Doing a Reverse Hex Dump

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If you work with the command line you've most likely used hexdump or od to dump binary files, but what do you do if you have a hex dump of something and you want to create the binary version of the data? Assuming your needs aren't too complex, the answer may be xxd. You can use xxd to dump binary files just like hexdump and od, but you can also use it to do the reverse: turn a hex dump back into binary.

If you run xxd with just a file name it dumps the data in a fairly standard hex dump format:

  # xxd bdata
  0000000: 0001 0203 0405                           ......
Now if you pipe the output back to xxd with the -r option and redirect that to a new file, you can convert the hex dump back to binary:
  # xxd bdata | xxd -r >bdata2
  # cmp bdata bdata2
  # xxd bdata2
  0000000: 0001 0203 0405                           ......
Note that when doing reverse conversions with xxd, the data needs to look like a hex dump: there needs to be an offset and the data needs to be formatted correctly. So, for example, this works:
  # echo 01: 01 02 03 04 | xxd -r >output
but this does not because the data is not formatted correctly:
  # echo 01: 1 2 3 4 | xxd -r >output

As a more concrete example, I recently had a need to create a Motorola S-Record file containing a MAC Address. First thing I needed was a way to create a binary file with the MAC Address so that I could use objcopy to convert it to an S-Record file. A bit of pondering produced no good ideas. Linux and its brethren have a lot of command line utilities for manipulating text but not many for manipulating binary data. Of course, I could have written a C program to create the binary file or to create the S-Record file itself, but that seemed a bit much considering I was only talking about 6 bytes of data.

After a fair bit of searching the net I came across xxd. In OpenSuSE xxd is part of the vim-base package. I'm not sure if that's where it is in all distros, since it doesn't seem to have any relationship to vim.

The script takes a MAC Address and outputs S-Record data:

  # sh macid.sh 00:11:22:33:44:55
  S00D0000333737382D322E746D703B
  S1090000001122334455F7
  S9030000FC
Optionally, you can specify an address for re-basing the S-Record file and an output file name:
  # sh macid.sh --address 0xffff0000 --output ma 00:11:22:33:44:55
  # cat ma
  S00D0000333733302D322E746D7047
  S30BFFFF0000001122334455F7
  S705FFFF0000FC
See the entire script is here

______________________

Mitch Frazier is an Associate Editor for Linux Journal.

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Wow

Alexander Janssen's picture

Mitch, I'm deeply impressed. I'm just wondering how I could have live without that tool? I remember several cases when I needed to do exactly that thing and I always ended up writing a program of some sort which was dumped afterwards...

Very cool!
Alex.

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