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printf – Format And Print Data

Unlike the other commands in this chapter, the printf command is not used for pipe- lines (it does not accept standard input) nor does it find frequent application directly on the command line (it’s mostly used in scripts). So why is it important? Because it is so widely used.

printf (from the phrase “print formatted”) was originally developed for the C pro- gramming language and has been implemented in many programming languages includ- ing the shell. In fact, in bash, printf is a builtin.

printf works like this:

printf “format” arguments

The command is given a string containing a format description which is then applied to a list of arguments. The formatted result is sent to standard output. Here is a trivial exam- ple:

[me@linuxbox ~]$ printf "I formatted the string: %s\n" foo

I formatted the string: foo

[me@linuxbox ~]$ printf "I formatted the string: %s\n" foo

I formatted the string: foo

The format string may contain literal text (like “I formatted the string:”), escape se- quences (such as \n, a newline character), and sequences beginning with the % character, which are called conversion specifications. In the example above, the conversion specifi- cation %s is used to format the string “foo” and place it in the command’s output. Here it is again:

[me@linuxbox ~]$ printf "I formatted '%s' as a string.\n" foo

I formatted 'foo' as a string.

[me@linuxbox ~]$ printf "I formatted '%s' as a string.\n" foo

I formatted 'foo' as a string.

As we can see, the %s conversion specification is replaced by the string “foo” in the com- mand’s output. The s conversion is used to format string data. There are other specifiers for other kinds of data. This table lists the commonly used data types:

Table 21-4: Common printf Data Type Specifiers

Specifier Description

Specifier Description

d Format a number as a signed decimal integer.

f Format and output a floating point number.

o Format an integer as an octal number.

s Format a string.

x Format an integer as a hexadecimal number using lowercase a-f where needed.

X Same as x but use uppercase letters.

% Print a literal % symbol (i.e., specify “%%”)

We’ll demonstrate the effect each of the conversion specifiers on the string “380”:

[me@linuxbox ~]$ printf "%d, %f, %o, %s, %x, %X\n" 380 380 380 380

380 380

380, 380.000000, 574, 380, 17c, 17C

[me@linuxbox ~]$ printf "%d, %f, %o, %s, %x, %X\n" 380 380 380 380

380 380

380, 380.000000, 574, 380, 17c, 17C

Since we specified six conversion specifiers, we must also supply six arguments for

printf to process. The six results show the effect of each specifier.

Several optional components may be added to the conversion specifier to adjust its out- put. A complete conversion specification may consist of the following:

%[flags][width][.precision]conversion_specification

Multiple optional components, when used, must appear in the order specified above to be properly interpreted. Here is a description of each:

Table 21-5: printf Conversion Specification Components

Component Description

Component Description

flags There are five different flags:

# – Use the “alternate format” for output. This varies by data type. For o (octal number) conversion, the output is prefixed with

0. For x and X (hexadecimal number) conversions, the output is prefixed with 0x or 0X respectively.

0–(zero) Pad the output with zeros. This means that the field will be filled with leading zeros, as in “000380”.

- – (dash) Left-align the output. By default, printf right-aligns output.

‘ ’ – (space) Produce a leading space for positive numbers.

+ – (plus sign) Sign positive numbers. By default, printf only

signs negative numbers.

width A number specifying the minimum field width.

.precision For floating point numbers, specify the number of digits of precision to be output after the decimal point. For string conversion, precision specifies the number of characters to output.

Here are some examples of different formats in action:

Table 21-6: print Conversion Specification Examples

Again, printf is used mostly in scripts where it is employed to format tabular data, rather than on the command line directly. But we can still show how it can be used to solve various formatting problems. First, let’s output some fields separated by tab charac- ters:

[me@linuxbox ~]$ printf "%s\t%s\t%s\n" str1 str2 str3

str1 str2 str3

[me@linuxbox ~]$ printf "%s\t%s\t%s\n" str1 str2 str3

str1 str2 str3

By inserting \t (the escape sequence for a tab), we achieve the desired effect. Next, some numbers with neat formatting:

[me@linuxbox ~]$ printf "Line: %05d %15.3f Result: %+15d\n" 1071 3.14156295 32589

Line: 01071 3.142 Result: +32589

[me@linuxbox ~]$ printf "Line: %05d %15.3f Result: %+15d\n" 1071 3.14156295 32589

Line: 01071 3.142 Result: +32589

This shows the effect of minimum field width on the spacing of the fields. Or how about formatting a tiny web page:

[me@linuxbox ~]$ printf "<html>\n\t<head>\n\t\t<title>%s</title>\n

\t</head>\n\t<body>\n\t\t<p>%s</p>\n\t</body>\n</html>\n" "Page Tit le" "Page Content"

<html>

<head>

<title>Page Title</title>

</head>

<body>

<p>Page Content</p>

</body>

</html>

[me@linuxbox ~]$ printf "<html>\n\t<head>\n\t\t<title>%s</title>\n

\t</head>\n\t<body>\n\t\t<p>%s</p>\n\t</body>\n</html>\n" "Page Tit le" "Page Content"

<html>

<head>

<title>Page Title</title>

</head>

<body>

<p>Page Content</p>

</body>

</html>

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