Automount USB drives with systemd

by Mike Blackwell   Last Updated September 13, 2017 14:00 PM

We're updating our servers from a very out-of-date distro to a modern Debian Jessie based system, including lightdm / xfce, and of course systemd (and udisks2). One sticking point is automounting USB drives. We used to accomplish this with some udev rules. The old rules almost still work - the mount point gets created and the drive is mounted fine, but after a few seconds systemd is doing something that breaks the mount, so subsequent access attempts result in "Transport endpoint is not connected" errors.

Manually mounting the drive via the command line works fine. So does letting a file manager (thunar and thunar-volman, which in turn uses udisks2). But those are not viable options - these systems mostly run headless, so thunar isn't normally running. We need to be able to plug in disk drives for unattended cron-based backups.

I thought that modifying the udev script to spawn a detached job which waits a few seconds before performing the mount might do the trick, but systemd seems to go out of its way to prevent this - it somehow still waits for the detached job to finish before continuing.

Perhaps having the udev script tickle udisks2 somehow is the right approach? I'm at a lose, so any advice greatly appreciated.



Answers 3


After several false starts I figured this out. The key is to add a systemd unit service between udev and a mounting script.

(For the record, I was not able to get this working using udisks2 (via something like udisksctl mount -b /dev/sdb1) called either directly from a udev rule or from a systemd unit file. There seems to be a race condition and the device node isn't quite ready, resulting in Error looking up object for device /dev/sdb1. Unfortunate, since udisks2 could take care of all the mount point messyness...)

The heavy lifting is done by a shell script, which takes care of creating and removing mount points, and mounting and unmounting the drives.

/usr/local/bin/usb-mount.sh

#!/bin/bash

# This script is called from our systemd unit file to mount or unmount
# a USB drive.

usage()
{
    echo "Usage: $0 {add|remove} device_name (e.g. sdb1)"
    exit 1
}

if [[ $# -ne 2 ]]; then
    usage
fi

ACTION=$1
DEVBASE=$2
DEVICE="/dev/${DEVBASE}"

# See if this drive is already mounted, and if so where
MOUNT_POINT=$(/bin/mount | /bin/grep ${DEVICE} | /usr/bin/awk '{ print $3 }')

do_mount()
{
    if [[ -n ${MOUNT_POINT} ]]; then
        echo "Warning: ${DEVICE} is already mounted at ${MOUNT_POINT}"
        exit 1
    fi

    # Get info for this drive: $ID_FS_LABEL, $ID_FS_UUID, and $ID_FS_TYPE
    eval $(/sbin/blkid -o udev ${DEVICE})

    # Figure out a mount point to use
    LABEL=${ID_FS_LABEL}
    if /bin/grep -q " /media/${LABEL} " /etc/mtab; then
        # Already in use, make a unique one
        LABEL+="-${DEVBASE}"
    fi
    MOUNT_POINT="/media/${LABEL}"

    echo "Mount point: ${MOUNT_POINT}"

    /bin/mkdir -p ${MOUNT_POINT}

    # Global mount options
    OPTS="rw,relatime"

    # File system type specific mount options
    if [[ ${ID_FS_TYPE} == "vfat" ]]; then
        OPTS+=",users,gid=100,umask=000,shortname=mixed,utf8=1,flush"
    fi

    if ! /bin/mount -o ${OPTS} ${DEVICE} ${MOUNT_POINT}; then
        echo "Error mounting ${DEVICE} (status = $?)"
        /bin/rmdir ${MOUNT_POINT}
        exit 1
    fi

    echo "**** Mounted ${DEVICE} at ${MOUNT_POINT} ****"
}

do_unmount()
{
    if [[ -z ${MOUNT_POINT} ]]; then
        echo "Warning: ${DEVICE} is not mounted"
    else
        /bin/umount -l ${DEVICE}
        echo "**** Unmounted ${DEVICE}"
    fi

    # Delete all empty dirs in /media that aren't being used as mount
    # points. This is kind of overkill, but if the drive was unmounted
    # prior to removal we no longer know its mount point, and we don't
    # want to leave it orphaned...
    for f in /media/* ; do
        if [[ -n $(/usr/bin/find "$f" -maxdepth 0 -type d -empty) ]]; then
            if ! /bin/grep -q " $f " /etc/mtab; then
                echo "**** Removing mount point $f"
                /bin/rmdir "$f"
            fi
        fi
    done
}

case "${ACTION}" in
    add)
        do_mount
        ;;
    remove)
        do_unmount
        ;;
    *)
        usage
        ;;
esac

The script, in turn, is called by a systemd unit file. We use the "@" filename syntax so we can pass the device name as an argument.

/etc/systemd/system/[email protected]

[Unit]
Description=Mount USB Drive on %i
[Service]
Type=oneshot
RemainAfterExit=true
ExecStart=/usr/local/bin/usb-mount.sh add %i
ExecStop=/usr/local/bin/usb-mount.sh remove %i

Finally, some udev rules start and stop the systemd unit service on hotplug/unplug:

/etc/udev/rules.d/99-local.rules

KERNEL=="sd[a-z][0-9]", SUBSYSTEMS=="usb", ACTION=="add", RUN+="/bin/systemctl start [email protected]%k.service"

KERNEL=="sd[a-z][0-9]", SUBSYSTEMS=="usb", ACTION=="remove", RUN+="/bin/systemctl stop [email protected]%k.service"

This seems to do the trick! A couple of useful commands for debugging stuff like this:

  • udevadm control -l debug turns on verbose logging to /var/log/syslog so you can see what's happening.
  • udevadm control --reload-rules after you modify files in the rules.d dir (may not be necessary, but can't hurt...).
  • systemctl daemon-reload after you modify systemd unit files.
Mike Blackwell
Mike Blackwell
March 30, 2016 15:26 PM

there is a new, succinct systemd auto-mount option which can be used with fstab which allows you to use all the standardized mount permission options, and it looks like this:

  x-systemd.automount

an example of it in an fstab line:

  /dev/sdd1   /mnt/hitachi-one     auto     noauto,x-systemd.automount     0 2

the noauto option will mean it will not attempt to be mounted at boot, as with older software autofs.

after adding a new x-systemd.automount line to fstab you then need to run:

  sudo systemctl daemon-reload

and then both, or one, of the following:

  sudo systemctl restart remote-fs.target
  sudo systemctl restart local-fs.target

for more infomation about it:

https://wiki.archlinux.org/index.php/Fstab#Automount_with_systemd

infinite-etcetera
infinite-etcetera
September 20, 2016 12:31 PM

thanks for this precious script but its not working for ntfs and ext4 fs, anyone help me for a perfect udev script ... actually i want to change my default mount point of /media to any other folder where i want.. just like default mountpoint of external drives, i tried all scripts and udev rules, but it will nt worked , sometimes permission problem , umount problem, and sometime it nt mounted , so plz could any one help me out of these problems , mainly i when i used udev rules, it will mounted only vfat fs and error shows for ntfs - END POINT NOT CONNECTED

Rahul Barde
Rahul Barde
September 13, 2017 13:52 PM

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