Suppose that you have two huge files (several GB) that you want to concatenate together, but that you have very little spare disk space (let's say a couple hundred MB). That is, given
file2, you want to end up with a single file which is the result of concatenating
file2 together byte-for-byte, and delete the original files.
You can't do the obvious
cat file2 >> file1; rm file2, since in between the two operations, you'd run out of disk space.
Solutions on any and all platforms with free or non-free tools are welcome; this is a hypothetical problem I thought up while I was downloading a Linux ISO the other day, and the download got interrupted partway through due to a wireless hiccup.
I think the difficulty is determining how the space can be recovered from the original files.
I think the following might work:
This all relies on sparse file support, and file truncation freeing space immediately.
If you actually wanted to do this then you should investigate the
ddcommand. which can do the copying step
Someone in another answer gave a neat solution that doesn't require sparse files, but does copy file2 twice: