On Fri, Feb 29, 2008 at 8:01 PM, James G. Sack (jim) <[EMAIL PROTECTED]> wrote:
> Carl Lowenstein wrote:
> > When I put a blank CD-R in the drive, after several seconds an icon
> > labeled "Blank CD-R Disc" appears on the desktop. Is there some
> > command-line way to recognize this?
> >
> > I am trying to make my CD-burning shell script more foolproof. If I
> > start the script too soon, the call to /usr/bin/cdrecord bombs out
> > with a message about "non-writeable disc". I think this really means
> > that the system hasn't gotten around to noticing the presence of the
> > blank. If I wait until the icon is visible, then there is no problem.
> >
>
> I have absolutely no reason to believe this is the right thing to do,
> but a couple of 'messin-around' experiments shows that
>
> dd if=/dev/cdrom bs=512 count=1 of=/dev/null
> mount /dev/cdrom /mnt
> both contain "No medium found" if the drive is empty (no disc present)
> file -s /dev/sr0
> reports /dev/sr0: writable, no read permission
>
> With a blank writable disc,
> dd reports Input/Output error
> mount reports a different error
> /dev/sr0: reports empty
>
> All of them first close the tray if it is open.
OK, you seem to have found something useful. You must have one of
those new-fangled kernels that maps the CD drive to /dev/sr0. My CD
drive is /dev/hdc, symlinked to /dev/cdrom.
# blank CD in drive, either open or shut
[EMAIL PROTECTED] tmp]$ file -sL /dev/cdrom; echo $?
/dev/cdrom: ERROR: cannot read `/dev/cdrom' (Input/output error)
0
# if drive is open, closes it. Does not return until after disc icon
is on screen.
# nothing in drive, either open or shut
[EMAIL PROTECTED] tmp]$ file -sL /dev/cdrom; echo $?
/dev/cdrom: writable, no read permission
0
# if drive is open, closes it. Returns immediately, at least to the
human observer.
Note that exit status of file command is 0 regardless, so that isn't much help.
But all is not lost:
[EMAIL PROTECTED] tmp]$ file -sL /dev/cdrom | grep -q "cannot read"; echo $?
1
[EMAIL PROTECTED] tmp]$ file -sL /dev/cdrom | grep -q "cannot read"; echo $?
0
I observe by the behavior of grep(1) that file(1) seems to write to
stdout. Interesting that file(1) should report "writable, no read
permission" for an empty drive, and "cannot read" for a blank disc.
Thanks for the ideas, I had run dry. No guarantee that this will work
for newer kernels etc. Will try newer distro's tomorrow some time.
Late-breaking news. Newer system, with blank CD in drive. file(1)
reports "ERROR: cannot read ... "
If run again, it reports "empty". So best to grep for "writable, no
read permission", I suppose.
carl
--
carl lowenstein marine physical lab u.c. san diego
[EMAIL PROTECTED]
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