The printf command, available in bash since version 2.02, has two parts (beyond the command name): a format string and a variable number of arguments:

printfformat-string [arguments]

format-string describes the format specifications; this is best supplied as a string constant in quotes. arguments is a list, such as a list of strings or variable values that correspond to the format specifications.

The format is reused as necessary to use up all of the arguments. If the format requires more arguments than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied.

A format specification is preceded by a percent sign (%), and the specifier is one of the characters described below. Two of the main format specifiers are %s for strings and %d for decimal integers (see Table A-13).

The printf command 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:

%flags 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 are right-justified by default. You must specify a flag of -to get left-justification (the rest of the flags are shown in the table). Thus, %-20s outputs a left-justified string in a field 20-characters wide. If the string is less than 20 characters, the field is padded with whitespace to fill. In the following examples, we put our format specifier between a pair of | in our format string so you can see the width of the field in the output. The first example right-justifies the text:

printf "|%10s|
" hello@

It produces:

|        hello|

The next example left-justifies the text:

printf "|%-10s|
" hello

It produces:

|hello        |

The precision modifier, used for decimal or floating-point values, controls the number of digits that appear in the result. For string values, it controls the maximum number of characters from the string that will be printed.

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

$ myvar=42.123456
$ mysig=6
$ printf "|%*.*G|
" 5 $mysig $myvar
|42.1235|

In this example, the width is 5, the precision is 6, and the value to print comes from the value of $myvar. The precision is optional and its exact meaning varies by control letter, as shown in Table A-14.

%b and %q are additions to bash (and other POSIX compliant shells) which provide useful features at the expense of nonportability to versions of the printf command found in some other shells and in other places in Unix. Here are two examples to make their functions a little clearer:

%q shell quotes:

$ printf "%q
" "greetings to the world"
greetings to the world

%b echo-style escapes:

$ printf "%s
" 'hello
world'
hello
world
$ printf "%b
" 'hello
world'
hello
world

Table A-15 shows the escape sequences that will be translated in a string printed with the %b format.

Finally, one or more flags may precede the field width and the precision in a printf format specifier. We’ve already seen the -flag for left-justification. The rest of the flags are shown in Table A-16.

PI=3.141592653589

$ printf "%b" "aRing terminal bell, then tab	 then newline
Then line 2.
"
Ring terminal bell, then tab → then newline
Then line 2.