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printf

⊕ Usage

printf format [ string ... ]

⊕ Purpose

To produce output from shell scripts. Since printf's behavior is defined by the POSIX standard, scripts that use it can be more portable than those that use echo.

⊕ Major options

None.

⊕ Behavior

printf uses the format string to control the output. Plain characters in the string are printed. Escape sequences as described for echo are interpreted. Format specifiers consisting of % and a letter direct formatting of corresponding argument strings. See text for details.

⊕ printf escape sequences

Sequence	Description
\a	Alert character, usually the ASCII BEL character.
\b	Backspace.
\c	Suppress any final newline in the output.[*] Furthermore, any characters left in the argument, any following arguments, and any characters left in the format string are ignored (not printed).
\f	Formfeed.
\n	Newline.
\r	Carriage return.
\t	Horizontal tab.
\v	Vertical tab.
\\	A literal backslash character.
\ddd	Character represented as a 1- to 3-digit octal value. Valid only in the format string.
\0ddd	Character represented as a 1- to 3-digit octal value.

[*] Valid only in argument strings under control of the %b format specifier.

Confusion of Printf Escape Sequences

By default, escape sequences are treated specially only in the format string.

Escape sequences appearing in argument strings are not interpreted:

$ printf "a string, no processing: <%s>\n" "A\nB" a string, no processing: <A\nB>

But, when the %b format specifier is used, printf does interpret escape sequences in argument strings:

$ printf "a string, with processing: <%b>\n" "A\nB" a string, with processing: <A B>

\c and \0ddd are only valid for use with %b,

\ddd is only interpreted in the format string.

⊕ printf format specifiers

Item	Description
%b	The corresponding argument is treated as a string containing escape sequences to be processed.
%c	ASCII character. Print the first character of the corresponding argument.
%d, %i	Decimal integer.
%e	Floating-point format ([-]d.precisione[+-]dd).
%E	Floating-point format ([-]d.precisionE[+-]dd).
%f	Floating-point format ([-]ddd.precision).
%g	%e or %f conversion, whichever is shorter, with trailing zeros removed.
%G	%E or %f conversion, whichever is shorter, with trailing zeros removed.
%o	Unsigned octal value.
%s	String.
%u	Unsigned decimal value.
%x	Unsigned hexadecimal number. Use a-f for 10 to 15.
%X	Unsigned hexadecimal number. Use A-F for 10 to 15.
%%	Literal %.

floating-point formats, %e, %E, %f, %g, and %G, are optional in POSIX standard.

⊕ Format Expression

To specify the width and alignment of output fields.

three optional modifiers following the % and preceding the format specifier:

%flags width.precision format-specifier

the contents of the field are right-justified by default.

use flag '-' to change to left justification.

"%-20s"

Example: right-justify $ printf "\|%10s\|\n" hello \| hello\| Example: right-justify $ printf "\|%-10s\|\n" hello \|hello \|

⊕ Precision

The precision modifier is optional.

For decimal or floating-point values, it 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.

Meaning of precision

Conversion	Precision means
%d, %i, %o, %u, %x, %X	The minimum number of digits to print. When the value has fewer digits, it is padded with leading zeros. The default precision is 1.
%e, %E	The minimum number of digits to print. When the value has fewer digits, it is padded with zeros after the decimal point. The default precision is 6. A precision of 0 inhibits printing of the decimal point.
%f	The number of digits to the right of the decimal point.
%g, %G	The maximum number of significant digits.
%s	The maximum number of characters to print.

Examples of the precision in action

$ printf "%.5d\n" 15 00015 $ printf "%.10s\n" "a very long string" a very lon $ printf "%.2f\n" 123.4567 123.46

The C library printf( ) function allows you to specify the width and precision dynamically, via additional values in the argument list.

The POSIX standard doesn't supply this, instead recommending the use of shell variable values in the format string.

Example:

$ width=5 prec=6 myvar=42.123456 $ printf "\|%${width}.${prec}G\|\n" $myvar # POSIX \|42.1235\| $ printf "\|%.G\|\n" 5 6 $myvar # ksh93 and bash \|42.1235\|

⊕ Flags for printf

one or more flags may precede the field width and the precision.

Flags for printf

Character	Description
-	Left-justify the formatted value within the field.
space	Prefix positive values with a space and negative values with a minus.
+	Always prefix numeric values with a sign, even if the value is positive.
#	Use an alternate form: %o has a preceding 0; %x and %X are prefixed with 0x and 0X, respectively; %e, %E, and %f always have a decimal point in the result; and %g and %G do not have trailing zeros removed.
0	Pad output with zeros, not spaces. This happens only when the field width is wider than the converted result. In the C language, this flag applies to all output formats, even nonnumeric ones. For the printf command, it applies only to the numeric formats.

   Example: 
 
$ printf "|%-10s| |%10s|\n" hello world  Left-, right-justified strings  
|hello | | world|
 $ printf "|% d| |% d|\n" 15 -15   #Space flag 
| 15| |-15|
 $ printf "%+d %+d\n" 15 -15   #+ flag 
+15 -15
 $ printf "%x %#x\n" 15 15   '#' flag 
f 0xf
 $ printf "%05d\n" 15  # 0 flag 
00015

Speical Treatment of %b, %c, and %s conversion specifiers the corresponding arguments are treated as strings.

Otherwise, they're interpreted as C-language numeric constants
(leading 0 for octal, and leading 0x or 0X for hexadecimal).
Furthermore, if an argument's first character is a single or double quote,
the corresponding numeric value is the ASCII value of the string's second character:

$ printf "%s is %d\n" a "'a" a is 97

When there are more arguments than format specifiers,

the format specifiers are reused as needed. This is convenient when the argument list is of unknown length, such as from a wildcard expression.

If there are more specifiers left in the format string than arguments,

the missing values are treated as zero for numeric conversions and as the empty string for string conversions.
(This seems to be only marginally useful. It's much better to make sure that you supply the same number of arguments as the format string expects.)

If printf cannot perform a format conversion,

it returns a nonzero exit status.