mount -a -t ext2 -O _netdevThe -t and -O options are cumulative in effect; that is, the command mounts all ext2 filesystems with the _netdev option, not all filesystems that are either ext2 or have the _netdev option specified.
mount /tmp/fdimage /mnt -t msdos -o loop=/dev/loop3,blocksize=1024This will set up the loop device /dev/loop3 to correspond to the file /tmp/fdimage, and then mount this device on /mnt.
mount --rbind olddir newdirThis call attaches only (part of) a single filesystem, not possible submounts. The entire file hierarchy including submounts is attached a second place using
mount -o remount,rw /dev/foo /dirAfter this call all old mount options are replaced and arbitrary stuff from fstab is ignored, except the loop=option which is internally generated and maintained by the mount command