awk 'BEGIN { printf ("Hello, world\n") }'

The main difference that you will notice at the outset is that, unlike print, printf does not automatically supply a newline. You must specify it explicitly as "\n".

The full syntax of the printf statement has two parts:

printf ( format-expression [, arguments] )

Table 7.6. Format Specifiers Used in printf

This example uses the printf statement to produce the output for rule 2 in the filesum program. It outputs a string and a decimal value found in two different fields:

printf("%d\t%s\n", $5, $9)

%-width.precision format-specifier

The width of the output field is a numeric value. When you specify a field width, the contents of the field will be right-justified by default. You must specify "-" to get left-justification. Thus, "%-20s" outputs a string left-justified in a field 20 characters wide. If the string is less than 20 characters, the field will be padded with whitespace to fill. In the following examples, a "|" is output to indicate the actual width of the field. The first example right-justifies the text:

printf("|%10s|\n", "hello")

It produces:

|     hello|

The next example left-justifies the text:

printf("|%-10s|\n", "hello")

It produces:

|hello     |

You can specify both the width and precision dynamically, via values in the printf or sprintf argument list. You do this by specifying asterisks, instead of literal values.

printf("%*.*g\n", 5, 3, myvar);

In this example, the width is 5, the precision is 3, and the value to print will come from myvar.

Using the full syntax of the format expression can solve the problem with filesum of getting fields and headings properly aligned. One reason we output the file size before the filename was that the fields had a greater chance of aligning themselves if they were output in that order. The solution that printf offers us is the ability to fix the width of output fields; therefore, each field begins in the same column.

Let's rearrange the output fields in the filesum report. We want a minimum field width so that the second field begins at the same position. You specify the field width place between the % and the conversion specification. "%-15s" specifies a minimum field width of 15 characters in which the value is left-justified. "%10d", without the hyphen, is right-justified, which is what we want for a decimal value.

printf("%-15s\t%10d\n", $9, $5)       # print filename and size

This will produce a report in which the data is aligned in columns and the numbers are right-justified. Look at how the printf statement is used in the END action:

printf("Total: %d bytes  (%d files)\n", sum, filenum)

The column header in the BEGIN rule is also changed appropriately. With the use of the printf statement, filesum now produces the following output:

$ filesum g*
FILE                 BYTES
g                       23
gawk                  2237
gawk.mail             1171
gawk.test               74
gawkro                 264
gfilesum               610
grades                  64
grades.awk             231
grepscript               6
Total: 4680 bytes  (9 files)