D.Millin has asked for the wisdom of the Perl Monks concerning the following question:

Hi Everyone,

I haven't much experience writting Perl code on Unix, as the company where I work as a Unix administrator only uses the Korn shell, Awk and Sed for everything. For my current project, I have to use Perl :-).As I have never seen or used perl in a system admin role on Unix, I haven't developed my own style. I would value the monks advise on the user of system() and programming style.

In my scripts I need to deport and import volume manger disk groups in order to take a point-in-time copy. Each command must be check to ensure that I get clean copy of the disk group. If I were using the Korn Shell, when I need to check the return code, the code would look something like: 

vxdg deport oracle
if [ $? ]
then
  echo "Could not deport oracle"
  exit 1
fi

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From digging through the Camel book and perldocs, I read that when executing a command using the system function, $? needs to be shifted by eight bytes to get the proper return code. From searching the net, I have seen this implemented in a variety different way, such as: 

system ("vxdg deport oracle");
if ($?){
  $rc = $_ >> 8;
  print "Could not deport oracle";
  exit 1;
}

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What I am unsure about is what is then best/efficient way of implementing this. Is it better to carry out the $rc = $_ >> 8; operation after every system call and then check the return code, or would it be better to encase system within the if statement?, and only do the $rc = $_ >> 8; where I absolutely need to use the proper return code.

Thanks,
Darren

Replies are listed 'Best First'.
Re: System and a question of style
by steves (Curate) on Feb 07, 2003 at 03:31 UTC

    Why don't you write a sub that does everything you want around each system call? That way you can change your mind later; add logging in one place, etc. 

    sub run_command
{
    my $command = shift;

    system($command);
    if ($?)
    {
        my $rc = $? >> 8;
        die "Failed to run '$command': exit code $rc\n";
    }
}

run_command('vxdg deport oracle');
run_command('something else ...');

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[reply]
[d/l]

      Good idea steves++ , I suppose it depends on one's perl version and personal preference, but being able to call system with list syntax (bypassing the shell hence no jiggery-pokery with escaping things) is IMHO so groovy it hurts doing it the old way!. I'd make a small modification to your subroutine.

      sub run_command
  {
      # invoke system(@list) if more than one arg passed.
      ($#_) ? system(@_) : system($_[0]);
      
      if ($?)
      {
          my $rc = $? >> 8;
          die "Failed to run '$command': exit code $rc\n";
      }
  }
  # Easy arg passing with a list;
  run_command('blah' , '-v' , '-q');
  # Pass as scalar for shell interpretation of metachars
  run_command('tar cvf /dev/null /tmp/*.sock');

      [download]

      warning - the args to run_command are untested but no harmful

      I can't believe it's not psellchecked
[reply]
[d/l]
        Perhaps you should put $! in the die string too.
[reply]

        Why don't force the list syntax? For example, doing a system(@_,"")?

        --bronto

        The very nature of Perl to be like natural language--inconsistant and full of dwim and special cases--makes it impossible to know it all without simply memorizing the documentation (which is not complete or totally correct anyway).
        --John M. Dlugosz
[reply]
[d/l]
Re: System and a question of style
by submersible_toaster (Chaplain) on Feb 07, 2003 at 01:18 UTC

    For starters I hope you meant $? and not $_ >> 8 in your second code snippet. You could wrap that bit shift any way you like such as

    $rc = sub { $? >> 8 };

# one way
system(@mysyscall) and die "@mysyscall returned ", &$rc;
# or
if (system(@mysyscall)) { die "@mysyscall returned ", &$rc };

    [download]
    But of course TIMTOWTDI!

    Update:BTW: Welcome to the Monastery.
    I can't believe it's not psellchecked
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[d/l]
Re: System and a question of style
by bsb (Priest) on Feb 07, 2003 at 06:43 UTC
    If most of the time you are just checking for success and only occasionally need the bit-shifted return code then I'd only do it in those cases.

    The rest of the time just use the normal 'system'.

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Re: System and a question of style
by D.Millin (Beadle) on Feb 07, 2003 at 09:59 UTC

    Thanks to everyone for replying to question, you given me plenty of food for thought.

    Thanks again,
    Darren
[reply]

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