Re: How to get child pid from backtics

[zejames]

If you require that you parent script have full control over the 'blastall' child, I really advise you to use fork, for exemple :

my $pid = fork;
die unless defined $pid;

if ($pid) {
  # Parent. Do whatever you want
} else {
  # Child
  do 'blastall.pl';
}

[Hena]

UPDATE: Ok, got it fixed. Shoud be $kid = waitpid(-1,0). I quite don't understand how it matches the correct PID, but this seems to be working.

I'm having problems with the execution and fork. Probably as I have never used it before. So is this correct way to use it? Where does the waitpid go and how does it work? How the program flow go from here?

#!/usr/bin/perl
#
# Wrapper takes is the arguments of the blastall binary
#  changes directory to binarys directory
#  adds binary name
#  runs it
# Needs to be able to trap normal signals and definedly kill the proce
+ss, 
# as it ignores some which it should not.
#
use warnings;
use strict;
use File::Basename;
use sigtrap qw(handler catch_signal normal-signals);
my $pid;        # blastalls pid after fork

sub catch_signal {
  print STDERR "SIGNAL:",shift," blast should be $pid\n";
  kill (9,$pid);
  exit;
}

print "Going\n";

my $dir=dirname($0);
chdir $dir;
unshift (@ARGV,"./blastall");

$pid = fork;
die "Can't fork: $!\n" unless defined $pid;

if ($pid) {
  # Parent. Do whatever you want
  print "In parent.\n";
  my $kid=0;
  do {
    waitpid ($pid,0);
  } until ($kid == -1);
} else {
  # Child
  print "In child.\n";
  exec (@ARGV) or die "Unable to exec: $!";
}

print "After fork if.\n";
# this was used after `@ARGV` to get error message if executing failed
# if ($?!=0) {
#  warn "@ARGV failed: ",$?,":",$?>>8,"\n";
#  exit $?;
# }

print "Exiting.\n";
exit;

[Hena]

I was kind of hoping that it would not involve fork :).

[zejames]

In fact, you are anyway. Backticks does not avoid it, it only does hide it from you...