How to tell without knowing the mount point that a device UUID is mounted?

Question

System: Linux Mint 19.1 Cinnamon 64-bit, based on Ubuntu 18.04 LTS.

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I would like to know if it is possible to get the following information:

Is this UUID (of a block device) mounted or not? (without knowing the mount point)

In spite, I have played along with this half a day, I can't figure it out.

I have at least created some working code below which un-mounts and powers off both of the USB hard drives.

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The current, temporary, version of my code looks like this:

dismount_and_poweroff_external_drives()
{
    name_external_drive_500gb_ntfs='500GB NTFS USB 2.0 HDD'
    name_external_drive_2_0tb_ext4='2.0TB Ext4 USB 3.0 HDD'
    uuid_external_drive_500gb_ntfs='xxxxxxxxxxxxxxxx' # censored
    uuid_external_drive_2_0tb_ext4='xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx' # censored
    path_external_drive_500gb_ntfs="/dev/disk/by-uuid/${uuid_external_drive_500gb_ntfs}"
    path_external_drive_2_0tb_ext4="/dev/disk/by-uuid/${uuid_external_drive_2_0tb_ext4}"

    tput bold; tput setaf 3; printf '%b' "\\n${name_external_drive_500gb_ntfs} un-mount\\n"; tput sgr0
    # info test ‘-b FILE’: True if FILE exists and is a block special device.
    if [ ! -b "${path_external_drive_500gb_ntfs}" ]
    then
        tput bold; tput setaf 4; printf '%b' "The device is not plugged in or powered on.\\n"; tput sgr0
    else
        if umount "${path_external_drive_500gb_ntfs}"
        then
            tput bold; tput setaf 2; printf '%b' "Un-mounting OK.\\n"; tput sgr0

            if udisksctl power-off --block-device "${path_external_drive_500gb_ntfs}"
            then
                tput bold; tput setaf 2; printf '%b' "Powering-off OK.\\n"; tput sgr0
            else
                tput bold; tput setaf 1; printf '%b' "Powering-off Failed.\\n"; tput sgr0
            fi

        else
            tput bold; tput setaf 1; printf '%b' "Un-mounting Failed.\\n"; tput sgr0
        fi
    fi

    printf '\n'

    tput bold; tput setaf 3; printf '%b' "\\n${name_external_drive_2_0tb_ext4} un-mount\\n"; tput sgr0
    # info test ‘-b FILE’: True if FILE exists and is a block special device.
    if [ ! -b "${path_external_drive_2_0tb_ext4}" ]
    then
        tput bold; tput setaf 4; printf '%b' "The device is not plugged in or powered on.\\n"; tput sgr0
    else
        if umount "${path_external_drive_2_0tb_ext4}"
        then
            tput bold; tput setaf 2; printf '%b' "Un-mounting OK.\\n"; tput sgr0

            if udisksctl power-off --block-device "${path_external_drive_2_0tb_ext4}"
            then
                tput bold; tput setaf 2; printf '%b' "Powering-off OK.\\n"; tput sgr0
            else
                tput bold; tput setaf 1; printf '%b' "Powering-off Failed.\\n"; tput sgr0
            fi

        else
            tput bold; tput setaf 1; printf '%b' "Un-mounting Failed.\\n"; tput sgr0
        fi
    fi

    printf '\n'
}

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I forgot to stress that the accepted solution must be POSIX-ly written.

Answer 1

Original Solution

UUID=<device_uuid>
mount | egrep $(readlink -f /dev/disk/by-uuid/${UUID}) && echo mounted

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Vlastimil's Notes

• It might be a good idea to use -e instead of -f, from readlink help:

  -e, --canonicalize-existing   canonicalize by following every symlink in
                                every component of the given name recursively,
                                all components must exist
  

  As compared to:

  -f, --canonicalize            canonicalize by following every symlink in
                                every component of the given name recursively;
                                all but the last component must exist
  

  As far as I understand it, with -e there is a guarantee that the whole path exists, might be better, needs additional verification or citation. Unfortunately, the -e option has been found not to be POSIX-compliant, so out of luck there. Leaving all the information here for future reference.

• There are no double quotes in the original solution, I'd recommend adding them along with one trailing space as a security measure to avoid matching e.g. sda11 or similarly.

• One could also take advantage of POSIX-defined fgrep to match for a fixed string, or even better match only line starting with this device using grep "^dev_name".

• As has been pointed out by Mark Plotnick, mount itself may not be POSIX-defined, again, a citation would be handy, but anyway I've changed the code to read from /proc/mounts directly.

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Plausible function

The resulting function for checking if UUID is mounted could look similar to:

is_uuid_mounted()
{
    readlink_output=$( readlink -f /dev/disk/by-uuid/"${1}" )
    [ -n "${readlink_output}" ] &&
        grep -F "${readlink_output} " /proc/mounts > /dev/null 2>&1
}

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Full working script

#!/bin/sh

set -eu

translate_uuid_to_device_name()
{
    # Linux-specific; needs *BSD revision
    readlink -f -n /dev/disk/by-uuid/"${1}"
}

is_uuid_mounted()
{
    device_name=$( translate_uuid_to_device_name "${1}" )

    if [ -n "${device_name}" ]
    then
        # 1. basic regex should be working across platfotms
        #    tested on FreeBSD, OpenBSD, NetBSD with success
        #    I prefer the starting with (^) rather than filtering throung all text
        # 2. /proc/mounts is not available on all *BSDs, needs revision
        proc_mounts=$( grep "^${device_name} " /proc/mounts )
        [ -n "${proc_mounts}" ]
    fi
}

# Simplest Usage Example
if is_uuid_mounted "PUT_SOME_UUID_IN_HERE"
then
    echo "This UUID is mounted."
else
    echo "This UUID isn't mounted."
fi

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Feel free to address further issues in the comments.

Answer 2

If findmnt is available, you can try :

test "$(findmnt -S UUID=$UUID)" || echo $UUID not mounted