Shell Programming and Scripting

Can someone explain the difference between backticks and system when
evaluated in these if statements:


sub getDate {
print "start date\n";
if ( system("/bin/date") ) {
print "can't get date\n";
exit(2);
}
print "finish date\n";
}

Returns the following:

start date
Thu Jul 28 12:13:59 EST 2005
finish date



While this:

sub getDate {
print "start date\n";
if ( `/bin/date` ) {
print "can't get date\n";
exit(2);
}
print "finish date\n";
}

Returns this:

start date
can't get date


Also - I'm not sure I understand the logic of the top if statement. If system() runs OK shouldn't the next two lines of the if statement then run i.e. the print and the exit?

In the first example: system('/bin/date');

prints the date to STDOUT and returns the exit status of /bin/date, in this case '0'. So the following if condition fails. You have already printed out the date simply by calling system('/bin/date').

In the second example `/bin/date' returns the string that you get by running the command but doesnt print anything to STDOUT. Since any non null string (except for the string that contains just '0') evaluates to true in perl your if clause prints 'Cant get date'.

At a rough guess its the second way you want to use but like this:

Code:

sub getDate
{
  my $date;
  print "start date\n";
  $date=`/bin/date`;
  if ( !$date )
  {
    print "can't get date\n";
    exit(2);
  }
  print $date;
  print "finish date\n";
}

Thanks for explaining that. Makes sense now.

The date thing was just an example. The actual code is from this tape changer script and I couldn't understand the logic if the chio command ran OK why the print and exit didn't run after it.

Code:

if ( system("/bin/chio -f $changerDevice move slot ".($tape-1)." drive 0
") ) {
    print "$progname: cannot '/bin/chio -f $changerDevice move' tape
 $tape into drive 0\n";
    exit(2);
}
print LOG &do_time(), ": leave: Load\n";

Just a note, when using system in perl, it returns the exit code multiplied by 256. So, if a command returns 1, system("command") will return 256. So, to get the real return value, divide by 256.

Yes, as tjlist said, the percecitonal way really should be

if ( ( system("/bin/chio -f $changerDevice move slot ".($tape-1)." drive 0
") ) / 256 ) {
print "$progname: cannot '/bin/chio -f $changerDevice move' tape
$tape into drive 0\n";
exit(2);
}
print LOG &do_time(), ": leave: Load\n";

And in the first example, the 2nd example would be

sub getDate {
print "start date\n";
my $buff = '';
if ( ($buff = `/bin/date`) eq ''){
print "can't get date\n";
exit(2);
}
print "finish date\n";
}

The OP is only checking if the return value from the function or call is not 0. So really the division by 256 is superfluous. Unless he gets return 0 (in which case the division will also be 0), anything else is an error.