How to determine the device file from which Debian Live was booted ?

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

is there a way to obtain the address of the disk-like device from which
Debian Live has booted ?

I would need shell commands with unambiguous outcome which surely list
the device address, if it is a disk-like device. Optical drives are not
of interest, but would not harm. They can be filtered out by the consumer
of the commands' output.

The overall goal is to make a script which proposes target candidates for
copying an ISO image file to a USB stick or memory card.

Of course it would be very undesirable if the script would choose the
device from which a Live system has booted and where it might have its
persistence filesystem.
Conventional system disks can be excluded by the kind of filesystems they
bear, or by the type of bus by which they are connected. But a Debian Live
stick intrinsically matches the description of a valid target device.

Given the eagerness of automounters i cannot use the mount status of the
candidate device as decisive criterion.

--------------------------------------------------------------------------
Examples of what the script does for now (mainly by help of lsblk):

  $ xorriso-dd-target -with_sudo \
                      -image_file debian-live-10.0.0-amd64-xfce.iso \
                      -list_all
  sda : NO  : not_usb- has_vfat+ has_ext4- : ATA Samsung SSD 850
  sdb : NO  : not_usb- has_swap- has_ext4- : ATA WDC WD20EFRX-68A
  sdc : YES : usb+ has_iso9660+ has_vfat+ : Intenso Ultra Line
  sdd : NO  : has_iso9660+ has_vfat+ has_ext2- : SanDisk Cruzer
  sde : NO  : has_iso9660+ has_vfat+ size_too_small- : INTENSO USB

When the recognition is trustworthy enough, i plan to enable this:

  $ xorriso-dd-target -with_sudo \
                      -image_file debian-live-10.0.0-amd64-xfce.iso \
                      -DO_WRITE
  sdc
  Would do:   sudo /bin/umount /dev/sdc /dev/sdc1 /dev/sdc2
  Would do:   sudo /bin/dd if='debian-live-10.0.0-amd64-xfce.iso' bs=1M of=/dev/sdc ; sync

This will come to action only if exactly one candidate is found:

  $ xorriso-dd-target -with_sudo \
                      -image_file debian-live-10.0.0-amd64-xfce.iso \
                      -look_for_iso \
                      -DO_WRITE
  sdc
  sdd
  WILL NOT WRITE ! More than one candidate found for target device:
  sdc : YES : usb+ has_iso9660+ has_vfat+ look_for_iso++ : Intenso Ultra Line
  sdd : YES : usb+ has_iso9660+ has_vfat+ has_ext2- look_for_iso++ : SanDisk Cruzer
  HINT: Unplug the unwanted devices from the machine,
        or add the desired name out of {sdc,sdd} as additional argument.

Following the hint:

  $ xorriso-dd-target -with_sudo \
                      -image_file debian-live-10.0.0-amd64-xfce.iso \
                      -look_for_iso \
                      -DO_WRITE \
                      sdc
  sdc : YES : usb+ has_iso9660+ has_vfat+ look_for_iso++
  Would do:   sudo /bin/umount /dev/sdc /dev/sdc1 /dev/sdc2
  Would do:   sudo /bin/dd if='debian-live-10.0.0-amd64-xfce.iso' bs=1M of=/dev/sdc ; sync

Unfug is not allowed:

  $ xorriso-dd-target -with_sudo \
                      -image_file debian-live-10.0.0-amd64-xfce.iso \
                      -look_for_iso \
                      -DO_WRITE \
                      sda
  sda : NO  : not_usb- has_vfat+ has_ext4- no_iso9660-
  NO CANDIDATE FOR TARGET DEVICE AMONG THE GIVEN NAMES !
  Overall available devices:
  sda : NO  : not_usb- has_vfat+ has_ext4- no_iso9660- : ATA Samsung SSD 850
  sdb : NO  : not_usb- has_swap- has_ext4- no_iso9660- : ATA WDC WD20EFRX-68A
  sdc : YES : usb+ has_iso9660+ has_vfat+ look_for_iso++ : Intenso Ultra Line
  sdd : YES : usb+ has_iso9660+ has_vfat+ has_ext2- look_for_iso++ : SanDisk Cruzer
  sde : NO  : has_iso9660+ has_vfat+ look_for_iso++ size_too_small- : INTENSO USB

--------------------------------------------------------------------------

Script text online:
  https://dev.lovelyhq.com/libburnia/libisoburn/raw/master/xorriso-dd-target/xorriso-dd-target

Reasons why i do not write this in C:
- lsblk -o TRAN is a smart piece of software, depending much on kernel
  aspects.
- Restricting superuser power to the commands which really need it
  is desirable but not a simple task in C.
- A C program would have to depend on libblkid directly.

The structure of script xorriso-dd-target looks more like BASIC than like
elaborate shell.
I would not deem an equivalent C program more readable than the script.

My main fear is that sudo can be talked into doing something unexpected
and the user can be tricked into authorizing this by the password.
(Sudo without password or running as superuser would offer easier ways to
 do unwanted things.)
I will try to find shell experts who can review the script for this danger.


Have a nice day :)

Thomas