This is quite wide requirement if you are running embedded linux on target board. Well The answer could be varied as question is more general. Lets limit the scope of question to USB devices.
Running external programs upon certain events to be specific want to run a program on USB device connection and on USB device removal.
Well I am going to do it with UDEV,
I have written simple udev rules file and kept in /etc/udev/rules.d/90-local.rules (off course you can give any name but with ext .rules and start with some no like 90 here); My udev rules are in 50-udev.rules files which defines the way to mount on USB connection and unmount on USB device disconnect.
I can't believe myself that with simple two lines rule placed in 90-local.rules file my job is done;
Here is 90-local.rules file
These two line says on connection of disconnection run the local file ie kept in /usr/sbin/.
You can write whatever you want in this exe; I am creating a default share on USB detection and making it to available for others to see the USB contents. On device removal I am deleting the shares.
The functionality introduced here allows you to run a program after the device node is put in place. This program can act on the device, however it must not run for any extended period of time, because udev is effectively paused while these programs are running. One workaround for this limitation is to make sure your program immediately detaches itself.
Here is an example rule which demonstrates the use of the RUN list assignment:
One observation if you enable printf inside your my_program you will not get the output on consol. Don't get panic, rather read the UDEV howto.KERNEL=="sdb", RUN+="/usr/bin/my_program"