Sounds strange, I have a shell script that it gets trigger by udev rules, to mount attached USB device to the system file tree. The script runs when an usb device attaches to system, so the rules seems to be fine. I monitor how the script progress by syslog, and it also goes fine, and even mount command returns zero, and it says:

 root[1023]: mount: /dev/sda1 mounted on /media/partitionlabel.

But at the end the device is not mounted, it is not listed in /etc/mtab – /proc/mounts – findmnt – mount. and if I run umount on device, it also says, the device is not mounted.

However If I run the script manually as a root from terminal, then it works perfect and device gets mount, but not when it runs by udev.

I've added 8 second sleep time to the start of the script, to make sure it's not a timing problem and also removed number from rules file name to make sure udevd would put the new rules at the bottom of rules queue, and script would run after other system rules, but no success.

The syslog:

(right after the device attached)

kernel: usb 1-1.2: new high-speed USB device number 12 using dwc_otg
kernel: usb 1-1.2: New USB device found, idVendor=058f, idProduct=6387
kernel: usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
kernel: usb 1-1.2: Product: Mass Storage
kernel: usb 1-1.2: Manufacturer: Generic
kernel: usb 1-1.2: SerialNumber: 24DCF568
kernel: usb-storage 1-1.2:1.0: USB Mass Storage device detected
kernel: scsi host6: usb-storage 1-1.2:1.0
kernel: scsi 6:0:0:0: Direct-Access     Generic  Flash Disk       8.07 PQ: 0 ANSI: 4
kernel: sd 6:0:0:0: [sda] 1968128 512-byte logical blocks: (1.00 GB/961 MiB)
kernel: sd 6:0:0:0: [sda] Write Protect is off
kernel: sd 6:0:0:0: [sda] Mode Sense: 23 00 00 00
kernel: sd 6:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
kernel:  sda: sda1
kernel: sda: p1 size 1968126 extends beyond EOD, enabling native capacity
kernel:  sda: sda1
kernel: sda: p1 size 1968126 extends beyond EOD, truncated
kernel: sd 6:0:0:0: [sda] Attached SCSI removable disk
root[1004]: /usr/local/sbin/udev-auto-mount.sh - status: started to automount sda1
root[1019]: /usr/local/sbin/udev-auto-mount.sh - status: Device Label is partitionlabel and Filesystem is vfat.
root[1021]: /usr/local/sbin/udev-auto-mount.sh - status: mounting the device sda1 by filesystem vfat to /media/partitionlabel.
root[1023]: mount: /dev/sda1 mounted on /media/partitionlabel.
root[1024]: /usr/local/sbin/udev-auto-mount.sh status: mount command proceed for vfat, retval is 0
root[1025]: /usr/local/sbin/udev-auto-mount.sh - status: succeed!

Configs:

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

The defined rule in udev is:

# /etc/udev/rules.d/local-rules
ENV{ID_BUS}=="usb",     ACTION=="add",  ENV{DEVTYPE}=="partition",      \
          RUN+="/usr/local/sbin/udev-automounter.sh %k $ENV{ID_FS_LABEL_ENC}"

udev-auto-mount.sh

The script starts by another script which defined in the udev rule.
It so straight, it makes mount point directory and mounts the usb device to the mount point using its file system type and some regular options. I've added "-v" option to the mount command to be more verbose and also redirected all outputs to syslog, so I can see how it runs, but it not says too much.

#!/bin/sh
## /usr/local/sbin/udec-auto-mount.sh
##

logger -s "$0 - status: started to automount ${1}"
DEVICE=$1
sleep 8 

#...
#...
# Checking inputs, getting filesystem type (ID_FS_TYPE), partition label
# (ID_FS_LABEL) and ...

mkdir "/media/${ID_FS_LABEL}"


logger -s "$0 - status: mounting the device ${DEVICE} by filesystem ${ID_FS_TYPE} to /media/${ID_FS_LABEL}."
case $ID_FS_TYPE in
    vfat)   mount -v -t vfat -o sync,noatime,nosuid,nodev /dev/${DEVICE} "/media/${ID_FS_LABEL}" 2>&1 | logger
        let retVal=$?
        logger -s "$0 status: mount command proceed for vfat, retval is ${retVal}"
        ;;

    *)  mount -v -t auto -o sync,noatime /dev/${DEVICE} "/media/${ID_FS_LABEL}"
        ;;
esac
if [ ${retVal} -eq 0 ]; then
    logger -s "$0 - status: succeed!"
    exit 0
else
    logger -s "$0 Error: unable to mount the device ${DEVICE}, retval is ${retVal}"
    rmdir "/media/${ID_FS_LABEL}"
fi

exit 0

Maybe it helps:

Sometimes, after the script fails to mount the USB device, when I detach the device, some error come to syslog like:

kernel: usb 1-1.2: USB disconnect, device number 11
systemd-udevd[143]: error: /dev/sda: No such file or directory
systemd-udevd[977]: inotify_add_watch(7, /dev/sda, 10) failed: No such file or directory

Edit:

This is 'mount' version:

$ mount -V:
mount from util-linux 2.27.1 (libmount 2.27.0: assert, debug)