UNIX, and therefore Linux, recognizes two different kinds of device: random-access block devices (such as disks), and character devices (such as tapes and serial lines) , some of which may be serial, and some random-access. Each supported device is represented in the filesystem as a device file. When you read or write a device file, the data comes from or goes to the device it represents. This way no special programs (and no special application programming methodology, such as catching interrupts or polling a serial port) are necessary to access devices; for example, to send a file to the printer, one could just say
$ cat filename > /dev/lp1 $
Since devices show up as files in the filesystem (in the /dev directory), it is easy to see just what device files exist, using ls or another suitable command. In the output of ls -l, the first column contains the type of the file and its permissions. For example, inspecting a serial device might give
$ ls -l /dev/ttyS0 crw-rw-r-- 1 root dialout 4, 64 Aug 19 18:56 /dev/ttyS0 $
Note that usually all device files exist even though the device itself might be not be installed. So just because you have a file /dev/sda, it doesn't mean that you really do have an SCSI hard disk. Having all the device files makes the installation programs simpler, and makes it easier to add new hardware (there is no need to find out the correct parameters for and create the device files for the new device).
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