6.11. Comparison Expressions

Comparison expressions match lines where the action is performed only if a certain condition is true. These expressions use relational operators and are used to compare numbers or strings.

6.11.1 Relational and Equality Operators

Table 6.8 provides a list of the relational operators. The value of the expression is 1 if the expression evaluates true, and 0 if false.

Example 6.54.

(The Database)

% cat employees

    Tom Jones       4423      5/12/66                543354

    Mary Adams      5346      11/4/63                28765

    Sally Chang     1654      7/22/54                650000

    Billy Black     1683      9/23/44                336500



(The Command Line)

1   % awk '$3 == 5346' employees

    Mary Adams        5346    11/4/63    28765



2   % awk '$3 > 5000{print $1} ' employees

    Mary



3   % awk '$2 ~ /Adam/ ' employees

    Mary Adams    5346    11/4/63       28765



4   % awk '$2 !~ /Adam/ ' employees

    Tom Jones         4423    5/12/66    543354

    Sally Chang    1654    7/22/54    650000

    Billy Black    1683    9/23/44    336500

Table 6.8. Relational Operators

Operator	Meaning	Example
<	Less than	x < y
<=	Less than or equal to	x <= y
==	Equal to	x == y
!=	Not equal to	x != y
>=	Greater than or equal to	x >= y
>	Greater than	x > y
~	Matched by regular expression	x ~ /y/
!~	Not matched by regular expression	x !~ /y/

EXPLANATION

6.11.2 Conditional Expressions

A conditional expression uses two symbols, the question mark and the colon, to evaluate expressions. It is really just a short way to achieve the same result as doing an if/else statement. The general format is as follows:

FORMAT

conditional expression1 ? expression2 : expression3

This produces the same result as the if/else shown here. (A complete discussion of the if/else construct is given later.)

{

if (expression1)

       expression2

else

       expression3

}

Example 6.55.

% awk '{max=($1 > $2) ? $1 : $2; print max}' filename

EXPLANATION

If the first field is greater than the second field, the value of the expression after the question mark is assigned to max; otherwise the value of the expression after the colon is assigned to max. This is comparable to

if ($1 > $2 )

        max=$1

else

        max=$2

6.11.3 Computation

Computation can be performed within patterns. Awk (all versions) performs all arithmetic in floating point. The arithmetic operators are provided in Table 6.9.

Example 6.56.

% awk '$3 * $4 > 500' filename

Table 6.9. Arithmetic Operators

Operator	Meaning	Example
+	Add	x + y
–	Subtract	x - y
*	Multiply	x * y
/	Divide	x / y
%	Modulus	x % y
^	Exponentiation	x ^ y

EXPLANATION

Awk will multiply the third field ($3) by the fourth field ($4), and if the result is greater than 500, it will display those lines. (Filename is assumed to be a file containing the input.)

6.11.4 Logical Operators and Compound Patterns

Logical operators test whether an expression or pattern is true or false. With the &&, logical AND operator, if both expressions are true, the entire expression is true. If one expression is false, the whole expression is false. With ||, the logical OR operator, only one expression or pattern must evaluate to true to make the whole expression true. If both expressions are false, the entire expression is false.

Compound patterns are expressions that combine patterns with logical operators (see Table 6.10). An expression is evaluated from left to right.

Example 6.57.

% awk '$2 > 5 && $2 <= 15' filename

Table 6.10. Logical Operators

Operator	Meaning	Example
&&	Logical AND	a && b
||	Logical OR	a || b
!	NOT	! a

EXPLANATION

Awk will display those lines that match both conditions; that is, where the second field ($2) is greater than 5 AND the second field ($2) is also less than or equal to 15. With the && operator, both conditions must be true. (Filename is assumed to be a file containing the input.)

Example 6.58.

% awk '$3 == 100 || $4 > 50' filename

EXPLANATION

Awk will display those lines that match one of the conditions; that is, where the third field is equal to 100 OR the fourth field is greater than 50. With the || operator, only one of the conditions must be true. (Filename is assumed to be a file containing the input.)

Example 6.59.

% awk '!($2 < 100 && $3 < 20)' filename

EXPLANATION

If both conditions are true, awk will negate the expression and display those lines—so the lines displayed will have one or both conditions false. The unary ! operator negates the result of the condition so that if the expression yields a true condition, the NOT will make it false, and vice versa. (Filename is assumed to be a file containing the input.)

6.11.5 Range Patterns

Range patterns match from the first occurrence of one pattern to the first occurrence of the second pattern, then match for the next occurrence of the first pattern to the next occurrence of the second pattern, and so on. If the first pattern is matched and the second pattern is not found, awk will display all lines to the end of the file.

Example 6.60.

% awk '/Tom/,/Suzanne/' filename

EXPLANATION

Awk will display all lines, inclusive, that range between the first occurrence of Tom and the first occurrence of Suzanne. If Suzanne is not found, awk will continue processing lines until the end of the file. If, after the range between Tom and Suzanne is printed, Tom appears again, awk will start displaying lines until another Suzanne is found or the file ends.

6.11.6 A Data Validation Program

Using the awk commands discussed so far, the password-checking program from the book The AWK Programming Language^[3] illustrates how the data in a file can be validated.

^[3] Aho, Weinberger, and Kernighan, The Awk Programming Language (Boston: Addison-Wesley, 1988).

Example 6.61.

(The Password Database)

1 % cat /etc/passwd

tooth:pwHfudo.eC9sM:476:40:Contract Admin.:/home/rickenbacker/tooth:/bin/csh

lisam:9JY7OuS2f3lHY:4467:40:Lisa M. Spencer:/home/fortune1/lisam:/bin/csh

goode:v7Ww.nWJCeSIQ:32555:60:Goodwill Guest User:/usr/goodwill:/bin/csh

bonzo:eTZbu6M2jM7VA:5101:911: SSTOOL Log account :/home/sun4/bonzo:/bin/csh

info:mKZsrioPtW9hA:611:41:Terri Stern:/home/chewie/info:/bin/csh

cnc:IN1IVqVj1bVv2:10209:41:Charles Carnell:/home/christine/cnc:/bin/csh

bee:*:347:40:Contract Temp.:/home/chanel5/bee:/bin/csh

friedman:oyuIiKoFTV0TE:3561:50:Jay Friedman:/home/ibanez/friedman:/bin/csh

chambers:Rw7R1k77yUY4.:592:40:Carol Chambers:/usr/callisto2/chambers:/bin/csh

gregc:nkLulOg:7777:30:Greg Champlin FE Chicago

ramona:gbDQLdDBeRc46:16660:68:RamonaLeininge MWA CustomerService Rep:/home/forsh:



(The Awk Commands)

2    % cat /etc/passwd | awk –F: '\

3    NF != 7{\

4    printf("line %d, does not have 7 fields: %s\n",NR,$0)} \

5    $1 !~ /[A–Za–z0–9]/{printf("line %d, nonalphanumeric user id: %s\n",NR,$0)} \

6    $2 == "*" {printf("line %d, no password: %s\n",NR,$0)} '



(The Output)

     line 7, no password: bee:*:347:40:Contract Temp.:/home/chanel5/bee:/bin/csh

     line 10, does not have 7 fields: gregc:nk2EYi7kLulOg:7777:30:Greg Champlin

     FE Chicago

     line 11, does not have 7 fields: ramona:gbDQLdDBeRc46:16660:68:Ramona

     Leininger MWA Customer Service Rep:/home/forsh:

EXPLANATION