5.5.2 Format-Control Letters

A format specifier starts with the character ‘%’ and ends with a format-control letter—it tells the printf statement how to output one item. The format-control letter specifies what kind of value to print. The rest of the format specifier is made up of optional modifiers that control how to print the value, such as the field width. Here is a list of the format-control letters:

%c

Print a number as an ASCII character; thus, ‘printf "%c", 65’ outputs the letter ‘A’. The output for a string value is the first character of the string.

NOTE: The POSIX standard says the first character of a string is printed. In locales with multibyte characters, gawk attempts to convert the leading bytes of the string into a valid wide character and then to print the multibyte encoding of that character. Similarly, when printing a numeric value, gawk allows the value to be within the numeric range of values that can be held in a wide character.

Other awk versions generally restrict themselves to printing the first byte of a string or to numeric values within the range of a single byte (0–255).

%d, %i

Print a decimal integer. The two control letters are equivalent. (The ‘%i’ specification is for compatibility with ISO C.)

%e, %E

Print a number in scientific (exponential) notation; for example:

printf "%4.3e\n", 1950

prints ‘1.950e+03’, with a total of four significant figures, three of which follow the decimal point. (The ‘4.3’ represents two modifiers, discussed in the next subsection.) ‘%E’ uses ‘E’ instead of ‘e’ in the output.

%f

Print a number in floating-point notation. For example:

printf "%4.3f", 1950

prints ‘1950.000’, with a total of four significant figures, three of which follow the decimal point. (The ‘4.3’ represents two modifiers, discussed in the next subsection.)

On systems supporting IEEE 754 floating point format, values representing negative infinity are formatted as ‘-inf’ or ‘-infinity’, and positive infinity as ‘inf’ and ‘infinity’. The special “not a number” value formats as ‘-nan’ or ‘nan’.

%F

Like ‘%f’ but the infinity and “not a number” values are spelled using uppercase letters.

The ‘%F’ format is a POSIX extension to ISO C; not all systems support it. On those that don’t, gawk uses ‘%f’ instead.

%g, %G

Print a number in either scientific notation or in floating-point notation, whichever uses fewer characters; if the result is printed in scientific notation, ‘%G’ uses ‘E’ instead of ‘e’.

%o

Print an unsigned octal integer (see section Octal and Hexadecimal Numbers).

%s

Print a string.

%u

Print an unsigned decimal integer. (This format is of marginal use, because all numbers in awk are floating-point; it is provided primarily for compatibility with C.)

%x, %X

Print an unsigned hexadecimal integer; ‘%X’ uses the letters ‘A’ through ‘F’ instead of ‘a’ through ‘f’ (see section Octal and Hexadecimal Numbers).

%%

Print a single ‘%’. This does not consume an argument and it ignores any modifiers.

NOTE: When using the integer format-control letters for values that are outside the range of the widest C integer type, gawk switches to the ‘%g’ format specifier. If ‘--lint’ is provided on the command line (see section Command-Line Options), gawk warns about this. Other versions of awk may print invalid values or do something else entirely. (d.c.)