Doing two things at once, fork question.

mhearse has asked for the wisdom of the Perl Monks concerning the following question:

I've a program which opens a system command which never finishes. Currently, I just:

open CMD, "/usr/local/bin/inotifywait |";

while (my $output = <CMD>) {
.......
}
[download]

Rather than block for the output, I would like to start the command and spool the output to an array. Simultaneously, I would then parse the output if certain conditions exist (system load, time of day). How could I do this? Could fork be used?

my @output;
while (my $line = <CMD>) {
        chomp $line;
        push @output, $line;
}
[download]

Then somewhere else...

for my $out (@output) {
        if (condition exists) {
                parse output;
        }
        .....
}
[download]

Comment on Doing two things at once, fork question. Select or Download Code

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Re: Doing two things at once, fork question. by BrowserUk (Patriarch) on Mar 07, 2008 at 20:04 UTC
With threads: `#! perl -slw use strict; use threads; use threads::shared; my @output :shared; my $pid :shared; my $t = async { $pid = open CMD, qq[ perl -le"\$\|++; print ~~localtime while sleep 1" \|] or die $!; while( <CMD> ) { chomp; lock @output; push @output, $_; } close CMD; }; sleep 10; ## do other stuff for my $out ( do{ lock @output; @output } ) { if( $out =~ m[6] ) { print $out; } } kill 9, $pid; $t->join;` [download] Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply] [d/l]
Re^2: Doing two things at once, fork question. by mhearse (Chaplain) on Mar 07, 2008 at 22:08 UTC
Thanks for the example. One question. I've read the threads perldoc. async expects an anonymous subroutine. All of my code is in a module. Is there anyway I can call async with a method? I'm not sure what the syntax would be...	[reply]
Re^3: Doing two things at once, fork question. (Updated!) by BrowserUk (Patriarch) on Mar 07, 2008 at 22:36 UTC
Is there anyway I can call async with a method? I'm not sure what the syntax would be... Yes. (Update: Corrected method syntax) `# normal sub - no parameters. my $thread = async \&subname; # sub with args my $thread = async \&subname, $arg1, $arg2; # method - no parameters. my $thread = async \&pClass::method, $obj; # method with args my $thread = async \&Classname::method, $obj, $arg1, $arg2;` [download] And note that `async` is just a functional alias for `my $thread = threads->create( \&sub, $arg1, $arg2 ); my $thread = threads->new( \&Classname::method, $obj, $arg1, $arg2 );` [download] But note. Calling objects (object methods) across threads probably won't work in most cases. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply] [d/l] [select]
Re^4: Doing two things at once, fork question. (thread + method) by tye (Sage) on Mar 07, 2008 at 23:25 UTC
Re^5: Doing two things at once, fork question. (thread + method) by BrowserUk (Patriarch) on Mar 07, 2008 at 23:52 UTC
Some notes below your chosen depth have not been shown here
Re: Doing two things at once, fork question. by zentara (Cardinal) on Mar 07, 2008 at 19:45 UTC
You can also use IPC::Open3. It's part of the faq questions. See "perldoc -q external command" There are all sorts of ways to do it, and the devil is in the details; so you need to be more specific about what type of command you need to run, and filter it's output. Some commands need a pty, and require special handling. I'm not really a human, but I play one on earth. Cogito ergo sum a bum	[reply]
Re^2: Doing two things at once, fork question. by mhearse (Chaplain) on Mar 07, 2008 at 19:53 UTC
The command is inotifywait.	[reply]
Re: Doing two things at once, fork question. (IPC) by tye (Sage) on Mar 07, 2008 at 19:25 UTC
No, if you fork, then the two processes are separate and won't share an array. You could use threads but there is a much sexier technique that seems a much better idea for this situation: Redirect the output to a file and then read and process the output from that file when you feel like it. - tye	[reply]
Re^2: Doing two things at once, fork question. (IPC) by mhearse (Chaplain) on Mar 07, 2008 at 19:42 UTC
Thanks for the input. I assume redirecting to a file would be done using a system call. I think I will do some reading on threads.pm. I'd like to get some experience with that.	[reply]