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Re: Add --dereference option to df

From: Pádraig Brady
Subject: Re: Add --dereference option to df
Date: Fri, 22 Nov 2013 01:52:13 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2

On 11/12/2013 07:45 AM, Bernhard Voelker wrote:
> On 05/15/2013 04:39 PM, Pádraig Brady wrote:
>> On 05/15/2013 03:32 PM, Ondrej Oprala wrote:
>>> 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).
>> Ah /dev/sda1 (/boot) isn't mounted on my system
>> (as I don't want that disk spinning up (long story)).
>> That's why I got the results I presented.
>> More thinking required...
> Isn't that what the findmnt(8) command achieves with
> the -S,--source option, i.e. it's not only about dereferencing
> per se but rather telling the tool that we're searching for
> the source of a mount instead of the target?
> Therefore, I think making the changes in get_point() is abusing the
> semantics of that function.  Instead, get_disk() should be enhanced
> in a similar way as in get_point() via canonicalize_file_name().

Makes sense.

> Another question is if such behavior could/should be made the default
> or if a new option has to be introduced.

I'd make it default to dereference. No new options needed I think.

> POSIX [1] does not explicitly mention symbolic link arguments:
> [1]
>   A pathname of a file within the hierarchy of the desired file system.
>   If a file other than a FIFO, a regular file, a directory,
>   [XSI] [Option Start]  or a special file representing the device
>   containing the file system (for example, /dev/dsk/0s1) [Option End]
>   is specified, the results are unspecified.
>   If the file operand names a file other than a special file containing
>   a file system, df shall write the amount of free space in the file system
>   containing the specified file operand.
>   [XSI] [Option Start]  Otherwise, df shall write the amount of free space
>   in that file system. [Option End]
> Well, we could either ask the OpenGroup for clarification.
> How do other df's behave? Any opinions?

I see FreeBSDs dereferences by default


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