Unix Utility of the Day: xxd

2018-08-02

I’m going to finally try out something mentioned in my first entry: a shorter post. Specifically, I’d like to take a second to introduce to a handy little tool that I’ve found myself using a lot lately: xxd!

xxd is a hexdump utility (much like the more-aptly-named hexdump). With no flags, it behaves pretty much like hexdump -C (or hd):

$ xxd sample.wav
00000000: 5249 4646 8d02 0100 5741 5645 666d 7420  RIFF....WAVEfmt 
00000010: 1000 0000 0100 0100 44ac 0000 cc04 0200  ........D.......
00000020: 0300 1800 6461 7461 6902 0100 9c2d 0014  ....datai....-..
00000030: 76ff a7ee ff53 e3ff 943d ffdb 0601 3e9a  v....S...=....>.
00000040: 0164 bbff b423 ff43 33ff ba17 0038 5300  .d...#.C3....8S.
00000050: aa84 0037 fa02 f763 0072 0af8 4e44 0059  ...7...c.r..ND.Y
...

However, xxd has several neat options lacking in other hexdump tools.

xxd -p

Plain mode. Just dumps hex continuously, byte-by-byte, with no fancy formatting. (Still includes newlines; pipe into tr -d '\n' to remove.)

$ xxd -p sample.wav
524946468d02010057415645666d7420100000000100010044ac0000cc04
02000300180064617461690201009c2d001476ffa7eeff53e3ff943dffdb
06013e9a0164bbffb423ff4333ffba1700385300aa840037fa02f7630072
0af84e440059...

Fun facts:

Has aliases -plain, as well as -ps and -postscript (because you can use this to embed data in a postscript file in-line).
You can do the same thing with hexdump -ve '1/1 "%.2x"'.

xxd -r

Reverse mode! Converts a hexdump back into the original bytes. Combine with -p to convert plain hex to bytes.

$ echo '00000000: 6865 6c6c 6f20 776f 726c 640a' | xxd -r
hello world

$ echo '68656c6c6f20776f726c640a' | xxd -r -p
hello world

xxd -i

Last but not least, include mode. Outputs valid C suitable for dumping in a header file (XBM-style), for when you want to include a blob in your program but don’t want to/can’t open it from a filesystem (for example, some embedded contexts).

$ xxd -i sample.wav
unsigned char sample_wav[] = {
  0x52, 0x49, 0x46, 0x46, 0x8d, 0x02, 0x01, 0x00, 0x57, 0x41, 0x56, 0x45,
  0x66, 0x6d, 0x74, 0x20, 0x10, 0x00, 0x00, 0x00, 0x01, 0x00, 0x01, 0x00,
  0x44, 0xac, 0x00, 0x00, 0xcc, 0x04, 0x02, 0x00, 0x03, 0x00, 0x18, 0x00,
  0x64, 0x61, 0x74, 0x61, 0x69, 0x02, 0x01, 0x00, 0x9c, 0x2d, 0x00, 0x14,
  0x76, 0xff, 0xa7, 0xee, 0xff, 0x53, 0xe3, 0xff, 0x94, 0x3d, 0xff, 0xdb,
  0x06, 0x01, 0x3e, 0x9a, 0x01, 0x64, 0xbb, 0xff, 0xb4, 0x23, 0xff, 0x43,
  0x33, 0xff, 0xba, 0x17, 0x00, 0x38, 0x53, 0x00, 0xaa, 0x84, 0x00, 0x37,
  0xfa, 0x02, 0xf7, 0x63, 0x00, 0x72, 0x0a, 0xf8, 0x4e, 0x44, 0x00, 0x59,
...
  0x00, 0x00, 0x4d, 0x01, 0x00, 0x61, 0x01, 0x00, 0x19, 0x01, 0x00, 0x95,
  0x01, 0x00, 0x92, 0x01, 0x00, 0xd0, 0x02, 0x00, 0xa0, 0x01, 0x00, 0x38,
  0x01, 0x00, 0x00, 0x00, 0x00
};
unsigned int sample_wav_len = 66197;

Note that this can’t be combined with the previous two options. xxd -r -i results in an apologetic error. If both -p and -i appear, whichever appears later takes precedence.