System and a question of style

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
[download]

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;
}
[download]

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

Comment on System and a question of style Select or Download Code

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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 $rc\n"; } } run_command('vxdg deport oracle'); run_command('something else ...');` [download]	[reply] [d/l]
Re: Re: System and a question of style by submersible_toaster (Chaplain) on Feb 07, 2003 at 05:42 UTC
Good idea steves++ , I suppose it depends on one's perl version and personal preference, but being able to call system with list syntax (bypassing the shell hence no jiggery-pokery with escaping things) is IMHO so groovy it hurts doing it the old way!. I'd make a small modification to your subroutine. `sub run_command { # invoke system(@list) if more than one arg passed. ($#_) ? 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]
Re: Re: Re: System and a question of style by bsb (Priest) on Feb 07, 2003 at 06:42 UTC
Perhaps you should put $! in the die string too.	[reply]
Re: Re: Re: System and a question of style by bronto (Priest) on Feb 07, 2003 at 11:24 UTC
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	[reply] [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'.	[reply]
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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