Linux scripting - how do I wait for CD/DVD medium to load before reading/mounting?

Question

I'm putting together a quick-and-dirty script to read 100s of CD/DVDs onto a NAS. It takes care of ejecting and loading the tray, but I haven't figured out a simple way to wait/block for the medium to become available to the OS after inserting the tray.

• eject -t exits as soon as the tray closes
• mount will fail with No medium found if it's run immediately after eject, but also when it's run 15 seconds later.

The discs are fairly old and some take considerable time to become available, so I'd like to avoid the "sleep for 2 minutes" non-solution. Any ideas? Using ubuntu 18.04.

Answer 1

Will this do?

while ! dd if=/dev/sr2 bs=2048 count=1 of=/dev/null 2>/dev/null; do sleep 1; done

Replace /dev/sr2 with your actual cd/dvd device.

You can make it more robust by checking for errors other than No medium found; untested, since I don't have any broken DVD at hand:

dev=/dev/sr2
while :; do
        err=$(dd if=$dev of=/dev/null bs=2048 status=none count=1 2>&1)
        case $err in
        "dd: failed to open '$dev': No medium found")
                sleep 1 ;;
        '')
                # successfully opened
                break ;;
        *)
                # unexpected error
                # play some SOUND in the speakers
                # and wait for user input to continue
                read wtf ;;
        esac
done

Update:

On drives that support it, linux will automatically close the tray on trying to open the device. While this could probably be disabled by ioctl(CDROM_CLEAR_OPTIONS, CDO_AUTO_CLOSE), I wasn't able to find any command line utility implementing it. Since that forces one to use C, better use C for the whole thing.

So, here's a small application that can check and poll for the cd/dvd drive status.

If called with a single argument:

cdstatus /dev/sr1

it will print the status of /dev/sr1: one of no_disc, tray_open, drive_not_ready or disc_ok.

If called with two arguments:

cdstatus /dev/sr1 1.3

it will poll /dev/sr1 every 1.3 seconds until its status is disk_ok.

It could be easily built with cc -Os -Wall cdstatus.c -s -o cdstatus, provided that the gcc and libc6-dev packages are installed.

cdstatus.c

#include <sys/ioctl.h>
#include <linux/cdrom.h>
#include <fcntl.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <poll.h>
int main(int argc, char **argv){
        int fd, s; int pt = -1;
        if(argc < 2)
                errx(1, "usage: %s /dev/srX [poll_secs] [verbose]", argv[0]);
        if((fd = open(argv[1], O_RDONLY|O_NONBLOCK)) == -1)
                err(1, "open %s", argv[1]);
        if(argc > 2 && ((pt = strtod(argv[2], 0) * 1000) < 1 || pt > 3600000))
                errx(1, "bad timeout '%s'", argv[2]);
redo:
        switch(s = ioctl(fd, CDROM_DRIVE_STATUS, 0)){
        case -1: err(1, "ioctl(CDROM_DRIVE_STATUS)");
        case CDS_NO_INFO: errx(1, "ioctl(CDROM_DRIVE_STATUS) not implemented");
        }
        if(pt < 0 || argc > 3)
                switch(s){
                case CDS_NO_DISC: printf("no_disc\n"); break;
                case CDS_TRAY_OPEN: printf("tray_open\n"); break;
                case CDS_DRIVE_NOT_READY: printf("drive_not_ready\n"); break;
                case CDS_DISC_OK: printf("disc_ok\n"); break;
                default: printf("status=%d\n", s); break;
                }
        if(pt > 0 && s != CDS_DISC_OK){
                if(poll(0, 0, pt) < 0) err(1, "poll");
                goto redo;
        }
        return s != CDS_DISC_OK;
}

Answer 2

In the end, a post in this vaguely-related thread on linuxquestions suggested to use wodim -atip (among other things) which turns out to work great to block until the medium is available.

#!/bin/bash

set -e

DRIVE=${DRIVE:-"sr0"}
DESTROOT=${DESTROOT:-"/mnt/newshare"}
DRV_FULLNAME="$(</sys/class/block/$DRIVE/device/vendor) $(</sys/class/block/$DRIVE/device/model)"

[ -d /tmp/$DRIVE ] || mkdir /tmp/$DRIVE
while true; do
  eject /dev/$DRIVE
  echo "$DRV_FULLNAME is ready."
  read -p "Enter next disc code: " DISC
  if [ -z "$DISC" ]; then
    echo "No disc code = we're done, exiting"
    break
  fi
  eject -t /dev/$DRIVE || {
    read -n1 -r -p "Unable to load tray. Push it in and hit Any key..."
    echo
  }
  wodim dev=/dev/$DRIVE -atip > /dev/null || {
    echo "Medium not detected."
    break
  }
  mount /dev/$DRIVE /tmp/$DRIVE -o ro ||  {
    echo "Couldn't mount medium."
    break
  }
  cp -ia /tmp/$DRIVE/. $DESTROOT/$DISC/ || {
    echo "COPY FAILED. Put it on the read-error pile and continue with next one."
  }
done