How to get mount point or device name by directory?

[Matrix Neo]

Hi, everybody:

     I want to get the mount point or device name (like /dev/sda1) by the directory name. I try my best only to find syscall.Stat_t struct but there is no field to

point the information i need. So is there any way to implement this ?  

[Ian Lance Taylor]

You neglected to say what kind of system you are running on. If you
are using GNU/Linux, then as far as I know this information is not
available using `stat`. You need to run the `findmnt` program or look
in `/proc/self/mountinfo`.

Ian

[Matrix Neo]

Lan,

    Thank you for  your reply. Yes, I'm using GNU/Linux. I lookup the

findmnt program your mentioned and find that doesn't meet the need.

For example, if /data is mounted on /dev/sda3 and the given dir is /data. 

It's ok to use findmnt to get /dev/sda3. But if the given dir is /data/test1, 

the findmnt returns nothing. 

在 2018年1月23日星期二 UTC+8下午10:39:58，Ian Lance Taylor写道：

[Ian Lance Taylor]

On Tue, Jan 23, 2018 at 5:35 PM, Matrix Neo <lanxis...@gmail.com> wrote:
> Lan,
> Thank you for your reply. Yes, I'm using GNU/Linux. I lookup the
> findmnt program your mentioned and find that doesn't meet the need.
> For example, if /data is mounted on /dev/sda3 and the given dir is /data.
> It's ok to use findmnt to get /dev/sda3. But if the given dir is
> /data/test1,
> the findmnt returns nothing.

Sorry the suggestion didn't help, but in any case this is a GNU/Linux
question, not a Go question as such.

Ian

> 在 2018年1月23日星期二 UTC+8下午10:39:58，Ian Lance Taylor写道：
>>
>> On Mon, Jan 22, 2018 at 10:11 PM, Matrix Neo <lanxis...@gmail.com> wrote:
>> >
>> > I want to get the mount point or device name (like /dev/sda1) by
>> > the
>> > directory name. I try my best only to find syscall.Stat_t struct but
>> > there
>> > is no field to
>> > point the information i need. So is there any way to implement this ?
>>
>> You neglected to say what kind of system you are running on. If you
>> are using GNU/Linux, then as far as I know this information is not
>> available using `stat`. You need to run the `findmnt` program or look
>> in `/proc/self/mountinfo`.
>>
>> Ian
>

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[Matt Harden]

syscall.Stat_t does have the Dev field, which is documented by POSIX: https://linux.die.net/man/2/stat: "The st_dev field describes the device on which this file resides. (The major(3) and minor(3) macros may be useful to decompose the device ID in this field.)"

In Go we don't have the major and minor macros but they are basically just shufflings of the bits. Unfortunately they are not platform independent, even between versions of Linux. Luckily that doesn't matter when you're searching for the device.

If you run os.Stat on each file in /dev, you will find for example "sda3", which will have a value in syscall.Stat_t.Rdev that will match the Dev value of any file/directory in a filesystem mounted on that device.

Note that there can possibly be more than one device file that has the same Rdev value, so you may get a different answer than what "df" does, for example.

If you have the tool "di", it can do all this for you:

% di /boot/grub
Filesystem         Mount               Size     Used    Avail %Used  fs Type
/dev/sda1          /boot             235.3M    68.0M   155.2M   34%  ext2
% di -n -f S /boot/grub
/dev/sda1