Everyone knows that gzip’ing a file will usually make it smaller, but gzip’ing it again will not. In other words, once you’ve compressed a file, it won’t compress further. But here’s an exception.
If you make a file of identical bytes, it can be gzip’ed several times and still become smaller. In the example below, we made a 2GB file which consisted of identical bytes, all equal 0. The first compression reduced the size by a factor of 1024. The second by a factor of 602. Next, by a factor of 14. The next compression, not shown, grew the file by about 20 bytes. The total compression, over the three generations, was a factor of 9,023,041 !
> dd if=/dev/zero of=2gb.zero bs=1024576 count=2048 > gzip -c 2gb.zero > 2gb.zero.gz > gzip -c 2gb.zero.gz > 2gb.zero.gz.gz > gzip -c 2gb.zero.gz.gz > 2gb.zero.gz.gz.gz > ls -lh 2gb.zero 2.0G 2gb.zero 2.0M 2gb.zero.gz 3.4K 2gb.zero.gz.gz 328B 2gb.zero.gz.gz.gz