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Formats

A format string contains any number of format units, separated by whitespace. A format unit contains up to three items: an iteration count, a byte count, and a format.

The iteration count is an optional positive integer, which defaults to one. Each format is applied iteration count times.

The byte count is an optional positive integer. If specified it defines the number of bytes to be interpreted by each iteration of the format.

If an iteration count and/or a byte count is specified, a single slash (‘/’) must be placed after the iteration count and/or before the byte count to disambiguate them. Any whitespace before or after the slash is ignored.

The format is required and must be surrounded by double quote (" ") marks (the quote mark is a special character in many shell programs, and may have to be escaped from the shell). It is interpreted as a fprintf-style format string (see fprintf(3)), with the following exceptions:

• An asterisk (*) may not be used as a field width or precision.
• A byte count or field precision is required for each ‘s’ conversion character (unlike the fprintf(3) default which prints the entire string if the precision is unspecified).
• The conversion characters ‘h’, ‘l’, ‘n’, ‘p’, and ‘q’ are not supported.
• The single character escape sequences described in the C standard are supported:

  NUL

  \0

  ⟨alert character⟩

  \a

  ⟨backspace⟩

  \b

  ⟨form-feed⟩

  \f

  ⟨newline⟩

  \n

  ⟨carriage return⟩

  \r

  ⟨tab⟩

  \t

  ⟨vertical tab⟩

  \v

hexdump also supports the following additional conversion strings:

_a[dox]

Display the input offset, cumulative across input files, of the next byte to be displayed. The appended characters d, o, and x specify the display base as decimal, octal or hexadecimal respectively.

_A[dox]

Identical to the _a conversion string except that it is only performed once, when all of the input data has been processed.

_c

Output characters in the default character set. Nonprinting characters are displayed in three character, zero-padded octal, except for those representable by standard escape notation (see above), which are displayed as two character strings.

_p

Output characters in the default character set. Nonprinting characters are displayed as a single dot ‘.’.

_u

Output US ASCII characters, with the exception that control characters are displayed using the following, lower-case, names. Other non-printable characters are displayed as hexadecimal strings.

000 nul  001 soh  002 stx  003 etx  004 eot  005 enq
006 ack  007 bel  008 bs   009 ht   00A lf   00B vt
00C ff   00D cr   00E so   00F si   010 dle  011 dc1
012 dc2  013 dc3  014 dc4  015 nak  016 syn  017 etb
018 can  019 em   01A sub  01B esc  01C fs   01D gs
01E rs   01F us   07F del

The default and supported byte counts for the conversion characters are as follows:

The amount of data interpreted by each format string is the sum of the data required by each format unit, which is the iteration count times the byte count, or the iteration count times the number of bytes required by the format if the byte count is not specified.

The input is manipulated in “blocks”, where a block is defined as the largest amount of data specified by any format string. Format strings interpreting less than an input block's worth of data, whose last format unit both interprets some number of bytes and does not have a specified iteration count, have the iteration count incremented until the entire input block has been processed or there is not enough data remaining in the block to satisfy the format string.

If, either as a result of user specification or hexdump modifying the iteration count as described above, an iteration count is greater than one, no trailing whitespace characters are output during the last iteration.

It is an error to specify a byte count as well as multiple conversion characters or strings unless all but one of the conversion characters or strings is _a or _A.

If, as a result of the specification of the -n option or end-of-file being reached, input data only partially satisfies a format string, the input block is zero-padded sufficiently to display all available data (i.e., any format units overlapping the end of data will display some number of the zero bytes).

Further output by such format strings is replaced by an equivalent number of spaces. An equivalent number of spaces is defined as the number of spaces output by an s conversion character with the same field width and precision as the original conversion character or conversion string but with any ‘+’, ‘ ’, ‘#’ conversion flag characters removed, and referencing a NULL string.

If no format strings are specified, the default display is equivalent to specifying the -x option.