6.1. Comparison Expressions

Comparison expressions match lines where the action is performed if the condition is true. These expressions use relational operators and are used to compare numbers or strings. Table 6.1 provides a list of the relational operators. The value of the expression is 1 if the expression evaluates true, and 0 if false.

6.1.1. Relational operators

Example 6.1.

(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
					

Explanation

6.1.2. 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:

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 6.2.

% 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

6.1.3. Computation

Computation can be performed within patterns. Awk performs all arithmetic in floating point. The arithmetic operators are provided in Table 6.2.

Example 6.3.

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

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.)

6.1.4. Compound Patterns

Compound patterns are expressions that combine patterns with logical operators (see Table 6.3). An expression is evaluated from left to right.

Example 6.4.

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

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 6.5.

% 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 6.6.

%  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.)

6.1.5. 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, etc. 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 6.7.

% 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 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.

6.1.6. A Data Validation Program

Using the awk commands discussed so far, the password-checking program from the book The AWK Programming Language ^[1] illustrates how the data in a file can be validated.

^[1] Aho, Weinberger, and Kernighan, The Awk Programming Language. Addison Wesley, 1988.

Example 6.8.

(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
",NR,$0)} 
5  $1 !~ /[A–Za–z0–9]/{printf("line %d, nonalphanumeric user id: %s
",NR,$0)} 
6  $2 == "*" {printf("line %d, no password: %s
",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