The s6-ftrig-listen1 program

s6-ftrig-listen1 subscribes to a fifodir, then spawns a program, then waits for a pattern of events to occur on the fifodir.

s6-ftrig-listen1 acts just as s6-ftrig-wait, except it can make sure that the process sending the notifications is actually started after there is a listener for those events.

Interface

     s6-ftrig-listen1 [ -t timeout ] fifodir regexp prog...

s6-ftrig-listen1 subscribes to fifodir
It then forks and exec prog... with all its arguments
It waits for the series of events received on fifodir to match regexp. regexp must be an Extended Regular Expression.
When the series of read events matches regexp, s6-ftrig-listen1 prints the last event it received to stdout (one byte followed by a newline), then exits 0.

Options

-t timeout : if the events on fifodir have not matched regexp after timeout milliseconds, print an error message on stderr and exit 1. By default, s6-ftrig-listen1 waits indefinitely for a matching series of events.

Usage example

The following sequence of shell commands has a race condition:

In terminal 1:

s6-mkfifodir /tmp/toto
s6-ftrig-wait /tmp/toto "message"

Then in terminal 2

s6-ftrig-notify /tmp/toto message

Depending on the operating system's scheduler, there is the possibility that the s6-ftrig-notify process starts sending "message" before the s6-ftrig-wait process has actually subscribed to /tmp/toto, in which case the notification will be missed. The following sequence of shell commands accomplishes the same goal in a reliable way, without the race condition:

s6-mkfifodir /tmp/toto
s6-ftrig-listen1 /tmp/toto "message" s6-ftrig-notify /tmp/toto message