Modifiers for printf Formats

A format specification can also include modifiers that can control how much of the item's value is printed and how much space it gets. The modifiers come between the `%' and the format-control letter. In the examples below, we use the bullet symbol "*" to represent spaces in the output. Here are the possible modifiers, in the order in which they may appear:

-

The minus sign, used before the width modifier (see below), 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 below), says to always supply a sign for numeric conversions, even if the data to be formatted 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 non-zero result. For `%e', `%E', and `%f', the result will always contain 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 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 be printed.

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 be expanded 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,

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

This is a number that specifies the precision to use when printing. For the `e', `E', and `f' formats, this specifies the number of digits you want printed to the right of the decimal point. For the `g', and `G' formats, it specifies the maximum number of significant digits. For the `d', `o', `i', `u', `x', and `X' formats, it specifies the minimum number of digits to print. For a string, it specifies the maximum number of characters from the string that should be printed. Thus,

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, you pass them 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 `l' and `h' flags in printf format strings. These are not valid in awk. Most awk implementations silently ignore these flags. If `--lint' is provided on the command line (see section Command Line Options), gawk will warn about their use. If `--posix' is supplied, their use is a fatal error.

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