@node Overview
@chapter Overview
@command{sat} is a simple alternative to @command{at}.
It implements the basic features of at @command{at}
in a sane less harmful way. It is hence by itself
incompatible with @command{at}.
@command{sat} is used to queue jobs for later execution.
It is daemon-based, the deamon being @command{satd}, and
using @command{satq}, @command{satrm}, and @command{satr}
you can list queued jobs, remove queued jobs, and run
queued jobs early, respectively. @command{satd} is launched
automatically and exits automatically when it has nothing
more to do.
If you are using @command{autohaltd}, you should set up
hooks for @command{sat} that pauses @command{autohaltd}
using @code{SIGSTOP} (and resume with @code{SIGCONT})
when there are queued jobs. This would prevent
@command{autohaltd} from shutting down the machine
while there are queued jobs but no logins. @command{sat}
cannot stop @command{autohaltd} by adding login entries,
because @command{autohaltd} will only recognise them if
they are in fact true logins.
@command{sat} stores the job queue in a file. However,
this file does not persist between reboot.
@command{sat} was written because @command{at} is far
too complex and has a lot of issues. @command{at}
sanitises the environment, @command{sat} does not do
this because you can do this with @command{env}, and
you will not lose environment variable that you my
want to keep, which sometimes happens with @command{at}.
@command{at} also do mailing, @command{sat} instead
lets you implement this with hooks, so that it is
less complex and less restrictive. @command{at} does
system load analysis, which @command{sat} does not,
instead you can use an external program for this.
@command{sat} also only has one queue, because you
do not need more than one queue, queues can easily
be emulated. Finally, the authors of @command{at} had
the brave idea to do natural language parsing,
@footnote{Good luck getting that to work properly
for even just English and the Gregorian calendar!}
@command{sat} does however do trivial parsing, but
is designed so that you can use any external parser.
Some think that @command{at} shall be merge into
@command{cron}. The authors of @command{sat} does
not think so because it is two wilding different
kind of job scheduling.
@command{sat} uses two clocks: @code{CLOCK_REALTIME}
for time specified in absolute time, and
@code{CLOCK_BOOTTIME} for time specified in relative
time. Note that this is not @code{CLOCK_REALTIME_ALARM}
and @code{CLOCK_BOOTTIME_ALARM}, if you need those
clocks, you can use the less powerful @command{sleep-until}
which supports all clocks your operating system supports.
