Automount USB as read only for MP3 player

Question

I've made a MP3 player for the Pi using a 16x2 LCD display and 5 buttons; what I would like to do next is to add the ability to automount any USB flash inserted and then tell my program (all written in C btw) so it can add any music on the flash to it's linked list / playlist. As I won't be changing anything on the flash, I'd just want to mount it ro mode (plus to avoid any corruption when the flash is removed)

Answer 1

Under /etc/udev/rules.d , create a file named like usb-ro.rules, with the following content.

KERNEL=="sd*", SUBSYSTEMS=="usb", MODE="0555"

This will mount all USB devices as read-only.

If you have something mounted on USB, unmount it all and issue command

sudo /etc/init.d/udev restart

I think to get RW access you need to unmount it while plugged in then remount it with mount. I am sure you can add that into your program to do this via the interface and calling bash command directly as su

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If you want to be able to "automount" any kind of USB device on any port with FAT32/NTFS/EXT it will require a bit more work.

You can simplify the /dev/sd location by always mounting any USB device to the same device namespace location by using udev again..(restart required also)

SUBSYSTEM=="usb", SYMLINK+="Playlist%n"

This will yield (pure example with multiple partitions)

ls -l /dev/Playlist*
lrwxrwxrwx 1 root root  3 2014-02-23 12:36 /dev/Playlist -> sdc
lrwxrwxrwx 1 root root 12 2014-02-23 12:36 /dev/Playlist0 -> bsg/14:0:0:0
lrwxrwxrwx 1 root root  4 2014-02-23 12:36 /dev/Playlist1 -> sdc1
lrwxrwxrwx 1 root root  3 2014-02-23 12:36 /dev/Playlist3 -> sg3

You can then install autofs which will detect the filesytem and mount it where you need.

apt-get install autofs

In the masterfile add this

/media/   /etc/auto.ext-usb --timeout=10,defaults,user,exec,uid=1000

Edit /etc/auto.ext-usb and add this (this will auto mount any 1st partition)

Playlist            -fstype=auto           :/dev/Playlist1

Restart

/etc/init.d/autofs restart

You still need to check if a USB was plugged in or not with your software.

/etc/init.d/autofs status

And if there is something to mount its as simple as command

mount