File: gawk.info,  Node: Boolean Ops,  Next: Conditional Exp,  Prev: Typing and Comparison,  Up: Truth Values and Conditions

6.3.3 Boolean Expressions
-------------------------

A "Boolean expression" is a combination of comparison expressions or
matching expressions, using the Boolean operators "or" ('||'), "and"
('&&'), and "not" ('!'), along with parentheses to control nesting.  The
truth value of the Boolean expression is computed by combining the truth
values of the component expressions.  Boolean expressions are also
referred to as "logical expressions".  The terms are equivalent.

   Boolean expressions can be used wherever comparison and matching
expressions can be used.  They can be used in 'if', 'while', 'do', and
'for' statements (*note Statements::).  They have numeric values (one if
true, zero if false) that come into play if the result of the Boolean
expression is stored in a variable or used in arithmetic.

   In addition, every Boolean expression is also a valid pattern, so you
can use one as a pattern to control the execution of rules.  The Boolean
operators are:

'BOOLEAN1 && BOOLEAN2'
     True if both BOOLEAN1 and BOOLEAN2 are true.  For example, the
     following statement prints the current input record if it contains
     both 'edu' and 'li':

          if ($0 ~ /edu/ && $0 ~ /li/) print

     The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is true.
     This can make a difference when BOOLEAN2 contains expressions that
     have side effects.  In the case of '$0 ~ /foo/ && ($2 == bar++)',
     the variable 'bar' is not incremented if there is no substring
     'foo' in the record.

'BOOLEAN1 || BOOLEAN2'
     True if at least one of BOOLEAN1 or BOOLEAN2 is true.  For example,
     the following statement prints all records in the input that
     contain _either_ 'edu' or 'li':

          if ($0 ~ /edu/ || $0 ~ /li/) print

     The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is false.
     This can make a difference when BOOLEAN2 contains expressions that
     have side effects.  (Thus, this test never really distinguishes
     records that contain both 'edu' and 'li'--as soon as 'edu' is
     matched, the full test succeeds.)

'! BOOLEAN'
     True if BOOLEAN is false.  For example, the following program
     prints 'no home!' in the unusual event that the 'HOME' environment
     variable is not defined:

          BEGIN { if (! ("HOME" in ENVIRON))
                      print "no home!" }

     (The 'in' operator is described in *note Reference to Elements::.)

   The '&&' and '||' operators are called "short-circuit" operators
because of the way they work.  Evaluation of the full expression is
"short-circuited" if the result can be determined partway through its
evaluation.

   Statements that end with '&&' or '||' can be continued simply by
putting a newline after them.  But you cannot put a newline in front of
either of these operators without using backslash continuation (*note
Statements/Lines::).

   The actual value of an expression using the '!' operator is either
one or zero, depending upon the truth value of the expression it is
applied to.  The '!' operator is often useful for changing the sense of
a flag variable from false to true and back again.  For example, the
following program is one way to print lines in between special
bracketing lines:

     $1 == "START"   { interested = ! interested; next }
     interested      { print }
     $1 == "END"     { interested = ! interested; next }

The variable 'interested', as with all 'awk' variables, starts out
initialized to zero, which is also false.  When a line is seen whose
first field is 'START', the value of 'interested' is toggled to true,
using '!'.  The next rule prints lines as long as 'interested' is true.
When a line is seen whose first field is 'END', 'interested' is toggled
back to false.(1)

   Most commonly, the '!' operator is used in the conditions of 'if' and
'while' statements, where it often makes more sense to phrase the logic
in the negative:

     if (! SOME CONDITION || SOME OTHER CONDITION) {
         ... DO WHATEVER PROCESSING ...
     }

     NOTE: The 'next' statement is discussed in *note Next Statement::.
     'next' tells 'awk' to skip the rest of the rules, get the next
     record, and start processing the rules over again at the top.  The
     reason it's there is to avoid printing the bracketing 'START' and
     'END' lines.

   ---------- Footnotes ----------

   (1) This program has a bug; it prints lines starting with 'END'.  How
would you fix it?