Messages in this thread | | | Date | Fri, 05 Nov 2010 23:30:11 +0300 | From | Michael Tokarev <> | Subject | Re: Detecting bind-mounts |
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05.11.2010 21:30, Michael Tokarev wrote: > 05.11.2010 13:24, Pádraig Brady wrote: >> On 04/11/10 20:45, Michael Tokarev wrote: > [] >>> There are 2 (mostly) different kinds of applications. One >>> is cp/tar/find with --same-filesystem option (or equivalent), >>> that should not cross mountpoints. And one more, apps like >>> mountpoint(1) from sysvinit - a utility to determine if a >>> given path is a mountpoint. >>> >>> Neither of the two work when two directores on the same >>> filesystem are bind-mounted. > [] >> The `stat` command recently got support for >> printing the mount point for a file: >> http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=commit;h=ddf6fb86 >> >> `stat` will output the alias for a bind mounted file >> while `df` will output the initial mount point of its backing device >> So you could do something like: >> >> file=. >> df_mnt=$(df -P "$file" | sed -n '2s/.* \([^ ]*$\)/\1/p') >> stat_mnt=$(stat -c%m "$file") >> test "$df_mnt" = "$stat_mnt" || echo "bind mount" > > This is incorrect in two ways. > > First of all, stat(1), even after that commit you quote, > still compares st_dev fields, which are the same for this > and parent directory in case of bind mount. So this version > of stat(1) does _not_ detect a bind mount, unfortunately.
And this statement, in turn, is untrue. I apologize for the misinformation, it wasn't intentional. The mentioned commit adds the ability to detect bind mounts indeed. but...
> Second, I asked for a low-level way to detect such a mount. > I know how to do it not as efficient as stat(2) and not as > reliable but much simpler than you propose above, in shell > or in C, and I already provided that way in my original > email: we just parse /proc/mounts file, this is faster and > more reliable than the above shell fragment which calls a > few external commands.
.. the way used by stat(1) is to enumerate /proc/mounts -- which is what I were able to come with initially. It is slow and unreliable. Hence I asked if a faster way exist.
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