscribe to it.
-f : force permissions. The presence or absence of the -P option (i.e. the public or private state of servicedir/event) will be enforced even if servicedir/event already exists. By default, s6-svlink exits with an error message if servicedir/event exists and its public/private state mismatches what is requested.
-t timeout : if the supervisor has not started after timeout milliseconds, s6-svlink will print a message to stderr and exit 99. By default, timeout is 0, which means no time limit.

Notes

Using s6-svlink to start services is a suboptimal pattern: it requires precise manipulations involving use of s6-ftrigrd in order to avoid race conditions, so it is relatively expensive. The simpler, more efficient pattern is to have all the supervisors already started at boot time, so the existence of the supervisor can be relied on, and starting the service becomes a trival and instant operation - this is, for instance, how the s6-rc service manager behaves. However, it can be difficult to implement this pattern with other services managers such as OpenRC; in those cases, s6-svlink, which starts the supervisors one at a time, can be used instead.
If servicedir is logged, i.e. servicedir/log is also a valid service directory, then s6-svlink will wait until supervisors have been spawned for both the service and its logger.
s6-svlink sends an s6-svscanctl -a command to scandir, which means that the system's view of services will be refreshed. Depending on what links exist in scandir, other services than servicedir may also appear.
The symmetrical program to s6-svlink is named s6-svunlink.