## 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 (see section Control Statements in Actions). They have numeric values (one if true, zero if false), which 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.

Here are descriptions of the three boolean operators, with examples.

`boolean1 && boolean2`
True if both boolean1 and boolean2 are true. For example, the following statement prints the current input record if it contains both `2400' and `foo'.
```if (\$0 ~ /2400/ && \$0 ~ /foo/) 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 `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 `2400' or `foo', or both.
```if (\$0 ~ /2400/ || \$0 ~ /foo/) print
```
The subexpression boolean2 is evaluated only if boolean1 is false. This can make a difference when boolean2 contains expressions that have side effects.
`! boolean`
True if boolean is false. For example, the following program prints all records in the input file `BBS-list' that do not contain the string `foo'.
```awk '{ if (! (\$0 ~ /foo/)) print }' BBS-list
```

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 part way through its evaluation.

You can continue a statement that uses `&&' or `||' simply by putting a newline after them. But you cannot put a newline in front of either of these operators without using backslash continuation (see section `awk` Statements Versus Lines).

The actual value of an expression using the `!' operator will be 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 }
interested == 1 { print }
\$1 == "END"     { interested = ! interested }
```

The variable `interested`, like 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.