A runwhen example

This example maintains a mirror of code.dogmap.org. in the directory ./mirror. It runs under Laurent Bercot's s6 system.

# cd /service/mirror-dogmap

# cat run
#!/bin/sh
exec 2>&1                           \
s6-setuidgid mirroruser             \
rw-add   n d1S             now1s    \
rw-match \$now1s ,H=2,M=30 wake     \
statfile -M finished                \
rw-add \$MTAI64N d36H      latest   \
rw-min \$wake \$latest     wake     \
statfile -M started                 \
rw-add \$MTAI64N d1H       earliest \
rw-max \$wake \$earliest   wake     \
sh -c '
  echo "@$wake" | s6-tai64nlocal | sed "s/^/next run time: /"
  exec "$@"' arg0                   \
rw-sleep \$wake                     \
./doit

# cat doit
#!/bin/sh
touch started &&
ftpcopy -x '/mirror/*' code.dogmap.org / ./mirror &&
touch finished

• setuidgid changes to the uid and gid of mirroruser, which should be able to write to the ./mirror directory.
• rw-add adds the current time and 1 second and stores the result in $now1s.
• rw-match computes the earliest time at or after $now1s that matches 2:30 AM (local time), and stores the result in $wake. (Adding 1 second in the previous step ensures that if ./doit runs in less than one second, rw-match won't schedule it to run again immediately; we'll schedule it for 2:30 AM the next day.)
• statfile (from the misc/fdtools package) finds the last-modified time of the file "finished" and stores it in $MTAI64N.
• rw-add adds $MTAI64N and 36 hours, and stores the result in $latest.
• rw-min computes the earlier of $wake and $latest, and stores the result in $wake. (This ensures that if the job fails, or was not run for some reason, it will be scheduled to run at most 36 hours after the last time it completed successfully, so the mirror will not be too far out of date.)
• statfile finds the last-modified time of the file "started" and stores it in $MTAI64N.
• rw-add adds $MTAI64N and 1 hour, and stores the result in $earliest.
• rw-max computes the later of $wake and $earliest, and stores the result in $wake. (This ensures that even if the job fails several times, it will not be attempted more often than once an hour, to avoid swamping a clogged network link.)
• The sh command logs $wake as the next scheduled run time.
• rw-sleep sleeps until $wake. (You can make it wake up immediately at any time by sending SIGALRM via svc -a /service/mirror-dogmap.)
• ./doit touches the file "started" to record that the job was attempted.
• ./doit updates the mirror with ftpcopy. If ftpcopy exits nonzero, the shell also exits, because of the -e option.
• ./doit touches the file "finished" to record that the job completed successfully.
• ./doit (which is running in the same process that was originally the run script) exits. supervise restarts ./run, and cycle begins again. (If you prefer to have your services never exit during normal operation, you could have ./doit end by exec'ing ./run.)