295 lines
5.1 KiB
HTML
295 lines
5.1 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
|
|
<HTML
|
|
><HEAD
|
|
><TITLE
|
|
>Two kinds of devices</TITLE
|
|
><META
|
|
NAME="GENERATOR"
|
|
CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
|
|
REL="HOME"
|
|
TITLE="Linux System Administrators Guide"
|
|
HREF="index.html"><LINK
|
|
REL="UP"
|
|
TITLE="Using Disks and Other Storage Media"
|
|
HREF="disk-usage.html"><LINK
|
|
REL="PREVIOUS"
|
|
TITLE="Using Disks and Other Storage Media"
|
|
HREF="disk-usage.html"><LINK
|
|
REL="NEXT"
|
|
TITLE="Hard disks"
|
|
HREF="hard-disk.html"></HEAD
|
|
><BODY
|
|
CLASS="SECT1"
|
|
BGCOLOR="#FFFFFF"
|
|
TEXT="#000000"
|
|
LINK="#0000FF"
|
|
VLINK="#840084"
|
|
ALINK="#0000FF"
|
|
><DIV
|
|
CLASS="NAVHEADER"
|
|
><TABLE
|
|
SUMMARY="Header navigation table"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TH
|
|
COLSPAN="3"
|
|
ALIGN="center"
|
|
>Linux System Administrators Guide: </TH
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="left"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="disk-usage.html"
|
|
ACCESSKEY="P"
|
|
>Prev</A
|
|
></TD
|
|
><TD
|
|
WIDTH="80%"
|
|
ALIGN="center"
|
|
VALIGN="bottom"
|
|
>Chapter 5. Using Disks and Other Storage Media</TD
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="right"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="hard-disk.html"
|
|
ACCESSKEY="N"
|
|
>Next</A
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"></DIV
|
|
><DIV
|
|
CLASS="SECT1"
|
|
><H1
|
|
CLASS="SECT1"
|
|
><A
|
|
NAME="BLOCK-CHAR-DEV"
|
|
></A
|
|
>5.1. Two kinds of devices</H1
|
|
><P
|
|
>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 <I
|
|
CLASS="GLOSSTERM"
|
|
>device file</I
|
|
>.
|
|
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
|
|
|
|
<TABLE
|
|
BORDER="1"
|
|
BGCOLOR="#E0E0E0"
|
|
WIDTH="100%"
|
|
><TR
|
|
><TD
|
|
><FONT
|
|
COLOR="#000000"
|
|
><PRE
|
|
CLASS="SCREEN"
|
|
><TT
|
|
CLASS="PROMPT"
|
|
>$</TT
|
|
> <TT
|
|
CLASS="USERINPUT"
|
|
><B
|
|
>cat filename > /dev/lp1</B
|
|
></TT
|
|
>
|
|
<TT
|
|
CLASS="PROMPT"
|
|
>$</TT
|
|
></PRE
|
|
></FONT
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
>
|
|
|
|
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 <B
|
|
CLASS="COMMAND"
|
|
>lpr</B
|
|
>
|
|
). 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.</P
|
|
><P
|
|
>Since devices show up as files in the filesystem (in the
|
|
<TT
|
|
CLASS="FILENAME"
|
|
>/dev</TT
|
|
>
|
|
directory), it is easy to see just what
|
|
device files exist, using <B
|
|
CLASS="COMMAND"
|
|
>ls</B
|
|
> or another suitable
|
|
command. In the output of <B
|
|
CLASS="COMMAND"
|
|
>ls -l</B
|
|
>, the first
|
|
column contains the type of the file and its permissions. For
|
|
example, inspecting a serial device might give
|
|
|
|
<TABLE
|
|
BORDER="1"
|
|
BGCOLOR="#E0E0E0"
|
|
WIDTH="100%"
|
|
><TR
|
|
><TD
|
|
><FONT
|
|
COLOR="#000000"
|
|
><PRE
|
|
CLASS="SCREEN"
|
|
> <TT
|
|
CLASS="PROMPT"
|
|
>$</TT
|
|
> <TT
|
|
CLASS="USERINPUT"
|
|
><B
|
|
>ls -l /dev/ttyS0</B
|
|
></TT
|
|
>
|
|
<TT
|
|
CLASS="COMPUTEROUTPUT"
|
|
> crw-rw-r-- 1 root dialout 4, 64 Aug 19 18:56 /dev/ttyS0
|
|
</TT
|
|
>
|
|
<TT
|
|
CLASS="PROMPT"
|
|
>$</TT
|
|
>
|
|
</PRE
|
|
></FONT
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
>
|
|
|
|
The first character in the first column, i.e.,
|
|
`<TT
|
|
CLASS="LITERAL"
|
|
>c</TT
|
|
>' in <TT
|
|
CLASS="LITERAL"
|
|
>crw-rw-rw-</TT
|
|
> above, tells
|
|
an informed user the type of the file, in this case a character
|
|
device. For ordinary files, the first character is
|
|
`<TT
|
|
CLASS="LITERAL"
|
|
>-</TT
|
|
>', for directories it is
|
|
`<TT
|
|
CLASS="LITERAL"
|
|
>d</TT
|
|
>', and for block devices
|
|
`<TT
|
|
CLASS="LITERAL"
|
|
>b</TT
|
|
>'; see the <B
|
|
CLASS="COMMAND"
|
|
>ls</B
|
|
> man page
|
|
for further information.</P
|
|
><P
|
|
>Note that usually all device files exist even though the
|
|
device itself might be not be installed. So just because you have a
|
|
file <TT
|
|
CLASS="FILENAME"
|
|
>/dev/sda</TT
|
|
>, 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).</P
|
|
></DIV
|
|
><DIV
|
|
CLASS="NAVFOOTER"
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"><TABLE
|
|
SUMMARY="Footer navigation table"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="disk-usage.html"
|
|
ACCESSKEY="P"
|
|
>Prev</A
|
|
></TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="index.html"
|
|
ACCESSKEY="H"
|
|
>Home</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="hard-disk.html"
|
|
ACCESSKEY="N"
|
|
>Next</A
|
|
></TD
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
>Using Disks and Other Storage Media</TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="disk-usage.html"
|
|
ACCESSKEY="U"
|
|
>Up</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
>Hard disks</TD
|
|
></TR
|
|
></TABLE
|
|
></DIV
|
|
></BODY
|
|
></HTML
|
|
> |