Is there any way in Linux to check if directory tree has changed?

Question

I'm creating a QEMU startup script in which I compile a rootfs to cpio.gz each time I launch the env. This is fine for the moment, but when the rootfs gets bigger in size, it is going to be a problem. Is there any way to check if a given directory structure and a compiled xxx.cpio.gz are different? I mean, if a file is added to the base rootfs directory structure, then a new cpio.gz must be created, but if its not the case, the one compiled in a previous launch would be fine.

Thanks in advance.

Answer 1

You can use diff between the rootfs dir and the cpio.gz to find files that exist only in the rootfs dir.

For example, given the following directories a and b:

a:
total 16
drwxr-xr-x  2 spiderpig spiderpig  4096 Oct  2 14:43 .
drwxr-xr-x 24 spiderpig spiderpig 12288 Oct  2 14:43 ..
-rw-r--r--  1 spiderpig spiderpig     0 Oct  2 14:43 in_both_dirs
-rw-r--r--  1 spiderpig spiderpig     0 Oct  2 14:43 only_in_a

b:
total 16
drwxr-xr-x  2 spiderpig spiderpig  4096 Oct  2 14:43 .
drwxr-xr-x 24 spiderpig spiderpig 12288 Oct  2 14:43 ..
-rw-r--r--  1 spiderpig spiderpig     0 Oct  2 14:43 in_both_dirs
-rw-r--r--  1 spiderpig spiderpig     0 Oct  2 14:43 only_in_b

the output will be:

spiderpig@linux-dev ~ $ diff --brief a b
Only in a: only_in_a
Only in b: only_in_b

Then you can know which files to copy instead of copying the whole directory. Note that diff has many options, so be sure running diff --help before using it.