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

I am trying to port a shell script to perl It was: 
/path/to/program/that/would/output/something/and/run/forever > /dev/nu
+ll 2>&1 &
for i in `seq 1 10`; do
    sleep 1;
    echo -n "."
done
kill -9 $!

[download]
The idea is to let the program capture some data and parse its logfile afterwards. So I need to start the program, put it in the background, output something to show I am alive, and kill the programm. However, I couldn't find how to do it.

Replies are listed 'Best First'.
Re: Executing a program, displaying some status, and killing it
by Roy Johnson (Monsignor) on Dec 08, 2004 at 00:47 UTC
    Check out fork and exec. Something like: 
    my $child = fork;
if ($child) {
  # I am in the parent...wait then kill
  for my $i in (1..10) {
    sleep 1;
    print ".";
  }
  kill -9, $child;
}
else {
  # Run the background program
  exec '/path/to/program/that/would/output/something/and/run/forever >
+ /dev/null 2>&1';
}

    [download]
    There is more to error checking for return values than I illustrate here. Do see the documentation for examples.
    Caution: Contents may have been coded under pressure.
[reply]
[d/l]

      I realize you were probably doing a quick port, but there are a few things I would like to point out.

      • You create an explicit variable with your for loop, but never use it.
      • There is a minor syntax error (for my $i in should just be for my $i).
      • You pass -9 to kill, but this should probably be 9 (the negative signal has a different meaning than it does to the shell command kill).

      Fixing these, I would probably write it as follows:

      my $child = fork;
if ($child) {
  # I am in the parent...wait then kill
  for (1..10) {
    sleep 1;
    print ".";
  }
  kill 9, $child;
}
else {
  # Run the background program
  exec '/path/to/program > /dev/null 2>&1';
}

      [download]

      Update: an alternative method that avoids explicit forking, and perhaps looks a bit more Perlish:

      my $pid = open my $cmdfh, "-|", '/path/to/program'
    or die "Cannot fork: $!\n";

for ( 1 .. 10 ) {
    print ".";
    sleep 1;
}

kill 9, $pid;

      [download]
[reply]
[d/l]
[select]
        Both versions work, but not the way I want... the status output (print ".") appears only after the background-program has been killed, which makes it pretty useless...
[reply]

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