Logical Operators

To combine comparisons, use the and, or and not logical operators. In some scripts, you might see their cryptic cousins, &&, || and !:

Lower precedence	Higher precedence	Description
$a and $b	$a && $b	TRUE if $a AND $b are TRUE
$a or $b	$a || $b	TRUE if either $a OR $b are TRUE
not $a	!$b	TRUE if $a is FALSE

if ($a < 100 and $a > 0) { print "a is the right size\n"; } else { die "out of bounds error, operation aborted!"; } if ($a < 100 && $a > 0) { print "a is the right size\n"; } else { die "out of bounds error, operation aborted!"; } if ($a >= 100 or $a <= 0) { die "out of bounds error, operation aborted!"; } if ($a >= 100 || $a <= 0) { die "out of bounds error, operation aborted!"; }

To Reverse Truth, use not or !

  $ok = ($a < 100 and $a > 0);
  print "a is too small\n" if not $ok;

  # same as this:
  print "a is too small\n" unless $ok;

  # and this:
  print "a is too small\n" if !$ok;

and vs &&, or vs ||

&& has higher precedence than and. || has higher precedence than or. This is an issue in assignments:

Low precedence operation:

$ok = $a < 100 and $a > 0; # This doesn't mean: $ok = ($a < 100 and $a > 0); # but: ($ok = $a < 100) and $a > 0;

High precedence operation:

$ok = $a < 100 && $a > 0; # This does mean $ok = ($a < 100 && $a > 0);

When in doubt, use parentheses.

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