[gentoo-amd64] Re: How to use &&

[Duncan]

Brett Johnson <brett@××××.com> posted 1231082331.30177.24.camel@homer,
excerpted below, on  Sun, 04 Jan 2009 09:18:51 -0600:

> I have never looked into multi threading a shell script, though I
> suppose it could be possible but fairly complex. You could execute
> multiple jobs in the background using '&' and capture each process id
> into an array with the ${!} parameter, which "Expands to the process ID
> of the most recently executed background (asynchronous) command". Once
> you have launched the multiple jobs, you could enter a loop that sleeps
> a few seconds and then checks the status of each process id in the
> array, waiting for them to disappear.

I haven't done anything that advanced, but I do use a relatively simple 
trick I learned from Mandrake's rc.sysinit (or whatever they call it, IDR 
for sure, see my immediately previous post for the story) to allow 
multiple background processes to execute in parallel, with a wait 
barrier.  Once you've seen it it's simple enough that perhaps you 
discounted that as proper multi-threading... or perhaps you've just not 
seen it done this way yet.

Here's how I did it in my esync script, which runs a esearch database 
update and auto-fetches both @system and @world in parallel.  There's 
also a parallel sync function I used to sync layman's overlays at the 
same time as portage, but I have the layman bit commented out now as I'm 
not using any overlays besides my own personal one ATM.  It's also worth 
noting the trick I used to get the eupdatedb to run at the same niceness 
portage uses from make.conf... and now that I look I see a nice example 
of using && in place of an if/then, as well. =:^)

The one problem, of course, is that handling it this way means jumbled up 
output.  There are ways around that, but this was a quick-hack enough 
that I didn't worry about doing the whole logging thing, etc, here.
"It works for me!" (TM)  The wait builtin simply waits until all 
background processes are completed before proceeding...

#!/bin/bash

sync-up () {
        emerge --sync $@ &
#       layman -S &
        wait
}

finish-up () {
#       emerge --regen
        emerge -uDf @world &
        emerge -uDf @system &

        # we need PORTAGE_NICENESSS from make.conf to run eupdatedb
        portnice=$(source /etc/make.conf;echo $PORTAGE_NICENESS)
        [ -z $portnice ] && portnice=0
        nice -n${portnice} eupdatedb &

        wait
}

sync-up
finish-up
# end of script

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman