Unix: Using the xxd Command on a ppm File

The three parts are the offset from the start of the input, in hex; the data in hex; and the data in characters, if they're printable. 16 bytes to a line, since it's a nice round number and fits nicely on a screen.

A display like that is very often used in cases where both the binary and the textual data may be important. E.g. tcpdump -X uses a similar one.

On your first line:

00000000: 5036 0a31 3131 2031 3332 0a32 3535 0a55  P6.111 132.255.U

The offset 00000000 is zero, since this is the beginning; 50, 36, and 0a are the character codes for P, 6, and the newline, and so on. Unprintable characters are printed as dots on the right hand column.

Note that in some variants, the bytes in the middle section may be "inverted" within each group. This happens if the program interprets the input as little-endian numbers, and displays them as such, instead of as individual bytes concatenated.

xxd -e does exactly this. Consider:

$ printf  "ABCD" | xxd -e -g2 
00000000: 4241 4443                                ABCD

41 for A, and 42 for B appear in an inverted order, as well as the two others. od from GNU coreutils also inverts the bytes within the groups. When in doubt, double-check, or aim to get a one-by-one grouping. (compare e.g. od -x and od -tx1 in this.)

Of course, the meaning of the data itself can be found out from the documentation for the ppm format. My system has the man page ppm(5). (the online version on die.net has broken formatting.)

From that first line we can tell it's an RGB picture of size 111x132 pixels with 8-bits per sample.

It's a hex dump, in a fairly common hexdump format, with each line of output containing the byte offset (in hexadecimal*), hex digits for 16 bytes, and the ASCII representation of those same bytes.

* Note how the byte offset increments by 0x10 hex (16 decimal) for each line.

From man xxd:

NAME

   xxd - make a hexdump or do the reverse.

SYNOPSIS

   xxd -h[elp]
   xxd [options] [infile [outfile]]
   xxd -r[evert] [options] [infile [outfile]]

DESCRIPTION

xxd creates a hex dump of a given file or standard input. It can also convert a hex dump back to its original binary form. Like uuencode(1) and uudecode(1) it allows the transmission of binary data in a 'mail-safe' ASCII representation, but has the advantage of decoding to standard output. Moreover, it can be used to perform binary file patching. [...]

hex dumps are extremely useful when you need to know exactly which bytes are in a file at some exact location. They can help to diagnose problems where, for example, there are unexpected bytes (e.g. NULs (0x00) or carriage-returns (0x0D)) in the file and, as @ilkkachu mentions in his answer if the binary file format is documented (or you happen to know it anyway), it is often possible to interpret specific meanings for bytes in specific offsets (locations).