7.9 Formatted Printing

Many of the scripts that we've written so far perform the data processing tasks just fine, but the output has not been formatted properly. That is because there is only so much you can do with the basic print statement. And since one of awk's most common functions is to produce reports, it is crucial that we be able to format our reports in an orderly fashion. The filesum program performs the arithmetic tasks well but the report lacks an orderly format.

Awk offers an alternative to the print statement, printf, which is borrowed from the C programming language. The printf statement can output a simple string just like the print statement.

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] )

The parentheses are optional. The first part is an expression that describes the format specifications; usually this is supplied as a string constant in quotes. The second part is an argument list, such as a list of variable names, that correspond to the format specifications. A format specification is preceded by a percent sign (%) and the specifier is one of the characters shown in Table 7.6. The two main format specifiers are s for strings and d for decimal integers.[11]

[11] The way printf does rounding is discussed in Appendix B.

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)

The value of $5 is to be output, followed by a tab (\t) and $9 and then a newline (\n).[12] For each format specification, you must supply a corresponding argument.

[12] Compare this statement with the print statement in the filesum program that prints the header line. The print statement automatically supplies a newline (the value of ORS); when using printf, you must supply the newline, it is never automatically provided for you.

This printf statement can be used to specify the width and alignment of output fields. A format expression can take three optional modifiers following "%" and preceding the format specifier:

%-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     |

The precision modifier, used for decimal or floating-point values, controls the number of digits that appear to the right of the decimal point. For string values, it controls the maximum number of characters from the string that will be printed. Note that the default precision for the output of numeric values is "%.6g".

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.

The default precision used by the print statement when outputting numbers can be changed by setting the system variable OFMT. For instance, if you are using awk to write reports that contain dollar values, you might prefer to change OFMT to "%.2f".

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)