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Modifiers for printf Formats

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 will 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:

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 @command{gawk} extension, intended for use in translating messages at runtime. See section Rearranging printf Arguments, which describes how and why to use positional specifiers. For now, we will not use them.

-

The minus sign, used before the width modifier (see further on in this table), 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 further on in this table), says to always supply a sign for numeric conversions, even if the data to format is positive. The `+' overrides the space modifier.

#

Use an "alternate 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', 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 that indicates that output should be padded with zeros instead of spaces. This applies even to non-numeric output formats. (d.c.) This flag only has an effect when the field width is wider than the value to print.

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:

%e, %E, %f

Number of digits to the right of the decimal point.

%g, %G

Maximum number of significant digits.

%d, %i, %o, %u, %x, %X

Minimum number of digits to print.

%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 (for example, "%*.*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 @command{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 `l', `L', and `h' modifiers in printf format strings. These are not valid in @command{awk}. Most @command{awk} implementations silently ignore these modifiers. If @option{--lint} is provided on the command line (see section Command-Line Options), @command{gawk} warns about their use. If @option{--posix} is supplied, their use is a fatal error.

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