Re: Add --dereference option to df

[Ondrej Oprala]

On 05/15/2013 04:19 PM, Pádraig Brady wrote:
> On 05/15/2013 02:55 PM, Ondrej Oprala wrote:
> > Hi, this patch adds the --dereference option to df, so if a symlink is specified
> > as an argument to df (possibly /dev/disk/by-uuid/*), df outputs info about the
> > filesystem the symlinked file is on instead. Now, I realize the option's name is
> > a bit misleading, since the arguments are stat-ed anyway, but IMHO that's 
> > exactly
> > what it looks like from a user's perspective.
> I don't get this sorry.
> Well the naming is very confusing since symlinks are already deferenced?
Yes, but since the best_match is mostly "guessed" the way it is, the 
user has
no way of knowing that.

> Also I don't see a change for your stated example:
>
> $ df -L /dev/disk/by-uuid/3030-3030
> Filesystem     1K-blocks  Used Available Use% Mounted on
> udev             1454732     0   1454732   0% /dev
>
> $ df $(realpath /dev/disk/by-uuid/3030-3030)
> Filesystem     1K-blocks  Used Available Use% Mounted on
> udev             1454732     0   1454732   0% /dev
>
> If I do create a symlink to a different FS,
> I'm not sure I get results you're expecting?
>
> $ ln -nsf /dev/shm shm
>
> $ src/df shm
> Filesystem     1K-blocks  Used Available Use% Mounted on
> tmpfs            1464412  6292   1458120   1% /dev/shm
>
> $ src/df -L shm
> Filesystem     1K-blocks  Used Available Use% Mounted on
> udev             1464412  6292   1458120   1% /dev
>
> thanks,
> Pádraig.

Let me explain by example from my system:

/dev/sda1 is mounted on /boot
/dev/disk/by-uuid/37a5c8d4-78ff-408f-8ae8-70555df51f7f is a symlink to 
/dev/sda1

#in the following example df prints info about the fs where the symlink 
file is.
$ df /dev/disk/by-uuid/37a5c8d4-78ff-408f-8ae8-70555df51f7f
Filesystem     1K-blocks  Used Available Use% Mounted on
devtmpfs         1013888     0   1013888   0% /dev

#with -L, information about the symlinked FS is printed.
$ df -L /dev/disk/by-uuid/37a5c8d4-78ff-408f-8ae8-70555df51f7f
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/sda1        1013888     0   1013888   0% /boot

So although the argument is stat-ed(dereferenced) in both cases, 
different parts
of the information are used to decide what to output if -L is specified 
(which is why
I find the option name misleading).

thanks,
Ondrej