Notes about Location-Aware Cron
-------------------------------

Location aware commands are documented in the xcrontab(1) man page
(search for the m option and @location).


INSTALLING LCRON
----------------

1. Build xcron normally (see README for details).
   Configure it with the "x" naming scheme
   (note that, for me, batch support doesn't build)
   and to be installed with a the cron library in
	/usr/local/lib/xcron/lib
   cron spool directory
	/usr/local/lib/xcron/spool
   batch spool directory
	/usr/local/lib/xcron/spool/batch
   putting the binaries in
	/usr/local/lib/xcron/bin
   and cron itself in
	/usr/local/lib/xcron/sbin

2. Install it with "make install".

3. Install the generic parts of lcron things with "make install.lcron".

4. Patch Lcron into RedHat 6.0:
	make install.lcron.redhat

	This will make xcron start on boot
	and patch it into apmd (redhat 6.1 or later)
	and the network (redhat 6.0 or later).

5. Start xcron with
	/etc/rc.d/init.d/xcron start

6. Set up lcron for your local system (see "USING LCRON" below).

	
Xcron needs to know when you get somewhere.  In RedHat 6 this happens
automatically through ifup/ifdown with the program "location_monitor".
If that doesn't work for you (you use an earlier version of RedHat, or
you don't use ifup/ifdown), then you should invoke location_monitor
after you move, or run it periodically (from cron).

Lcron makes location determinations by detecting changes in your
attached network interfaces.  (In the long run other things are
planed, but for now that's all there is.)  For lcron to be useful your
computer must change the list of attached network interfaces when you
add/remove your network pccard.


USING LCRON
-----------

For simple things you can use standard crontab format entries, with an
@location field following the day-of-week column.  Initially LOCATION
is specified as IP address of your gateway.

Thus, a simple lcron entry would be

	0,30 * * * * @128.125.187.254 fetchmail darkstar.isi.edu

which says every 30 minutes when I'm at the the place with
gateway 128.125.187.254 (USC), get my mail from ISI.
You can find the gateway you use with the command "netstat -rn".

Of course, remembering the IP address of the gateway is a pain.
You can map these numbers of names using the table in
/usr/local/lib/xcron/lib/netstat_mapping.  Here's mine:

	128.9.160.7 isi
	128.9.128.3 isi-netwave
	128.125.187.254 usc
	128.9.97.33 home
	128.9.32.13 ppp

Kuenning's xcron (upon which lcron is built) also supports a
non-standard (but easier) syntax for specifying things.
See the man page for crontab for the definition.
For example, 

	- :0,30 @isi rdist -f office.distfile

says to run rdist every 30 minutes to replicate my files
in a Kuenning-style entry.

In addition to period jobs which are dependent on location, lcron also
supports edge-triggered jobs which run only when you first connect to
a new location.  An example:

	m @isi /usr/local/lib/lcron/sbin/printcap_set_default ps11d_d
	m @usc /usr/local/lib/lcron/sbin/printcap_set_default sal224

The m says this job runs when you (m)ove (i.e., is edge triggered),
@isi says where, and the rest runs a program which sets the printcap
entry named default to point to the printer ps11d_d, while the second
one sets the default printer to be sal224 when I'm at USC.

(A digression on printcap_set_default:
It copies an existing printcap entry over the one named "default".
Thus you can set the PRINTER environment variable to "default"
which will then be bound to whatever local printer you want.
To use it, manually create a printcap entry named default
(the contents of the entry don't matter),
and create entries for your other printers (ps11d_d and sal224
in my case), then run printcap_set_default as above.)

Sometimes you want to run jobs every so many hours when you're at a
place.  For example, I back up my computer every 12 hours when I'm at
home.  Ideally, a backup should run shortly after I get home, then
periodically when I've been home a while.  Here's how I do it:

- :0,30 @home /usr/local/lib/lcron/bin/run_every 12h rdist home_file


Sometimes you want to do a one-shot job when something happens (like
you're connected to the network).  Just use a location-specific at-job:

xat @home <<END
/usr/ucb/Mail -s 'take out the trash' johnh </dev/null
END

or more succinctly with the program designed for that exact purpose

tomorrow @home <<END
take out the trash
END


LCRON CODE
----------

In the code, it should all be ifdef'ed on LOC_CRON.
In addition, there are some sub-options:

	LOC_MOVE: support for edge-detection on location.
		Should work and always be enabled
		(if LOC_CRON is enabled).

	LOC_EVERY: support for running commands at most
		every N seconds.  Currently this functionality
		can be gotten with the "run_every" perl program,
		The C code is not functional.

	LOC_EVENTSDIR: support arbitrary events in files in 
		lib/events.

	LOC_ATLOC: support location-triggered at jobs.

All of these are on by default except LOC_EVERY.

The lcron modifications to Kuenning's xcron were made by 
Dhaval Shah and John Heidemann.


   -John Heidemann
    <johnh@isi.edu>
     9-Jul-99