enhex and dehex are a stand-alone hex encoder/decoder pair.
To compile:

  cc -o enhex enhex.c
  cc -o dehex dehex.c

I wrote these as freely distributable (non-copyleft) programs for
doing the hex encoding which is an optional nrrd encoding, because I
only want to add optional encodings for which there are freely
available command-line converters.  A google search didn't reveal
simple special-purpose tools like these, so I wrote them.  Nothing
fancy.

Gordon Kindlmann