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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="DEVPROC"
></A
>Chapter 29. <TT
CLASS="FILENAME"
>/dev</TT
> and <TT
CLASS="FILENAME"
>/proc</TT
></H1
><P
><A
NAME="DEVPROCREF"
></A
></P
><P
>A Linux or UNIX filesystem typically has the
      <TT
CLASS="FILENAME"
>/dev</TT
> and
	<TT
CLASS="FILENAME"
>/proc</TT
> special-purpose
	directories.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="DEVREF1"
></A
>29.1. <TT
CLASS="FILENAME"
>/dev</TT
></H1
><P
><A
NAME="DEVFILEREF"
></A
>The <TT
CLASS="FILENAME"
>/dev</TT
> directory contains entries for
	the <I
CLASS="FIRSTTERM"
>physical devices</I
> that may or may not
	be present in the hardware.

	   <A
NAME="AEN19045"
HREF="#FTN.AEN19045"
>[1]</A
>
	
	Appropriately enough, these are called <I
CLASS="FIRSTTERM"
>device
	files</I
>.
	As an example, the hard drive partitions containing
	the mounted filesystem(s) have entries in <TT
CLASS="FILENAME"
>/dev</TT
>, as <A
HREF="system.html#DFREF"
>df</A
> shows.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>df</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>Filesystem           1k-blocks      Used Available Use%
 Mounted on
 /dev/hda6               495876    222748    247527  48% /
 /dev/hda1                50755      3887     44248   9% /boot
 /dev/hda8               367013     13262    334803   4% /home
 /dev/hda5              1714416   1123624    503704  70% /usr</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
        </P
><P
><A
NAME="LOOPBACKREF"
></A
>Among other things, the <TT
CLASS="FILENAME"
>/dev</TT
> directory
	contains <I
CLASS="FIRSTTERM"
>loopback</I
> devices, such as
	<TT
CLASS="FILENAME"
>/dev/loop0</TT
>. A loopback device is a gimmick
	that allows an ordinary file to be accessed as if it were a
	block device.

	  <A
NAME="AEN19065"
HREF="#FTN.AEN19065"
>[2]</A
>

	This permits mounting an entire filesystem within a
	single large file. See <A
HREF="system.html#CREATEFS"
>Example 17-8</A
> and <A
HREF="system.html#ISOMOUNTREF"
>Example 17-7</A
>.</P
><P
>A few of the pseudo-devices in <TT
CLASS="FILENAME"
>/dev</TT
>
	 have other specialized uses, such as <A
HREF="zeros.html#ZEROSREF"
><TT
CLASS="FILENAME"
>/dev/null</TT
></A
>, <A
HREF="zeros.html#ZEROSREF1"
><TT
CLASS="FILENAME"
>/dev/zero</TT
></A
>, <A
HREF="randomvar.html#URANDOMREF"
><TT
CLASS="FILENAME"
>/dev/urandom</TT
></A
>,
	 <TT
CLASS="FILENAME"
>/dev/sda1</TT
> (hard drive partition),
	 <TT
CLASS="FILENAME"
>/dev/udp</TT
> (<I
CLASS="FIRSTTERM"
>User
	 Datagram Packet</I
> port), and <A
HREF="devproc.html#DEVTCP"
><TT
CLASS="FILENAME"
>/dev/tcp</TT
></A
>.</P
><P
>For instance:</P
><P
>To manually <A
HREF="system.html#MOUNTREF"
>mount</A
>
         a USB flash drive, append the following line to
         <A
HREF="system.html#FSTABREF"
><TT
CLASS="FILENAME"
>/etc/fstab</TT
></A
>.

	      <A
NAME="AEN19093"
HREF="#FTN.AEN19093"
>[3]</A
>

	   <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;/dev/sda1    /mnt/flashdrive    auto    noauto,user,noatime    0 0</PRE
></TD
></TR
></TABLE
>

         (See also <A
HREF="contributed-scripts.html#USBINST"
>Example A-23</A
>.)</P
><P
>Checking whether a disk is in the CD-burner
         (soft-linked to <TT
CLASS="FILENAME"
>/dev/hdc</TT
>):

	 <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;head -1 /dev/hdc
   2&nbsp;
   3&nbsp;
   4&nbsp;#  head: cannot open '/dev/hdc' for reading: No medium found
   5&nbsp;#  (No disc in the drive.)
   6&nbsp;
   7&nbsp;#  head: error reading '/dev/hdc': Input/output error
   8&nbsp;#  (There is a disk in the drive, but it can't be read;
   9&nbsp;#+  possibly it's an unrecorded CDR blank.)   
  10&nbsp;
  11&nbsp;#  Stream of characters and assorted gibberish
  12&nbsp;#  (There is a pre-recorded disk in the drive,
  13&nbsp;#+ and this is raw output -- a stream of ASCII and binary data.)
  14&nbsp;#  Here we see the wisdom of using 'head' to limit the output
  15&nbsp;#+ to manageable proportions, rather than 'cat' or something similar.
  16&nbsp;
  17&nbsp;
  18&nbsp;#  Now, it's just a matter of checking/parsing the output and taking
  19&nbsp;#+ appropriate action.</PRE
></TD
></TR
></TABLE
></P
><P
><A
NAME="SOCKETREF"
></A
></P
><P
>When executing a command on a
         <TT
CLASS="FILENAME"
>/dev/tcp/$host/$port</TT
> pseudo-device file, Bash
	 opens a TCP connection to the associated
	 <I
CLASS="FIRSTTERM"
>socket</I
>.</P
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN19112"
></A
><P
>A <I
CLASS="FIRSTTERM"
>socket</I
> is a
	      communications node associated with a specific I/O
	      port. (This is analogous to a <I
CLASS="FIRSTTERM"
>hardware
	      socket</I
>, or <I
CLASS="FIRSTTERM"
>receptacle</I
>,
	      for a connecting cable.) It permits data transfer between
	      hardware devices on the same machine, between machines
	      on the same network, between machines across different
	      networks, and, of course, between machines at different
	      locations on the Internet.</P
></DIV
></TD
></TR
></TABLE
><P
><A
NAME="NPREF"
></A
>The following examples assume an active Internet
         connection.</P
><P
>Getting the time from <TT
CLASS="FILENAME"
>nist.gov</TT
>:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cat &#60;/dev/tcp/time.nist.gov/13</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>53082 04-03-18 04:26:54 68 0 0 502.3 UTC(NIST) *</TT
>
 	      </PRE
></TD
></TR
></TABLE
><P
>[Mark contributed this example.]</P
><P
>Generalizing the above into a script:</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# This script must run with root permissions.
   3&nbsp;
   4&nbsp;URL="time.nist.gov/13"
   5&nbsp;
   6&nbsp;Time=$(cat &#60;/dev/tcp/"$URL")
   7&nbsp;UTC=$(echo "$Time" | awk '{print$3}')   # Third field is UTC (GMT) time.
   8&nbsp;# Exercise: modify this for different time zones.
   9&nbsp;
  10&nbsp;echo "UTC Time = "$UTC""</PRE
></TD
></TR
></TABLE
></P
><P
><A
NAME="NW001"
></A
>Downloading a URL:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>exec 5&#60;&#62;/dev/tcp/www.net.cn/80</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo -e "GET / HTTP/1.0\n" &#62;&#38;5</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cat &#60;&#38;5</B
></TT
>
 	      </PRE
></TD
></TR
></TABLE
><P
>[Thanks, Mark and Mihai Maties.]</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="DEVTCP"
></A
><P
><B
>Example 29-1. Using <TT
CLASS="FILENAME"
>/dev/tcp</TT
> for
	      troubleshooting</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# dev-tcp.sh: /dev/tcp redirection to check Internet connection.
   3&nbsp;
   4&nbsp;# Script by Troy Engel.
   5&nbsp;# Used with permission.
   6&nbsp; 
   7&nbsp;TCP_HOST=news-15.net       # A known spam-friendly ISP.
   8&nbsp;TCP_PORT=80                # Port 80 is http.
   9&nbsp;  
  10&nbsp;# Try to connect. (Somewhat similar to a 'ping' . . .) 
  11&nbsp;echo "HEAD / HTTP/1.0" &#62;/dev/tcp/${TCP_HOST}/${TCP_PORT}
  12&nbsp;MYEXIT=$?
  13&nbsp;
  14&nbsp;: &#60;&#60;EXPLANATION
  15&nbsp;If bash was compiled with --enable-net-redirections, it has the capability of
  16&nbsp;using a special character device for both TCP and UDP redirections. These
  17&nbsp;redirections are used identically as STDIN/STDOUT/STDERR. The device entries
  18&nbsp;are 30,36 for /dev/tcp:
  19&nbsp;
  20&nbsp;  mknod /dev/tcp c 30 36
  21&nbsp;
  22&nbsp;&#62;From the bash reference:
  23&nbsp;/dev/tcp/host/port
  24&nbsp;    If host is a valid hostname or Internet address, and port is an integer
  25&nbsp;port number or service name, Bash attempts to open a TCP connection to the
  26&nbsp;corresponding socket.
  27&nbsp;EXPLANATION
  28&nbsp;
  29&nbsp;   
  30&nbsp;if [ "X$MYEXIT" = "X0" ]; then
  31&nbsp;  echo "Connection successful. Exit code: $MYEXIT"
  32&nbsp;else
  33&nbsp;  echo "Connection unsuccessful. Exit code: $MYEXIT"
  34&nbsp;fi
  35&nbsp;
  36&nbsp;exit $MYEXIT</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="MUSICSCR"
></A
><P
><B
>Example 29-2. Playing music</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# music.sh
   3&nbsp;
   4&nbsp;# Music without external files
   5&nbsp;
   6&nbsp;# Author: Antonio Macchi
   7&nbsp;# Used in ABS Guide with permission.
   8&nbsp;
   9&nbsp;
  10&nbsp;#  /dev/dsp default = 8000 frames per second, 8 bits per frame (1 byte),
  11&nbsp;#+ 1 channel (mono)
  12&nbsp;
  13&nbsp;duration=2000       # If 8000 bytes = 1 second, then 2000 = 1/4 second.
  14&nbsp;volume=$'\xc0'      # Max volume = \xff (or \x00).
  15&nbsp;mute=$'\x80'        # No volume = \x80 (the middle).
  16&nbsp;
  17&nbsp;function mknote ()  # $1=Note Hz in bytes (e.g. A = 440Hz ::
  18&nbsp;{                   #+ 8000 fps / 440 = 16 :: A = 16 bytes per second)
  19&nbsp;  for t in `seq 0 $duration`
  20&nbsp;  do
  21&nbsp;    test $(( $t % $1 )) = 0 &#38;&#38; echo -n $volume || echo -n $mute
  22&nbsp;  done
  23&nbsp;}
  24&nbsp;
  25&nbsp;e=`mknote 49`
  26&nbsp;g=`mknote 41`
  27&nbsp;a=`mknote 36`
  28&nbsp;b=`mknote 32`
  29&nbsp;c=`mknote 30`
  30&nbsp;cis=`mknote 29`
  31&nbsp;d=`mknote 27`
  32&nbsp;e2=`mknote 24`
  33&nbsp;n=`mknote 32767`
  34&nbsp;# European notation.
  35&nbsp;
  36&nbsp;echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \
  37&nbsp;$n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" &#62; /dev/dsp
  38&nbsp;# dsp = Digital Signal Processor
  39&nbsp;
  40&nbsp;exit      # A "bonny" example of an elegant shell script!</PRE
></TD
></TR
></TABLE
><HR></DIV
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN19045"
HREF="devproc.html#AEN19045"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The entries in <TT
CLASS="FILENAME"
>/dev</TT
>
	     provide mount points for physical and virtual devices. These
	     entries use very little drive space.</P
><P
>Some devices, such as <TT
CLASS="FILENAME"
>/dev/null</TT
>,
	   <TT
CLASS="FILENAME"
>/dev/zero</TT
>,
	   and <TT
CLASS="FILENAME"
>/dev/urandom</TT
> are virtual. They
	   are not actual physical devices and exist only in
	   software.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN19065"
HREF="devproc.html#AEN19065"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><A
NAME="BLOCKDEVREF"
></A
>A <I
CLASS="FIRSTTERM"
>block
	    device</I
> reads and/or writes data in chunks,
	    or <I
CLASS="FIRSTTERM"
>blocks</I
>, in contrast to a <A
NAME="CHARDEVREF"
></A
><I
CLASS="FIRSTTERM"
>character device</I
>,
	    which acesses data in <I
CLASS="FIRSTTERM"
>character</I
>
	    units. Examples of block devices are hard drives, CDROM
	    drives, and flash drives. Examples of character devices are
	    keyboards, modems, sound cards.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN19093"
HREF="devproc.html#AEN19093"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Of course, the mount point
	        <TT
CLASS="FILENAME"
>/mnt/flashdrive</TT
> must exist. If not,
		then, as <I
CLASS="FIRSTTERM"
>root</I
>, <B
CLASS="COMMAND"
>mkdir
		/mnt/flashdrive</B
>.</P
><P
>To actually mount the drive, use the following command:
	       <B
CLASS="COMMAND"
>mount /mnt/flashdrive</B
></P
><P
>Newer Linux distros automount flash drives in the
	       <TT
CLASS="FILENAME"
>/media</TT
>
	       directory without user intervention.</P
></TD
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