Re: Sequential background tasks

[Digby Tarvin <digbyt@acm.org>]

On Mon, Jul 17, 2006 at 06:36:55AM -0400, Gary R. Leaf wrote:
> >hi
> >
> >I find that in many cases I need my background tasks to be executed in
> >sequence. Ie, I need background task-b to start right after background
> >task-a has properly started. 
> >
> >So far I haven't found a good way to do it. I used
> >
> >task-a & sleep 2; task-b &
> >
> >but that 'sleep 2' has changed to 'sleep 5' and still sometimes task-b
> >starts before task-a. I can raise the wait time, but it means that task-b
> >would normally start too late... 
> >
> >Any good way? 
> >
> >thanks
> >
> >
> >
> Set up a script that checks the return codes.
> 
> if  return code of task A is 0 then
>   task B
> else
>  do something else
> 
> if rturn code of task B is 0 then
>   task C
> else
>   do something else
> 
> and so on
> 
> execute the controlling script in the background

You don't need a script - just use the 'conditional and' operator
of the shell:

	(cmd1 && cmd2 && cmd3)&

will execute cmd1 to cmd3 in sequence, aborting if any fail. If you
don't want to abort on error, use ';' to separate the commands...

from the 'sh' manpage:
       ``&&'' and ``||'' are AND-OR list operators.  ``&&'' exe-
       cutes the first command, and then executes the second com-
       mand iff the exit status of the first command is zero.
       ``||'' is similar, but executes the second command iff the
       exit status of the first command is nonzero.  ``&&'' and
       ``||'' both have the same priority.

DigbyT
-- 
Digby R. S. Tarvin                                          digbyt(at)digbyt.com
http://www.digbyt.com