Hey there! I see you're not using JavaScript. Just FYI, I use MathJax to display things that deserve to look like math.


Computer "scientist"

Alex Clemmer is a computer programmer. Other programmers love Alex, excitedly describing him as "employed here" and "the boss's son".

Alex is also a Hacker School alum. Surely they do not at all regret admitting him!

Recovering deleted files using only grep

June 24, 2014

In my college systems class we were required to implement malloc.

I spent a week or so on it. No version control — I was both youthful and arrogant.

After ironing out all the little systems bugs, I began cleaning up the directory to package up and send off for grading. I went to remove something in the same directory that also started with the letter m, and when I hit tab, zsh helpfully completed this to malloc.c.

By the time I had noticed, it was too late.

I had just run rm malloc.c.

grep to the rescue

I knew that grep works on a lot of the virtual filesystems available in nix, like /proc. I thought, “why not `/dev/` too?”

I figured I could use a command like this to grep over raw disk data — not over files or anything like that, just on the raw data on disk — and if successful, I’d have a resonable shot at recovering my homework.

I now doubt that this idea is even remotely original — grep was probably designed to do exactly this sort of thing.

Anyway, after RTFM'ing for awhile, eventually I ran a command that looked vaguely like this:

$ grep --binary-files=text --context=x 'stringfromyourfile' \
    /dev/whateverPartition > someFile.txt

The gist of what we’re doing here is running grep over the partition /dev/whateverPartition, finding the string 'stringfromyourfile', and grabbing the x lines bookending that string. If you pick x to be big enough, you should get the entire file, plus a bit of junk around the edges. [EDIT: HN user sparkie points out that this is actually not entirely accurate!] Though, of course, the string has to be unique on your disk, or this will fail.

The key to this is actually the flag --binary-files=text; that tells grep to run on the binary disk data even though it doesn’t really make any sense.

In the end, this proved good enough and I got my homework back. It’s a nifty hack that I don’t expect to need to use again, though it’s worth knowing that in principle it can be done, especially if you need to look for something else in some binary file, somewhere.

comments powered by Disqus