gawk: Format Modifiers
5.5.3 Modifiers for 'printf' Formats
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A format specification can also include "modifiers" that can control how
much of the item's value is printed, as well as how much space it gets.
The modifiers come between the '%' and the format-control letter. We
use the bullet symbol "*" in the following examples to represent spaces
in the output. Here are the possible modifiers, in the order in which
they may appear:
'N$'
An integer constant followed by a '$' is a "positional specifier".
Normally, format specifications are applied to arguments in the
order given in the format string. With a positional specifier, the
format specification is applied to a specific argument, instead of
what would be the next argument in the list. Positional specifiers
begin counting with one. Thus:
printf "%s %s\n", "don't", "panic"
printf "%2$s %1$s\n", "panic", "don't"
prints the famous friendly message twice.
At first glance, this feature doesn't seem to be of much use. It
is in fact a 'gawk' extension, intended for use in translating
messages at runtime. 
Printf Ordering, which describes how
and why to use positional specifiers. For now, we ignore them.
'-' (Minus)
The minus sign, used before the width modifier (see later on in
this list), says to left-justify the argument within its specified
width. Normally, the argument is printed right-justified in the
specified width. Thus:
printf "%-4s", "foo"
prints 'foo*'.
SPACE
For numeric conversions, prefix positive values with a space and
negative values with a minus sign.
'+'
The plus sign, used before the width modifier (see later on in this
list), says to always supply a sign for numeric conversions, even
if the data to format is positive. The '+' overrides the space
modifier.
'#'
Use an "alternative form" for certain control letters. For '%o',
supply a leading zero. For '%x' and '%X', supply a leading '0x' or
'0X' for a nonzero result. For '%e', '%E', '%f', and '%F', the
result always contains a decimal point. For '%g' and '%G',
trailing zeros are not removed from the result.
'0'
A leading '0' (zero) acts as a flag indicating that output should
be padded with zeros instead of spaces. This applies only to the
numeric output formats. This flag only has an effect when the
field width is wider than the value to print.
'''
A single quote or apostrophe character is a POSIX extension to ISO
C. It indicates that the integer part of a floating-point value, or
the entire part of an integer decimal value, should have a
thousands-separator character in it. This only works in locales
that support such characters. For example:
$ cat thousands.awk Show source program
-| BEGIN { printf "%'d\n", 1234567 }
$ LC_ALL=C gawk -f thousands.awk
-| 1234567 Results in "C" locale
$ LC_ALL=en_US.UTF-8 gawk -f thousands.awk
-| 1,234,567 Results in US English UTF locale
For more information about locales and internationalization issues,
see Locales.
NOTE: The ''' flag is a nice feature, but its use complicates
things: it becomes difficult to use it in command-line
programs. For information on appropriate quoting tricks, see
Quoting.
WIDTH
This is a number specifying the desired minimum width of a field.
Inserting any number between the '%' sign and the format-control
character forces the field to expand to this width. The default
way to do this is to pad with spaces on the left. For example:
printf "%4s", "foo"
prints '*foo'.
The value of WIDTH is a minimum width, not a maximum. If the item
value requires more than WIDTH characters, it can be as wide as
necessary. Thus, the following:
printf "%4s", "foobar"
prints 'foobar'.
Preceding the WIDTH with a minus sign causes the output to be
padded with spaces on the right, instead of on the left.
'.PREC'
A period followed by an integer constant specifies the precision to
use when printing. The meaning of the precision varies by control
letter:
'%d', '%i', '%o', '%u', '%x', '%X'
Minimum number of digits to print.
'%e', '%E', '%f', '%F'
Number of digits to the right of the decimal point.
'%g', '%G'
Maximum number of significant digits.
'%s'
Maximum number of characters from the string that should
print.
Thus, the following:
printf "%.4s", "foobar"
prints 'foob'.
The C library 'printf''s dynamic WIDTH and PREC capability (e.g.,
'"%*.*s"') is supported. Instead of supplying explicit WIDTH and/or
PREC values in the format string, they are passed in the argument list.
For example:
w = 5
p = 3
s = "abcdefg"
printf "%*.*s\n", w, p, s
is exactly equivalent to:
s = "abcdefg"
printf "%5.3s\n", s
Both programs output '**abc'. Earlier versions of 'awk' did not support
this capability. If you must use such a version, you may simulate this
feature by using concatenation to build up the format string, like so:
w = 5
p = 3
s = "abcdefg"
printf "%" w "." p "s\n", s
This is not particularly easy to read, but it does work.
C programmers may be used to supplying additional modifiers ('h',
'j', 'l', 'L', 't', and 'z') in 'printf' format strings. These are not
valid in 'awk'. Most 'awk' implementations silently ignore them. If
'--lint' is provided on the command line (Options), 'gawk' warns
about their use. If '--posix' is supplied, their use is a fatal error.
Info Catalog
gawk: Control Letters
gawk: Printf
gawk: Printf Examples