shell 6.doc

Chapter 6. The awk Utility: awk Programming Constructs ?6.1 Comparison Expressions ?6.2 Review ?UNIX TOOLS LAB EXERCISE ? 6.1 Comparison Expressions Comparison expressions match lines where if the condition is true, the action is performed. 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. 比较表达式匹配行，在如果条件下为真的时候，进行action。这些表达式使用操作关系是使用比较字符串的数量。表格6.1提供一个表格关系操作符。 6.1.1 Relational Operators Table 6.1. 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/ Example 6.1 (The Database) % cat employee 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 % nawk $3 == 5346 employees Mary Adams 5346 11/4/63 28765 2 % nawk $3 5000{print $1} employees Mary 3 % nawk $2 ~ /Adam/ employees Mary Adams 5346 11/4/63 28765 4 % nawk $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 If the third field is equal to 5346, the condition is true and awk will perform the default action—print the line. When an if condition is implied, it is a conditional pattern test. If the third field is greater than 5000, awk prints the first field. If the second field matches the regular expression Adam, the record is printed. If the second field does not match the regular expression Adam, the record is printed. If an expression is a numeric value and is being compared to a string val