Swap (exchange) two big files in limited environment

Question

Let's say I have two flash memory devices 1GB capacity each. Let's also say each device contains one big file ~ 1GB size. Assuming, I have limited (64MB) RAM and no hard disk drive, what is the shortest way to swap this two files ?

I mean: if file a is on device A and b is on device B, I want swap operation to place a on B and b on A.

I know there is no exact linux tool to do that according to this discussion.

I am interested in shortest way to achieve my goal. If that way will be too long, I probably will have to implement my own dedicated solution that will perform swap chunk by chunk, am I right ?

Answer 1

As long as both drives are full, or almost full, I doubt there is a pretty solution. It should be possible to loop over dd commands though. Something like

#This code is completely untested, 
#do NOT just copy paste and use it without proper testing
while [[ $((i*chunkSize)) -lt fileSize ]]; do
  dd skip=$i seek=$i bs=$chunkSize count=1 if=fileA of=tmpFileInMemory
  dd skip=$i seek=$i bs=$chunkSize count=1 if=fileB of=FileA
  dd skip=$i seek=$i bs=$chunkSize count=1 if=tmpFileInMemory of=fileB
done

Some checks are needed for when fileA and fileB have different sizes.