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File real location

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  File real location JimR <NotReallyMyEmail@example.net> - 2011-04-26 13:06 -0400
    Re: File real location Richard Kettlewell <rjk@greenend.org.uk> - 2011-04-26 18:24 +0100
      Re: File real location The Natural Philosopher <tnp@invalid.invalid> - 2011-04-26 18:28 +0100
        Re: File real location notbob <notbob@notbob.invalid> - 2011-04-26 18:08 +0000
          Re: File real location Richard Kettlewell <rjk@greenend.org.uk> - 2011-04-26 19:14 +0100
          Re: File real location Tim Watts <tw@dionic.net> - 2011-04-26 19:25 +0100
          Re: File real location unruh <unruh@wormhole.physics.ubc.ca> - 2011-04-26 18:43 +0000
          Re: File real location Chris Davies <chris-usenet@roaima.co.uk> - 2011-04-27 16:31 +0100
      Re: File real location Tim Watts <tw@dionic.net> - 2011-04-26 19:08 +0100
      Re: File real location JimR <NotReallyMyEmail@example.net> - 2011-04-26 14:43 -0400
        Re: File real location Richard Kettlewell <rjk@greenend.org.uk> - 2011-04-26 20:28 +0100
          Re: File real location JimR <NotReallyMyEmail@example.net> - 2011-04-26 17:00 -0400
    Re: File real location The Natural Philosopher <tnp@invalid.invalid> - 2011-04-26 18:27 +0100
    Re: File real location "Chris F.A. Johnson" <cfajohnson@gmail.com> - 2011-04-26 13:44 -0400
    Re: File real location unruh <unruh@wormhole.physics.ubc.ca> - 2011-04-26 17:52 +0000

#916 — File real location

This applies to Unix as well as Linux, but has been bugging me for awhile.

Supposed I have a filesystem that is optionally mounted.  Let's says the 
mount point is
/foo

Now I create a file on it, let's call it bar.  Now I have
/foo/bar

In an alternative case, the filesystem is not mounted.  I can still 
create a file bar in the directory foo, but it will fall onto the 
default filesystem (likely root filesystem).

How can I tell the difference between these 2 files, i.e., on which 
filesystem did the file land?

Thanks,
JimR

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#918

JimR <NotReallyMyEmail@example.net> writes:

> This applies to Unix as well as Linux, but has been bugging me for awhile.
>
> Supposed I have a filesystem that is optionally mounted.  Let's says
> the mount point is
> /foo
>
> Now I create a file on it, let's call it bar.  Now I have
> /foo/bar
>
> In an alternative case, the filesystem is not mounted.  I can still
> create a file bar in the directory foo, but it will fall onto the
> default filesystem (likely root filesystem).
>
> How can I tell the difference between these 2 files, i.e., on which
> filesystem did the file land?

stat() it and look at st_dev.

-- 
http://www.greenend.org.uk/rjk/

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#920

Richard Kettlewell wrote:
> JimR <NotReallyMyEmail@example.net> writes:
> 
>> This applies to Unix as well as Linux, but has been bugging me for awhile.
>>
>> Supposed I have a filesystem that is optionally mounted.  Let's says
>> the mount point is
>> /foo
>>
>> Now I create a file on it, let's call it bar.  Now I have
>> /foo/bar
>>
>> In an alternative case, the filesystem is not mounted.  I can still
>> create a file bar in the directory foo, but it will fall onto the
>> default filesystem (likely root filesystem).
>>
>> How can I tell the difference between these 2 files, i.e., on which
>> filesystem did the file land?
> 
> stat() it and look at st_dev.
> 
I bow to a superior solution...

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#925

On 2011-04-26, The Natural Philosopher <tnp@invalid.invalid> wrote:
> Richard Kettlewell wrote:
>> JimR <NotReallyMyEmail@example.net> writes:

>>> Now I create a file on it, let's call it bar.  Now I have
>>> /foo/bar
>>>
>>> In an alternative case, the filesystem is not mounted.  I can still
>>> create a file bar in the directory foo, but it will fall onto the
>>> default filesystem (likely root filesystem).
>>>
>>> How can I tell the difference between these 2 files, i.e., on which
>>> filesystem did the file land?
>> 
>> stat() it and look at st_dev.
>> 
> I bow to a superior solution...


Ummm.... let's back up a bit.  Nevermind I haven't a clue what stat()
and st_dev are.  If a "filesystem is not mounted", how could there be
any root (/) dir?  If the dir foo has not been mounted how can one put
a file into a dir that doesn't exist ...in a filesystem that hasn't
been mounted?  Is there some editor ....cat, vi, emacs... that will
create a filesystem and dir if a nonexistent dir/file is specified and
doesn't exist?

nb --confused

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#927

notbob <notbob@notbob.invalid> writes:
> On 2011-04-26, The Natural Philosopher <tnp@invalid.invalid> wrote:
>> Richard Kettlewell wrote:
>>> JimR <NotReallyMyEmail@example.net> writes:

>>>> Now I create a file on it, let's call it bar.  Now I have
>>>> /foo/bar
>>>>
>>>> In an alternative case, the filesystem is not mounted.  I can still
>>>> create a file bar in the directory foo, but it will fall onto the
>>>> default filesystem (likely root filesystem).
>>>>
>>>> How can I tell the difference between these 2 files, i.e., on which
>>>> filesystem did the file land?
>>> 
>>> stat() it and look at st_dev.
>>> 
>> I bow to a superior solution...
>
> Ummm.... let's back up a bit.  Nevermind I haven't a clue what stat()
> and st_dev are.  If a "filesystem is not mounted", how could there be
> any root (/) dir?  If the dir foo has not been mounted how can one put
> a file into a dir that doesn't exist ...in a filesystem that hasn't
> been mounted?  Is there some editor ....cat, vi, emacs... that will
> create a filesystem and dir if a nonexistent dir/file is specified and
> doesn't exist?

Mount points are directories.  If the filesystem usually mounted on /foo
isn't mounted then the directory /foo nevertheless generally exists (on
the root filesystem in this instance) and all the usual rules about
putting files in it apply.

-- 
http://www.greenend.org.uk/rjk/

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#928

notbob wrote:

> On 2011-04-26, The Natural Philosopher <tnp@invalid.invalid> wrote:
>> Richard Kettlewell wrote:
>>> JimR <NotReallyMyEmail@example.net> writes:
> 
>>>> Now I create a file on it, let's call it bar.  Now I have
>>>> /foo/bar
>>>>
>>>> In an alternative case, the filesystem is not mounted.  I can still
>>>> create a file bar in the directory foo, but it will fall onto the
>>>> default filesystem (likely root filesystem).
>>>>
>>>> How can I tell the difference between these 2 files, i.e., on which
>>>> filesystem did the file land?
>>> 
>>> stat() it and look at st_dev.
>>> 
>> I bow to a superior solution...
> 
> 
> Ummm.... let's back up a bit.  Nevermind I haven't a clue what stat()
> and st_dev are. 

man -S2 stat

returns a lot of what there is to know about a file/devicenode/other 
including size, timestamps, owner and st_dev which is the block device the 
file lives on. It doesn't matter for the question here what the actual 
st_dev numbers are - only whether they are the same (same block device ie 
same disk/partition) or different.

> If a "filesystem is not mounted", how could there be
> any root (/) dir?  

There's (almost) always at least one filesystem mounted and that is / (even 
if the mount is a fudge from a packed special file or embedded in the 
kernel).

Every mount *must* happen on an existing directory - so if there is no 
filesystem mounted on /x/y/z/ and you stick a file in /x/y/z/ then the file 
lives in the filesystem that z/ is a member of which may or may not be the 
FS that / is from. If you mount another FS on /x/y/z/ and the stick a file 
in z/ then the file exists on the FS you just mounted there.

> If the dir foo has not been mounted how can one put
> a file into a dir that doesn't exist ...in a filesystem that hasn't
> been mounted?  Is there some editor ....cat, vi, emacs... that will
> create a filesystem and dir if a nonexistent dir/file is specified and
> doesn't exist?

Sometimes. There are tools that allow (v)fat/msdos filesystem devices to be 
manipulated without mounting. In principle you can manipulate any raw device 
to add a file to its contained FS if any - whether there was any point to 
anyone writing such a tool is another matter altogether.

If you are hard core, you could use hexedit on any FS device - but you would 
have to intimately know the data structures involved such as free 
block/extent lists, directory structures, inode structures and more.

HTH

Tim

-- 
Tim Watts

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#929

On 2011-04-26, notbob <notbob@notbob.invalid> wrote:
> On 2011-04-26, The Natural Philosopher <tnp@invalid.invalid> wrote:
>> Richard Kettlewell wrote:
>>> JimR <NotReallyMyEmail@example.net> writes:
>
>>>> Now I create a file on it, let's call it bar.  Now I have
>>>> /foo/bar
>>>>
>>>> In an alternative case, the filesystem is not mounted.  I can still
>>>> create a file bar in the directory foo, but it will fall onto the
>>>> default filesystem (likely root filesystem).
>>>>
>>>> How can I tell the difference between these 2 files, i.e., on which
>>>> filesystem did the file land?
>>> 
>>> stat() it and look at st_dev.
>>> 
>> I bow to a superior solution...
>
>
> Ummm.... let's back up a bit.  Nevermind I haven't a clue what stat()
> and st_dev are.  If a "filesystem is not mounted", how could there be
> any root (/) dir?  If the dir foo has not been mounted how can one put
> a file into a dir that doesn't exist ...in a filesystem that hasn't
> been mounted?  Is there some editor ....cat, vi, emacs... that will
> create a filesystem and dir if a nonexistent dir/file is specified and
> doesn't exist?

A mountpoint of a filesystem is a directory on another filesystem, often
the so called root filesystem (because it contains the /directory). If
the system is running at all it has a / filesystem already mounted. Now
in the OP case, that contains a directory called /foo. That is just a
regular directory, but on top of that directory (hiding anything which
happens to be already in that directory) can be mounted another
filesystem. Thus, contents of /foo can exist both on the original / filesystem and on
the new mounted filesystem. If the mounted filesystem is actually
mounted, all original contents of /foo on the / filesystem are
inaccessible and hidden, until the new mounted filesystem is unmounted. 


>
> nb --confused

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#943

notbob <notbob@notbob.invalid> wrote:
> and st_dev are.  If a "filesystem is not mounted", how could there be
> any root (/) dir?  If the dir foo has not been mounted how can one put
> a file into a dir that doesn't exist ...in a filesystem that hasn't
> been mounted?

    $ df -h /mnt
    Filesystem            Size  Used Avail Use% Mounted on
    /dev/sda5             464M  180M  261M  41% /
    $ echo 'this is on root' > /mnt/foo
    $ ls -l /mnt/foo
    -rw-r--r-- 1 chris chris 16 Apr 27 16:26 /mnt/foo
    $ sudo mount /dev/mapper/vg-mnt /mnt
    $ df -h /mnt
    Filesystem            Size  Used Avail Use% Mounted on
    /dev/mapper/vg-mnt   1008M   34M  924M   4% /mnt
    $ ls -l /mnt/foo
    ls: cannot access /mnt/foo: No such file or directory
    $ echo 'this is on mnt' > /mnt/foo
    $ ls -l /mnt/foo
    -rw-r--r-- 1 chris chris 15 Apr 27 16:28 /mnt/foo
    $ cat /tmp/foo
    this is on root
    $ sudo umount /mnt
    $ ls -l /mnt/foo
    -rw-r--r-- 1 chris chris 16 Apr 27 16:26 /mnt/foo
    $ cat /tmp/foo
    this is on root

Chris

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#926

Richard Kettlewell wrote:

> JimR <NotReallyMyEmail@example.net> writes:
> 
>> This applies to Unix as well as Linux, but has been bugging me for
>> awhile.
>>
>> Supposed I have a filesystem that is optionally mounted.  Let's says
>> the mount point is
>> /foo
>>
>> Now I create a file on it, let's call it bar.  Now I have
>> /foo/bar
>>
>> In an alternative case, the filesystem is not mounted.  I can still
>> create a file bar in the directory foo, but it will fall onto the
>> default filesystem (likely root filesystem).
>>
>> How can I tell the difference between these 2 files, i.e., on which
>> filesystem did the file land?
> 
> stat() it and look at st_dev.
> 

Wot he said ^

-- 
Tim Watts

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#930

On 04/26/2011 01:24 PM, Richard Kettlewell wrote:
{snip}

> stat() it and look at st_dev.
>

OK, did stat with and without -f.  Got the results below.  The device ID 
is different in each case.  How do I relate that number back to the 
filesystem?

Thanks,

JimR

on mounted fs

stat  /mnt/backups/me
   File: `/mnt/backups/me'
   Size: 0               Blocks: 0          IO Block: 4096   regular 
empty file
Device: 806h/2054d      Inode: 18          Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2011-04-26 14:29:12.105661329 -0400
Modify: 2011-04-26 14:29:12.105661329 -0400
Change: 2011-04-26 14:29:12.105661329 -0400
===
on unmounted fs

stat /mnt/backups/me
   File: `/mnt/backups/me'
   Size: 0               Blocks: 0          IO Block: 4096   regular 
empty file
Device: 808h/2056d      Inode: 132711      Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2011-04-26 14:27:06.083765346 -0400
Modify: 2011-04-26 14:27:06.083765346 -0400
Change: 2011-04-26 14:27:06.083765346 -0400

===
stat -f

on mounted

stat -f  /mnt/backups/me
   File: "/mnt/backups/me"
     ID: a9599914aa7bb43c Namelen: 255     Type: ext2/ext3
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 36910968   Free: 24913212   Available: 23038230
Inodes: Total: 9379840    Free: 9379599

===

on umounted
stat -f  /mnt/backups/me
   File: "/mnt/backups/me"
     ID: f2e3d0e7c6681d85 Namelen: 255     Type: ext2/ext3
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 4921158    Free: 3560882    Available: 3310898
Inodes: Total: 1250928    Free: 1073436

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#931

JimR <NotReallyMyEmail@example.net> writes:
> Richard Kettlewell wrote:

>> stat() it and look at st_dev.
>
> OK, did stat with and without -f.  Got the results below.  The device
> ID is different in each case.  How do I relate that number back to the
> filesystem?

You can find it in the 'ls -l' output for the physical device underlying
the filesystem.  For instance, 806h corresponds to 8, 6 (probably sda6);
in the example below fe00h = fe, 00 = 254, 0.

  richard@araminta:~/src/rsbackup$ stat /etc/motd | grep Device
  Device: fe00h/65024d	Inode: 301455      Links: 1
  richard@araminta:~/src/rsbackup$ mount | grep 'on / '
  /dev/mapper/araminta-root on / type ext3 (rw,noatime,errors=remount-ro)
  richard@araminta:~/src/rsbackup$ ls -lL /dev/mapper/araminta-root 
  brw-rw---- 1 root disk 254, 0 Apr  4 18:57 /dev/mapper/araminta-root

-- 
http://www.greenend.org.uk/rjk/

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#933

On 04/26/2011 03:28 PM, Richard Kettlewell wrote:
> JimR<NotReallyMyEmail@example.net>  writes:
>> Richard Kettlewell wrote:
>
>>> stat() it and look at st_dev.
>>
>> OK, did stat with and without -f.  Got the results below.  The device
>> ID is different in each case.  How do I relate that number back to the
>> filesystem?
>
> You can find it in the 'ls -l' output for the physical device underlying
> the filesystem.  For instance, 806h corresponds to 8, 6 (probably sda6);
> in the example below fe00h = fe, 00 = 254, 0.
>
>    richard@araminta:~/src/rsbackup$ stat /etc/motd | grep Device
>    Device: fe00h/65024d	Inode: 301455      Links: 1
>    richard@araminta:~/src/rsbackup$ mount | grep 'on / '
>    /dev/mapper/araminta-root on / type ext3 (rw,noatime,errors=remount-ro)
>    richard@araminta:~/src/rsbackup$ ls -lL /dev/mapper/araminta-root
>    brw-rw---- 1 root disk 254, 0 Apr  4 18:57 /dev/mapper/araminta-root
>

Perfect, thanks for your help.

JimR

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#919

JimR wrote:
> This applies to Unix as well as Linux, but has been bugging me for awhile.
> 
> Supposed I have a filesystem that is optionally mounted.  Let's says the 
> mount point is
> /foo
> 
> Now I create a file on it, let's call it bar.  Now I have
> /foo/bar
> 
> In an alternative case, the filesystem is not mounted.  I can still 
> create a file bar in the directory foo, but it will fall onto the 
> default filesystem (likely root filesystem).
> 
> How can I tell the difference between these 2 files, i.e., on which 
> filesystem did the file land?
> 

parse /etc/mtab?

> Thanks,
> JimR

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#923

On 2011-04-26, JimR wrote:
> This applies to Unix as well as Linux, but has been bugging me for awhile.
>
> Supposed I have a filesystem that is optionally mounted.  Let's says the 
> mount point is
> /foo
>
> Now I create a file on it, let's call it bar.  Now I have
> /foo/bar
>
> In an alternative case, the filesystem is not mounted.  I can still 
> create a file bar in the directory foo, but it will fall onto the 
> default filesystem (likely root filesystem).
>
> How can I tell the difference between these 2 files, i.e., on which 
> filesystem did the file land?

df /foo/bar

-- 
   Chris F.A. Johnson, <http://cfajohnson.com>
   Author:
   Pro Bash Programming: Scripting the GNU/Linux Shell (2009, Apress)
   Shell Scripting Recipes: A Problem-Solution Approach (2005, Apress)

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#924

On 2011-04-26, JimR <NotReallyMyEmail@example.net> wrote:
> This applies to Unix as well as Linux, but has been bugging me for awhile.
>
> Supposed I have a filesystem that is optionally mounted.  Let's says the 
> mount point is
> /foo
>
> Now I create a file on it, let's call it bar.  Now I have
> /foo/bar
>
> In an alternative case, the filesystem is not mounted.  I can still 
> create a file bar in the directory foo, but it will fall onto the 
> default filesystem (likely root filesystem).
>
> How can I tell the difference between these 2 files, i.e., on which 
> filesystem did the file land?

By looking to see if there was a filesystem mounted on the /foo
directory.

>
> Thanks,
> JimR

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