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smokris
Asked: 2009-09-18 14:36:25 +0800 CST

In *nix, how to determine which filesystem a particular file is on?

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In a generic, modern unix environment (say, GNU/Linux, GNU/Solaris, or Mac OS X), is there a good way to determine which mountpoint and filesystem-type a particular absolute file path is on?

I suppose I could execute the mount command and manually parse the output of that and string-compare it with my file path, but before I do that I'm wondering if there's a more elegant way.

I'm developing a BASH script that makes use of extended attributes, and want to make it Do The Right Thing (to the small extent that it is possible) for a variety of filesystems and host environments.

unix

7 Answers

  1. Best Answer

    The command df(1) takes one or more arguments and will return the mountpoint and device on which that file or directory exists, as well as usage information. You can then use the path or device to look up the filesystem type in the output of mount -v or similar.

    Unfortunately, the output format of both df and mount are system-dependent; there is no apparent standard, at least as I can see between Solaris, NetBSD and Mac OS X.

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  2. You could use stat. The command stat --printf '%d' filename.txt will return the device number as hex/decimal.

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  3. For just a specific file it's as easy as

    df -T "${FILE}" | awk '{print $2}' | tail -n1
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  4. Hm. For the mount point, you can go up the hierarchy until the st_dev changes (then you have just crossed over a mount boundary); there's GNU stat for bash scripts; however, I don't know how you can guess the filesystem type without parsing /proc/mounts or by trial and error (i.e. handle failures after setting extended attributes)

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  5. One gotcha with using df is that if the device name in the output is long it's line will wrap so you can't just grab the last line. Instead strip off the first line and then grab the new first line and then print the first field:

    #!/usr/bin/env bash

path=$1
curdir=$(pwd)
cd $path
df . | tail -n +2 | head -1 | awk '{print $1}'
cd $curdir
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  6. There seems to be a catch with df and btrfs on Linux. When you ask df to locate the mount point for a mounted btrfs volume, it will do the right thing. In this case, joe is a sub-directory of /m/whale/backup.

    # df /srv/backup/joe
Filesystem      1K-blocks      Used  Available Use% Mounted on
/dev/md126     2930135488 307676684 2619663252  11% /m/whale/backup

    But if the directory being referenced is a sub-volume, it won't tell you the mount point anymore.

    # df /srv/backup/joe/code
Filesystem      1K-blocks      Used  Available Use% Mounted on
-              2930135488 307676684 2619663252  11% /a/whale/backup/joe/code

    The /a/whale/backup is the only mount point according to the kernel.

    # mount | grep whale
/dev/md126 on /a/whale/backup type btrfs (rw,relatime,space_cache)

    FWIW, stat does the same thing:

    # stat --printf '%m\n' /srv/backup/joe/code
/a/whale/backup/joe/code
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  7. From https://stackoverflow.com/questions/2167558/give-the-mount-point-of-a-path:

     df -P $path  | tail -1 | awk '{ print $NF}'

    works everywhere I have tested, for both *BSDs and sysVs, and for wacky automounted directories. I'd be delighted to hear of a case where it fails.

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