How can I test whether a block device is read-only from /sys or /proc?

Question

On a CentOS6 system I have /dev/block/sr0 (read only) and /dev/block/sda (read/write). How can I find out programmatically which devices are read-only? There is a ro file in there, but it's '0' for both devices.

There are some hacks I could use (compare names ^sr vs ^sd, look for cdrom ownership of device node), but I'd like to have a generic solution.

Answer 1

If /sys/block/xxx/ro is 1, then the kernel guarantees it's read-only

$ sudo losetup /dev/loop0 a
$ sudo losetup -r /dev/loop1 b
$ cat /sys/block/loop0/ro
0
$ cat /sys/block/loop1/ro
1

If it's not, then the kernel may let you try to write on it, but the writing may be blocked at a lower level.

For CDs/DVDs, you may find out the type of media (read-only, writable, rewritable, open session...), with cdrdao disk-info.

Now the device may be writable, but you may not have write permission to the device file, but that's another story and can be easily checked with [ -w "$dev" ]

Answer 2

How about look in /proc/mounts (or output of mount command).

my /dev/sr0 entry says ro,....
my /dev/sda1 entry says rw,....

Often your root partition will be mounted using the UUID, or, like in my case, the volume label. Have to be smart about that part, but the cdrom and additional drives are listed as /dev/...

Need to remember, you're looking for RO/RW status on the FILESYSTEM, not the DEVICE.