UNIX, and therefore Linux, recognises 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
$ |
and the contents of the file are printed (the file must, of course,
be in a form that the printer understands). However, since it is
not a good idea to have several people cat their files to the
printer at the same time, one usually uses a special program to send
the files to be printed (usually
lpr). This
program makes sure that only one file is being printed at a time,
and will automatically send files to the printer as soon as it
finishes with the previous file. Something similar is needed for
most devices. In fact, one seldom needs to worry
about device files at all.
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
$ |
The first character in the first column, i.e.,
`
c' in
crw-rw-rw- above, tells
an informed user the type of the file, in this case a character
device. For ordinary files, the first character is
`
-', for directories it is
`
d', and for block devices
`
b'; see the
ls man page
for further information.
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).
