如何从父命令并行运行两个命令？

Question

我有两个代码

1.

use File::Temp qw(tempfile);
$tmp = new File::Temp( UNLINK => 0 );
system("tv_grab_au | tv_sort >> $file");
system("cp $file $HOME/.xmltv/listings.xml");

unlink($file);

2.

while (-e $file) {
sleep 2;
system("tvtime-command DISPLAY_MESSAGE \'Updating TV Guide. Please wait this might take a several minutes...\'");
}

我想结合这两个代码来运行tv_grab_au xmltv捕获(更新电视指南)，同时，发送命令到tvtime显示消息‘更新电视指南。请等待这可能需要几分钟...’，每两秒，直到$file存在。

我试一下这个：

use strict;
use warnings;
use File::Temp qw(tempfile);
my $file = new File::Temp( UNLINK => 0 );
use POSIX qw(:sys_wait_h);
$|++;

defined(my $pid = fork) or die "Couldn't fork: $!";

if (!$pid) {    
    system("tv_grab_huro | tv_sort >> $file");
    unlink($file);
}
else { 
    while (! waitpid($pid, WNOHANG)) {
        system("tvtime-command DISPLAY_MESSAGE \'Updating TV Guide. Please wait this might take a several minutes...\'");
        sleep 2;
        }
}

谢谢。

Answer 1

内置的fork函数在一个新的后台进程中创建当前程序的副本。然后，原始进程和“子”进程将同时运行。所以你可以这样做：

use File::Temp qw(tempfile);
my $file = new File::Temp( UNLINK => 0 );

my $new_pid = fork();
die "fork failed $!" unless defined $new_pid;   # this is uncommon

# Task 1 - in the background
if ($new_pid == 0) {
    system("tv_grab_au | tv_sort >> $file");
    system("cp $file $HOME/.xmltv/listings.xml");    
    unlink($file);
    exit;            # don't forget this part!
}

# Task 2 - in the foreground
while (-e $file) {
    print "...";
    sleep 2;
}

使用$file作为第一个任务何时完成的指示器有一些缺点。如果子代码有一些运行时错误怎么办？如果子进程被中断怎么办？子进程可能会在有机会删除$file之前退出。那么父进程中的while循环将永远不会结束。

内置的waitpid命令可以检查子进程是否仍在运行，并且可以处理子进程异常终止的情况。

# Task 2 
use POSIX ':sys_wait_h';
while (! waitpid $new_pid, &WNOHANG) {   # WNOHANG => non-blocking wait
    print "...";
    sleep 2;
}

Answer 2

使用fork()。我添加了额外的sleep()调用，这样您就可以看到进程同时运行和工作。在实践中，crontab更新可能运行得足够快，以至于监视器循环根本不运行，或者只运行一次。我用了“除非(...)”因为在我看来，这样做可以使代码更清晰；要记住的是，fork()将pid返回给父进程，而将零返回给子进程。因此，看不到pid的进程是一个子进程。(如前所述，如果fork失败，则fork将返回undef，并且代码将在原始进程中执行。在我们的例子中，这只是意味着监控在写入完成后启动，所以我们唯一失去的就是监控。)

my $file = "/tmp/.$$.crontab.txt";
my $crontab = <<EOS;
# Crontab lines here. Inserted at @{[scalar localtime()]}
EOS

my ($writer_pid, $monitor_pid);

$|++;

# Open file BEFORE launching processes. The monitor depends on the file's
# presence or absence, so if we opened it in the writer process, there'd be a
# chance the monitor process would check before we created it, and exit without
# monitoring.
die "Cannot open temp file\n" unless open(WRITE, ">" . $file);

# Crontab file handle gets passed to the forked process, so we can just use it.
# Altered so we can see the process do its thing.
unless ($writer_pid = fork()) {
        print WRITE $crontab."\n";
        close WRITE;
        print("crontab -l |grep -v backup >> $file");
        sleep 20;
        print("crontab $file");
        sleep 10;
        unlink($file);
        print "done!\n";
        exit;
}

# Either file will exist, or the previous process will
# have completed. If it exists, we monitor. If not,
# we exit immediately.
unless ($monitor_pid = fork()) {
    # Child: monitor the writer.
    my $waitcount = 1;
    while ( -e $file ) {
       sleep 2;
       print  "($waitcount) installing crontab...";
       $waitcount++;
    }
    print "installed\n";
    exit;
}

waitpid($monitor_pid, 0);
waitpid($writer_pid,0);
print "both processes done\n";