how do i execute dependant system commands in perl (i m trying waitpid but does not work)

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

Hi i am executing two commands from my perl script.
1. zcat ... | gzip huge file. &
it takes lots of time
2. java huge file &
after above 1. is done.
how can i do this?
here is what I am doing but does not work:

system("zcat....> file1 &");
my $pid = fork(); 
waitpid($pid,0);
system("java file1 &");
[download]

idea is : i want to execute java file1 after zcat is done
Please help.
Thanks,
SDJ

Comment on how do i execute dependant system commands in perl (i m trying waitpid but does not work) - please help Download Code

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Re: how do i execute dependant system commands in perl (i m trying waitpid but does not work) - please help by GrandFather (Saint) on Jan 27, 2012 at 10:21 UTC
In what way doesn't it work? system already waits for the executed command to complete so you shouldn't need anything more than: `system("zcat....> file1 &"); system("java file1 &");` [download] to have the commands run one after the other. True laziness is hard work	[reply] [d/l]
Re^2: how do i execute dependant system commands in perl (i m trying waitpid but does not work) - please help by BrowserUk (Patriarch) on Jan 27, 2012 at 10:26 UTC
system already waits for the executed command to complete Don't the ampersands on the ends of those commands mean that the command processes will be backgrounded by the shell and system will return immediately? With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' 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. The start of some sanity?	[reply]
Re^3: how do i execute dependant system commands in perl (i m trying waitpid but does not work) - please help by GrandFather (Saint) on Jan 27, 2012 at 10:32 UTC
Argh! Quite right - I didn't notice them. :( In fact that will be why it "doesn't work" for the OP. Probably a little cargo culting going on. True laziness is hard work	[reply]
Re: how do i execute dependant system commands in perl (i m trying waitpid but does not work) - please help by ikegami (Patriarch) on Jan 27, 2012 at 22:00 UTC
`&` does exactly the opposite of what you want. It causes the shell to not wait for its child to finish. `system` is basically `fork`+`exec`+`waitpid` mixed into one. `system("zcat....> file1"); system("java file1");` [download] Or if you don't want to wait for `java` to finish, `system("( zcat....> file1 && java file1 )&");` [download]	[reply] [d/l] [select]
Re: how do i execute dependant system commands in perl (i m trying waitpid but does not work) - please help by fisher (Priest) on Jan 27, 2012 at 10:27 UTC
just don't fork(), system() will not return control until job's done.	[reply]
Re: how do i execute dependant system commands in perl (i m trying waitpid but does not work) - please help by i5513 (Pilgrim) on Jan 27, 2012 at 15:40 UTC
Play with sh ! `system ("(zcat ... > file1 & wait; java file1)&"); your next sentence;` [download] Now you need to know when java process end .., but it should be another question, right ?	[reply] [d/l]