Table 7.1. Escape Sequences

x = 1

x is the name of the variable, = is an assignment operator, and 1 is a numeric constant.

The following expression assigns the string "Hello" to the variable z:

z = "Hello"

z = "Hello" "World"

concatenates the two strings and assigns "HelloWorld" to the variable z.

The dollar sign ($) operator is used to reference fields. The following expression assigns the value of the first field of the current input record to the variable w:

w = $1

Table 7.2. Arithmetic Operators

Once a variable has been assigned a value, that value can be referenced using the name of the variable. The following expression adds 1 to the value of x and assigns it to the variable y:

y = x + 1

So, evaluate x, add 1 to it, and put the result into the variable y. The statement:

print y

prints the value of y. If the following sequence of statements appears in a script:

x = 1
y = x + 1
print y

then the value of y is 2.

We could reduce these three statements to two:

x = 1
print x + 1

Table 7.3. Assignment Operators

Look at the following example, which counts each blank line in a file.

# Count blank lines.
/^$/ { 
	print x += 1 
     }

Although we didn't initialize the value of x, we can safely assume that its value is 0 up until the first blank line is encountered. The expression "x += 1" is evaluated each time a blank line is matched and the value of x is incremented by 1. The print statement prints the value returned by the expression. Because we execute the print statement for every blank line, we get a running count of blank lines.

There are different ways to write expressions, some more terse than others. The expression "x += 1" is more concise than the following equivalent expression:

x = x + 1

But neither of these expressions is as terse as the following expression:

++x

++x	Increment x before returning value (prefix)
x++	Increment x after returning value (postfix)

For instance, if our example was written:

/^$/ { 
	print x++
     }

When the first blank line is matched, the expression returns the value "0"; the second blank line returns "1", and so on. If we put the increment operator before x, then the first time the expression is evaluated, it will return "1."

Let's implement that expression in our example. In addition, instead of printing a count each time a blank line is matched, we'll accumulate the count as the value of x and print only the total number of blank lines. The END pattern is the place to put the print that displays the value of x after the last input line is read.

# Count blank lines.
/^$/ { 
	++x
}
END {
	print x
}

Let's try it on the sample file that has three blank lines in it.

$ awk -f awkscr test
3

The script outputs the number of blank lines.