6.3.2.2 Comparison Operators

Comparison expressions compare strings or numbers for relationships such as equality. They are written using relational operators, which are a superset of those in C. Table 6.3 describes them.

Comparison expressions have the value one if true and zero if false. When comparing operands of mixed types, numeric operands are converted to strings using the value of CONVFMT (see section Conversion of Strings and Numbers).

Strings are compared by comparing the first character of each, then the second character of each, and so on. Thus, "10" is less than "9". If there are two strings where one is a prefix of the other, the shorter string is less than the longer one. Thus, "abc" is less than "abcd".

It is very easy to accidentally mistype the ‘==’ operator and leave off one of the ‘=’ characters. The result is still valid awk code, but the program does not do what is intended:

if (a = b)   # oops! should be a == b
   …
else
   …

Unless b happens to be zero or the null string, the if part of the test always succeeds. Because the operators are so similar, this kind of error is very difficult to spot when scanning the source code.

The following list of expressions illustrates the kinds of comparisons awk performs, as well as what the result of each comparison is:

1.5 <= 2.0

Numeric comparison (true)

"abc" >= "xyz"

String comparison (false)

1.5 != " +2"

String comparison (true)

"1e2" < "3"

String comparison (true)

a = 2; b = "2"

a == b

String comparison (true)

a = 2; b = " +2"

a == b

String comparison (false)

In this example:

$ echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }'
-| false

the result is ‘false’ because both $1 and $2 are user input. They are numeric strings—therefore both have the strnum attribute, dictating a numeric comparison. The purpose of the comparison rules and the use of numeric strings is to attempt to produce the behavior that is “least surprising,” while still “doing the right thing.”

String comparisons and regular expression comparisons are very different. For example:

x == "foo"

has the value one, or is true if the variable x is precisely ‘foo’. By contrast:

x ~ /foo/

has the value one if x contains ‘foo’, such as "Oh, what a fool am I!".

The righthand operand of the ‘~’ and ‘!~’ operators may be either a regexp constant (/…/) or an ordinary expression. In the latter case, the value of the expression as a string is used as a dynamic regexp (see section How to Use Regular Expressions; also see section Using Dynamic Regexps).

A constant regular expression in slashes by itself is also an expression. /regexp/ is an abbreviation for the following comparison expression:

$0 ~ /regexp/

One special place where /foo/ is not an abbreviation for ‘$0 ~ /foo/’ is when it is the righthand operand of ‘~’ or ‘!~’. See section Using Regular Expression Constants, where this is discussed in more detail.