Hotplugging usb for linux

hotplug-udev.txt

# HOTPLUGGING W/ UDEV

[Udev rule]
// Create udev rule file    
    sudo touch /etc/udev/rules.d/66-hotplug.rules
// Fill the file with:
    KERNEL=="sd[a-z][0-9]*", SUBSYSTEM=="block", ACTION=="add",    RUN+="/usr/local/bin/mount.sh &" 
    KERNEL=="sd[a-z][0-9]*", SUBSYSTEM=="block", ACTION=="remove", RUN+="/usr/local/bin/umount.sh &"
// Scripts will be called whenever you plug/unplug(add/remove) usb.

[Mount/Umount scripts]
// create mount.sh & umount.sh:
sudo touch /usr/local/bin/mount.sh /usr/local/bin/umount.sh 
// Fill mount.sh with:
    #!/bin/sh
    mount /dev/sda1 /mnt/tmp
// Fill umount.sh with:
    #!/bin/sh
    umount /dev/sda1 /mnt/tmp
// Give executable permission to these files:
    sudo chmod +x /usr/local/bin/mount.sh /usr/local/bin/umount.sh

The original udev command has been replaced by systemd-udevd (see its man page). One of the differences is that it creates its own filesystem namespace, so your mount is done, but it is not visible in the principal namespace. Check with:
// Get pid of service
    systemctl status systemd-udevd 
// See if it is detected
    cat /proc/<pid-of-systemd-udevd>/mounts | grep sda
// Edit the service file:
    sudo systemctl edit systemd-udevd
// insert to it:
    [Service]
    MountFlags=shared (on Ubuntu 20.04, replace MountFlags=shared with PrivateMounts=no.)
// Reload systemd manager configuration
    sudo systemctl daemon-reload
// Restart service
    sudo service systemd-udevd --full-restart

check with df cmd

Tested both 18.04 & 20.04 Ubuntu

[For more]
https://unix.stackexchange.com/questions/330094/udev-rule-to-mount-disk-does-not-work