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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="filearchiv.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
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>Chapter 16. External Filters, Programs and Commands</TD
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><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="COMMUNICATIONS"
></A
>16.6. Communications Commands</H1
><P
>Certain of the following commands find use in
	  network data transfer and analysis, as well as in
	  <A
HREF="writingscripts.html#CSPAMMERS"
>chasing spammers</A
>.</P
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="COMMUNINFO1"
></A
>Information and Statistics</B
></P
><DL
><DT
><A
NAME="HOSTREF"
></A
><B
CLASS="COMMAND"
>host</B
></DT
><DD
><P
>Searches for information about an Internet host by name or
	      IP address, using DNS.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>host surfacemail.com</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>surfacemail.com. has address 202.92.42.236</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="IPCALCREF"
></A
><B
CLASS="COMMAND"
>ipcalc</B
></DT
><DD
><P
>Displays IP information for a host.
	      With the <TT
CLASS="OPTION"
>-h</TT
> option,
	      <B
CLASS="COMMAND"
>ipcalc</B
> does a reverse DNS lookup, finding
	      the name of the host (server) from the IP address.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ipcalc -h 202.92.42.236</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>HOSTNAME=surfacemail.com</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="NSLOOKUPREF"
></A
><B
CLASS="COMMAND"
>nslookup</B
></DT
><DD
><P
>Do an Internet <SPAN
CLASS="QUOTE"
>"name server lookup"</SPAN
>
	      on a host by IP address. This is essentially equivalent
	      to <B
CLASS="COMMAND"
>ipcalc -h</B
> or <B
CLASS="COMMAND"
>dig -x
	      </B
>. The command may be run either interactively
	      or noninteractively, i.e., from within a script.</P
><P
>The <B
CLASS="COMMAND"
>nslookup</B
> command has allegedly
	      been <SPAN
CLASS="QUOTE"
>"deprecated,"</SPAN
> but it is still useful.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>nslookup -sil 66.97.104.180</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>nslookup kuhleersparnis.ch
 Server:         135.116.137.2
 Address:        135.116.137.2#53

 Non-authoritative answer:
 Name:   kuhleersparnis.ch</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	  </P
></DD
><DT
><A
NAME="DIGREF"
></A
><B
CLASS="COMMAND"
>dig</B
></DT
><DD
><P
><B
CLASS="COMMAND"
>D</B
>omain <B
CLASS="COMMAND"
>I</B
>nformation
	      <B
CLASS="COMMAND"
>G</B
>roper. Similar to
	      <B
CLASS="COMMAND"
>nslookup</B
>, <I
CLASS="FIRSTTERM"
>dig</I
> does
	      an Internet <I
CLASS="FIRSTTERM"
>name server lookup</I
> on a host.
	      May be run from the command-line or from within a script.</P
><P
>Some interesting options to <I
CLASS="FIRSTTERM"
>dig</I
> are
	      <TT
CLASS="OPTION"
>+time=N</TT
> for setting a query timeout to
	      <TT
CLASS="PARAMETER"
><I
>N</I
></TT
> seconds, <TT
CLASS="OPTION"
>+nofail</TT
> for
	      continuing to query servers until a reply is received, and
	      <TT
CLASS="OPTION"
>-x</TT
> for doing a reverse address lookup.</P
><P
>Compare the output of <B
CLASS="COMMAND"
>dig -x</B
> with
	      <B
CLASS="COMMAND"
>ipcalc -h</B
> and
	      <B
CLASS="COMMAND"
>nslookup</B
>.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>dig -x 81.9.6.2</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>;; Got answer:
 ;; -&#62;&#62;HEADER&#60;&#60;- opcode: QUERY, status: NXDOMAIN, id: 11649
 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

 ;; QUESTION SECTION:
 ;2.6.9.81.in-addr.arpa.         IN      PTR

 ;; AUTHORITY SECTION:
 6.9.81.in-addr.arpa.    3600    IN      SOA     ns.eltel.net. noc.eltel.net.
 2002031705 900 600 86400 3600

 ;; Query time: 537 msec
 ;; SERVER: 135.116.137.2#53(135.116.137.2)
 ;; WHEN: Wed Jun 26 08:35:24 2002
 ;; MSG SIZE  rcvd: 91</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	  </P
><P
><A
NAME="SPAMLOOKUP_0"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SPAMLOOKUP"
></A
><P
><B
>Example 16-40. Finding out where to report a spammer</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# spam-lookup.sh: Look up abuse contact to report a spammer.
   3&nbsp;# Thanks, Michael Zick.
   4&nbsp;
   5&nbsp;# Check for command-line arg.
   6&nbsp;ARGCOUNT=1
   7&nbsp;E_WRONGARGS=85
   8&nbsp;if [ $# -ne "$ARGCOUNT" ]
   9&nbsp;then
  10&nbsp;  echo "Usage: `basename $0` domain-name"
  11&nbsp;  exit $E_WRONGARGS
  12&nbsp;fi
  13&nbsp;
  14&nbsp;
  15&nbsp;dig +short $1.contacts.abuse.net -c in -t txt
  16&nbsp;# Also try:
  17&nbsp;#     dig +nssearch $1
  18&nbsp;#     Tries to find "authoritative name servers" and display SOA records.
  19&nbsp;
  20&nbsp;# The following also works:
  21&nbsp;#     whois -h whois.abuse.net $1
  22&nbsp;#           ^^ ^^^^^^^^^^^^^^^  Specify host.  
  23&nbsp;#     Can even lookup multiple spammers with this, i.e."
  24&nbsp;#     whois -h whois.abuse.net $spamdomain1 $spamdomain2 . . .
  25&nbsp;
  26&nbsp;
  27&nbsp;#  Exercise:
  28&nbsp;#  --------
  29&nbsp;#  Expand the functionality of this script
  30&nbsp;#+ so that it automatically e-mails a notification
  31&nbsp;#+ to the responsible ISP's contact address(es).
  32&nbsp;#  Hint: use the "mail" command.
  33&nbsp;
  34&nbsp;exit $?
  35&nbsp;
  36&nbsp;# spam-lookup.sh chinatietong.com
  37&nbsp;#                A known spam domain.
  38&nbsp;
  39&nbsp;# "crnet_mgr@chinatietong.com"
  40&nbsp;# "crnet_tec@chinatietong.com"
  41&nbsp;# "postmaster@chinatietong.com"
  42&nbsp;
  43&nbsp;
  44&nbsp;#  For a more elaborate version of this script,
  45&nbsp;#+ see the SpamViz home page, http://www.spamviz.net/index.html.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="ISSPAMMER_0"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ISSPAMMER"
></A
><P
><B
>Example 16-41. Analyzing a spam domain</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#! /bin/bash
   2&nbsp;# is-spammer.sh: Identifying spam domains
   3&nbsp;
   4&nbsp;# $Id: is-spammer, v 1.4 2004/09/01 19:37:52 mszick Exp $
   5&nbsp;# Above line is RCS ID info.
   6&nbsp;#
   7&nbsp;#  This is a simplified version of the "is_spammer.bash
   8&nbsp;#+ script in the Contributed Scripts appendix.
   9&nbsp;
  10&nbsp;# is-spammer &#60;domain.name&#62;
  11&nbsp;
  12&nbsp;# Uses an external program: 'dig'
  13&nbsp;# Tested with version: 9.2.4rc5
  14&nbsp;
  15&nbsp;# Uses functions.
  16&nbsp;# Uses IFS to parse strings by assignment into arrays.
  17&nbsp;# And even does something useful: checks e-mail blacklists.
  18&nbsp;
  19&nbsp;# Use the domain.name(s) from the text body:
  20&nbsp;# http://www.good_stuff.spammer.biz/just_ignore_everything_else
  21&nbsp;#                       ^^^^^^^^^^^
  22&nbsp;# Or the domain.name(s) from any e-mail address:
  23&nbsp;# Really_Good_Offer@spammer.biz
  24&nbsp;#
  25&nbsp;# as the only argument to this script.
  26&nbsp;#(PS: have your Inet connection running)
  27&nbsp;#
  28&nbsp;# So, to invoke this script in the above two instances:
  29&nbsp;#       is-spammer.sh spammer.biz
  30&nbsp;
  31&nbsp;
  32&nbsp;# Whitespace == :Space:Tab:Line Feed:Carriage Return:
  33&nbsp;WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D'
  34&nbsp;
  35&nbsp;# No Whitespace == Line Feed:Carriage Return
  36&nbsp;No_WSP=$'\x0A'$'\x0D'
  37&nbsp;
  38&nbsp;# Field separator for dotted decimal ip addresses
  39&nbsp;ADR_IFS=${No_WSP}'.'
  40&nbsp;
  41&nbsp;# Get the dns text resource record.
  42&nbsp;# get_txt &#60;error_code&#62; &#60;list_query&#62;
  43&nbsp;get_txt() {
  44&nbsp;
  45&nbsp;    # Parse $1 by assignment at the dots.
  46&nbsp;    local -a dns
  47&nbsp;    IFS=$ADR_IFS
  48&nbsp;    dns=( $1 )
  49&nbsp;    IFS=$WSP_IFS
  50&nbsp;    if [ "${dns[0]}" == '127' ]
  51&nbsp;    then
  52&nbsp;        # See if there is a reason.
  53&nbsp;        echo $(dig +short $2 -t txt)
  54&nbsp;    fi
  55&nbsp;}
  56&nbsp;
  57&nbsp;# Get the dns address resource record.
  58&nbsp;# chk_adr &#60;rev_dns&#62; &#60;list_server&#62;
  59&nbsp;chk_adr() {
  60&nbsp;    local reply
  61&nbsp;    local server
  62&nbsp;    local reason
  63&nbsp;
  64&nbsp;    server=${1}${2}
  65&nbsp;    reply=$( dig +short ${server} )
  66&nbsp;
  67&nbsp;    # If reply might be an error code . . .
  68&nbsp;    if [ ${#reply} -gt 6 ]
  69&nbsp;    then
  70&nbsp;        reason=$(get_txt ${reply} ${server} )
  71&nbsp;        reason=${reason:-${reply}}
  72&nbsp;    fi
  73&nbsp;    echo ${reason:-' not blacklisted.'}
  74&nbsp;}
  75&nbsp;
  76&nbsp;# Need to get the IP address from the name.
  77&nbsp;echo 'Get address of: '$1
  78&nbsp;ip_adr=$(dig +short $1)
  79&nbsp;dns_reply=${ip_adr:-' no answer '}
  80&nbsp;echo ' Found address: '${dns_reply}
  81&nbsp;
  82&nbsp;# A valid reply is at least 4 digits plus 3 dots.
  83&nbsp;if [ ${#ip_adr} -gt 6 ]
  84&nbsp;then
  85&nbsp;    echo
  86&nbsp;    declare query
  87&nbsp;
  88&nbsp;    # Parse by assignment at the dots.
  89&nbsp;    declare -a dns
  90&nbsp;    IFS=$ADR_IFS
  91&nbsp;    dns=( ${ip_adr} )
  92&nbsp;    IFS=$WSP_IFS
  93&nbsp;
  94&nbsp;    # Reorder octets into dns query order.
  95&nbsp;    rev_dns="${dns[3]}"'.'"${dns[2]}"'.'"${dns[1]}"'.'"${dns[0]}"'.'
  96&nbsp;
  97&nbsp;# See: http://www.spamhaus.org (Conservative, well maintained)
  98&nbsp;    echo -n 'spamhaus.org says: '
  99&nbsp;    echo $(chk_adr ${rev_dns} 'sbl-xbl.spamhaus.org')
 100&nbsp;
 101&nbsp;# See: http://ordb.org (Open mail relays)
 102&nbsp;    echo -n '   ordb.org  says: '
 103&nbsp;    echo $(chk_adr ${rev_dns} 'relays.ordb.org')
 104&nbsp;
 105&nbsp;# See: http://www.spamcop.net/ (You can report spammers here)
 106&nbsp;    echo -n ' spamcop.net says: '
 107&nbsp;    echo $(chk_adr ${rev_dns} 'bl.spamcop.net')
 108&nbsp;
 109&nbsp;# # # other blacklist operations # # #
 110&nbsp;
 111&nbsp;# See: http://cbl.abuseat.org.
 112&nbsp;    echo -n ' abuseat.org says: '
 113&nbsp;    echo $(chk_adr ${rev_dns} 'cbl.abuseat.org')
 114&nbsp;
 115&nbsp;# See: http://dsbl.org/usage (Various mail relays)
 116&nbsp;    echo
 117&nbsp;    echo 'Distributed Server Listings'
 118&nbsp;    echo -n '       list.dsbl.org says: '
 119&nbsp;    echo $(chk_adr ${rev_dns} 'list.dsbl.org')
 120&nbsp;
 121&nbsp;    echo -n '   multihop.dsbl.org says: '
 122&nbsp;    echo $(chk_adr ${rev_dns} 'multihop.dsbl.org')
 123&nbsp;
 124&nbsp;    echo -n 'unconfirmed.dsbl.org says: '
 125&nbsp;    echo $(chk_adr ${rev_dns} 'unconfirmed.dsbl.org')
 126&nbsp;
 127&nbsp;else
 128&nbsp;    echo
 129&nbsp;    echo 'Could not use that address.'
 130&nbsp;fi
 131&nbsp;
 132&nbsp;exit 0
 133&nbsp;
 134&nbsp;# Exercises:
 135&nbsp;# --------
 136&nbsp;
 137&nbsp;# 1) Check arguments to script,
 138&nbsp;#    and exit with appropriate error message if necessary.
 139&nbsp;
 140&nbsp;# 2) Check if on-line at invocation of script,
 141&nbsp;#    and exit with appropriate error message if necessary.
 142&nbsp;
 143&nbsp;# 3) Substitute generic variables for "hard-coded" BHL domains.
 144&nbsp;
 145&nbsp;# 4) Set a time-out for the script using the "+time=" option
 146&nbsp;     to the 'dig' command.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>For a much more elaborate version of the above script, see 
	    <A
HREF="contributed-scripts.html#ISSPAMMER2"
>Example A-28</A
>.</P
></DD
><DT
><A
NAME="TRACEROUTEREF"
></A
><B
CLASS="COMMAND"
>traceroute</B
></DT
><DD
><P
>Trace the route taken by packets sent to a remote host. This
	      command works within a LAN, WAN, or over the
	      Internet. The remote host may be specified by an IP
	      address. The output of this command may be filtered
	      by <A
HREF="textproc.html#GREPREF"
>grep</A
> or <A
HREF="sedawk.html#SEDREF"
>sed</A
> in a pipe.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>traceroute 81.9.6.2</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets
 1  tc43.xjbnnbrb.com (136.30.178.8)  191.303 ms  179.400 ms  179.767 ms
 2  or0.xjbnnbrb.com (136.30.178.1)  179.536 ms  179.534 ms  169.685 ms
 3  192.168.11.101 (192.168.11.101)  189.471 ms  189.556 ms *
 ...</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	  </P
></DD
><DT
><A
NAME="PINGREF"
></A
><B
CLASS="COMMAND"
>ping</B
></DT
><DD
><P
>Broadcast an <TT
CLASS="REPLACEABLE"
><I
>ICMP
	      ECHO_REQUEST</I
></TT
> packet to another machine,
              either on a local or remote network. This is a
              diagnostic tool for testing network connections,
              and it should be used with caution.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ping localhost</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.
 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec
 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec

 --- localhost.localdomain ping statistics ---
 2 packets transmitted, 2 packets received, 0% packet loss
 round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>A successful <I
CLASS="FIRSTTERM"
>ping</I
> returns
	      an <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
> of
	      <SPAN
CLASS="ERRORCODE"
>0</SPAN
>. This can be tested for in a
	      script.</P
><P
><A
NAME="PING0"
></A
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;  HNAME=news-15.net  # Notorious spammer.
   2&nbsp;# HNAME=$HOST     # Debug: test for localhost.
   3&nbsp;  count=2  # Send only two pings.
   4&nbsp;
   5&nbsp;if [[ `ping -c $count "$HNAME"` ]]
   6&nbsp;then
   7&nbsp;  echo ""$HNAME" still up and broadcasting spam your way."
   8&nbsp;else
   9&nbsp;  echo ""$HNAME" seems to be down. Pity."
  10&nbsp;fi</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><A
NAME="WHOISREF"
></A
><B
CLASS="COMMAND"
>whois</B
></DT
><DD
><P
>Perform a DNS (Domain Name System) lookup.
	      The <TT
CLASS="OPTION"
>-h</TT
> option permits specifying which
	      particular <I
CLASS="FIRSTTERM"
>whois</I
> server to query. See
	      <A
HREF="othertypesv.html#EX18"
>Example 4-6</A
> and <A
HREF="communications.html#SPAMLOOKUP"
>Example 16-40</A
>.</P
></DD
><DT
><A
NAME="FINGERREF"
></A
><B
CLASS="COMMAND"
>finger</B
></DT
><DD
><P
>Retrieve information about users on a
	      network. Optionally, this command can display
	      a user's <TT
CLASS="FILENAME"
>~/.plan</TT
>,
	      <TT
CLASS="FILENAME"
>~/.project</TT
>, and
	      <TT
CLASS="FILENAME"
>~/.forward</TT
> files, if present.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>finger</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>Login  Name           Tty      Idle  Login Time   Office     Office Phone
 bozo   Bozo Bozeman   tty1        8  Jun 25 16:59                (:0)
 bozo   Bozo Bozeman   ttyp0          Jun 25 16:59                (:0.0)
 bozo   Bozo Bozeman   ttyp1          Jun 25 17:07                (:0.0)</TT
>
 
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>finger bozo</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>Login: bozo                             Name: Bozo Bozeman
 Directory: /home/bozo                   Shell: /bin/bash
 Office: 2355 Clown St., 543-1234
 On since Fri Aug 31 20:13 (MST) on tty1    1 hour 38 minutes idle
 On since Fri Aug 31 20:13 (MST) on pts/0   12 seconds idle
 On since Fri Aug 31 20:13 (MST) on pts/1
 On since Fri Aug 31 20:31 (MST) on pts/2   1 hour 16 minutes idle
 Mail last read Tue Jul  3 10:08 2007 (MST) 
 No Plan.</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Out of security considerations, many networks disable
	      <B
CLASS="COMMAND"
>finger</B
> and its associated daemon.
	          <A
NAME="AEN13320"
HREF="#FTN.AEN13320"
>[1]</A
>
	      </P
></DD
><DT
><A
NAME="CHFNREF"
></A
><B
CLASS="COMMAND"
>chfn</B
></DT
><DD
><P
>Change information disclosed by the
	      <B
CLASS="COMMAND"
>finger</B
> command.</P
></DD
><DT
><A
NAME="VRFYREF"
></A
><B
CLASS="COMMAND"
>vrfy</B
></DT
><DD
><P
>Verify an Internet e-mail address.</P
><P
>This command seems to be missing from newer Linux
	      distros.</P
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="COMMREMOTE1"
></A
>Remote Host Access</B
></P
><DL
><DT
><A
NAME="RXREF"
></A
><B
CLASS="COMMAND"
>sx</B
>, <B
CLASS="COMMAND"
>rx</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>sx</B
> and <B
CLASS="COMMAND"
>rx</B
>
	      command set serves to transfer files to and from a remote
	      host using the <I
CLASS="FIRSTTERM"
>xmodem</I
> protocol. These
	      are generally part of a communications package, such as
	      <B
CLASS="COMMAND"
>minicom</B
>.</P
></DD
><DT
><A
NAME="RZREF"
></A
><B
CLASS="COMMAND"
>sz</B
>, <B
CLASS="COMMAND"
>rz</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>sz</B
> and <B
CLASS="COMMAND"
>rz</B
>
	      command set serves to transfer files to and from a remote
	      host using the <I
CLASS="FIRSTTERM"
>zmodem</I
> protocol.
	      <I
CLASS="FIRSTTERM"
>Zmodem</I
> has certain advantages over
	      <I
CLASS="FIRSTTERM"
>xmodem</I
>, such as faster transmission
	      rate and resumption of interrupted file transfers.
	      Like <B
CLASS="COMMAND"
>sx</B
> and <B
CLASS="COMMAND"
>rx</B
>,
	      these are generally part of a communications package.</P
></DD
><DT
><A
NAME="FTPREF"
></A
><B
CLASS="COMMAND"
>ftp</B
></DT
><DD
><P
>Utility and protocol for uploading / downloading
	      files to or from a remote host. An ftp session can be automated
	      in a script (see <A
HREF="here-docs.html#EX72"
>Example 19-6</A
> and <A
HREF="contributed-scripts.html#ENCRYPTEDPW"
>Example A-4</A
>).</P
></DD
><DT
><A
NAME="UUCPREF"
></A
><B
CLASS="COMMAND"
>uucp</B
>, <A
NAME="UUXREF"
></A
><B
CLASS="COMMAND"
>uux</B
>, <A
NAME="CUREF"
></A
><B
CLASS="COMMAND"
>cu</B
></DT
><DD
><P
><B
CLASS="COMMAND"
>uucp</B
>: <I
CLASS="FIRSTTERM"
>UNIX to UNIX
	      copy</I
>. This is a communications package for
	      transferring files between UNIX servers. A shell script
	      is an effective way to handle a <B
CLASS="COMMAND"
>uucp</B
>
	      command sequence.</P
><P
>Since the advent of the Internet and e-mail,
	      <B
CLASS="COMMAND"
>uucp</B
> seems to have faded into obscurity,
	      but it still exists and remains perfectly workable in
	      situations where an Internet connection is not available
	      or appropriate. The advantage of <B
CLASS="COMMAND"
>uucp</B
>
	      is that it is fault-tolerant, so even if there is a service
	      interruption the copy operation will resume where it left
	      off when the connection is restored.</P
><P
>---</P
><P
><B
CLASS="COMMAND"
>uux</B
>: <I
CLASS="FIRSTTERM"
>UNIX to UNIX
	      execute</I
>. Execute a command on a remote system.
	      This command is part of the <B
CLASS="COMMAND"
>uucp</B
>
	      package.</P
><P
>---</P
><P
><B
CLASS="COMMAND"
>cu</B
>: <B
CLASS="COMMAND"
>C</B
>all
	      <B
CLASS="COMMAND"
>U</B
>p a remote system and connect as a
	      simple terminal. It is a sort of dumbed-down version of
	      <A
HREF="communications.html#TELNETREF"
>telnet</A
>. This command is
	      part of the <B
CLASS="COMMAND"
>uucp</B
> package.</P
></DD
><DT
><A
NAME="TELNETREF"
></A
><B
CLASS="COMMAND"
>telnet</B
></DT
><DD
><P
>Utility and protocol for connecting to a remote host.</P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <I
CLASS="FIRSTTERM"
>telnet</I
> protocol
	    contains security holes and should therefore probably be
	    avoided. Its use within a shell script is
	    <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> recommended.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="WGETREF"
></A
><B
CLASS="COMMAND"
>wget</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>wget</B
> utility
	      <I
CLASS="FIRSTTERM"
>noninteractively</I
> retrieves or
	      downloads files from a Web or ftp site. It works well in a
	      script.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;wget -p http://www.xyz23.com/file01.html
   2&nbsp;#  The -p or --page-requisite option causes wget to fetch all files
   3&nbsp;#+ required to display the specified page.
   4&nbsp;
   5&nbsp;wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE
   6&nbsp;#  The -r option recursively follows and retrieves all links
   7&nbsp;#+ on the specified site.
   8&nbsp;
   9&nbsp;wget -c ftp://ftp.xyz25.net/bozofiles/filename.tar.bz2
  10&nbsp;#  The -c option lets wget resume an interrupted download.
  11&nbsp;#  This works with ftp servers and many HTTP sites.</PRE
></TD
></TR
></TABLE
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="QUOTEFETCH"
></A
><P
><B
>Example 16-42. Getting a stock quote</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# quote-fetch.sh: Download a stock quote.
   3&nbsp;
   4&nbsp;
   5&nbsp;E_NOPARAMS=86
   6&nbsp;
   7&nbsp;if [ -z "$1" ]  # Must specify a stock (symbol) to fetch.
   8&nbsp;  then echo "Usage: `basename $0` stock-symbol"
   9&nbsp;  exit $E_NOPARAMS
  10&nbsp;fi
  11&nbsp;
  12&nbsp;stock_symbol=$1
  13&nbsp;
  14&nbsp;file_suffix=.html
  15&nbsp;# Fetches an HTML file, so name it appropriately.
  16&nbsp;URL='http://finance.yahoo.com/q?s='
  17&nbsp;# Yahoo finance board, with stock query suffix.
  18&nbsp;
  19&nbsp;# -----------------------------------------------------------
  20&nbsp;wget -O ${stock_symbol}${file_suffix} "${URL}${stock_symbol}"
  21&nbsp;# -----------------------------------------------------------
  22&nbsp;
  23&nbsp;
  24&nbsp;# To look up stuff on http://search.yahoo.com:
  25&nbsp;# -----------------------------------------------------------
  26&nbsp;# URL="http://search.yahoo.com/search?fr=ush-news&#38;p=${query}"
  27&nbsp;# wget -O "$savefilename" "${URL}"
  28&nbsp;# -----------------------------------------------------------
  29&nbsp;# Saves a list of relevant URLs.
  30&nbsp;
  31&nbsp;exit $?
  32&nbsp;
  33&nbsp;# Exercises:
  34&nbsp;# ---------
  35&nbsp;#
  36&nbsp;# 1) Add a test to ensure the user running the script is on-line.
  37&nbsp;#    (Hint: parse the output of 'ps -ax' for "ppp" or "connect."
  38&nbsp;#
  39&nbsp;# 2) Modify this script to fetch the local weather report,
  40&nbsp;#+   taking the user's zip code as an argument.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>See also <A
HREF="contributed-scripts.html#WGETTER2"
>Example A-30</A
> and <A
HREF="contributed-scripts.html#BASHPODDER"
>Example A-31</A
>.</P
></DD
><DT
><A
NAME="LYNXREF"
></A
><B
CLASS="COMMAND"
>lynx</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>lynx</B
> Web and file browser
	      can be used inside a script (with the
	      <TT
CLASS="OPTION"
>-dump</TT
> option) to retrieve a file from a Web or 
	      ftp site noninteractively.</P
><P
>	   <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;lynx -dump http://www.xyz23.com/file01.html &#62;$SAVEFILE</PRE
></TD
></TR
></TABLE
>
            </P
><P
>With the <TT
CLASS="OPTION"
>-traversal</TT
> option,
	      <B
CLASS="COMMAND"
>lynx</B
> starts at the HTTP URL specified
	      as an argument, then <SPAN
CLASS="QUOTE"
>"crawls"</SPAN
> through all
	      links located on that particular server. Used together
	      with the <TT
CLASS="OPTION"
>-crawl</TT
> option, outputs page text
	      to a log file.</P
></DD
><DT
><A
NAME="RLOGINREF"
></A
><B
CLASS="COMMAND"
>rlogin</B
></DT
><DD
><P
><TT
CLASS="REPLACEABLE"
><I
>Remote login</I
></TT
>, initates a
	      session on a remote host. This command has security issues,
	      so use <A
HREF="communications.html#SSHREF"
>ssh</A
> instead.</P
></DD
><DT
><A
NAME="RSHREF"
></A
><B
CLASS="COMMAND"
>rsh</B
></DT
><DD
><P
><TT
CLASS="REPLACEABLE"
><I
>Remote shell</I
></TT
>, executes
	      command(s) on a remote host. This has security issues,
	      so use <B
CLASS="COMMAND"
>ssh</B
> instead.</P
></DD
><DT
><A
NAME="RCPREF"
></A
><B
CLASS="COMMAND"
>rcp</B
></DT
><DD
><P
><TT
CLASS="REPLACEABLE"
><I
>Remote copy</I
></TT
>, copies files
	      between two different networked machines.</P
></DD
><DT
><A
NAME="RSYNCREF"
></A
><B
CLASS="COMMAND"
>rsync</B
></DT
><DD
><P
><TT
CLASS="REPLACEABLE"
><I
>Remote synchronize</I
></TT
>, updates
	    (synchronizes) files
	      between two different networked machines.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>rsync -a ~/sourcedir/*txt /node1/subdirectory/</B
></TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="FC4UPD"
></A
><P
><B
>Example 16-43. Updating FC4</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# fc4upd.sh
   3&nbsp;
   4&nbsp;# Script author: Frank Wang.
   5&nbsp;# Slight stylistic modifications by ABS Guide author.
   6&nbsp;# Used in ABS Guide with permission.
   7&nbsp;
   8&nbsp;
   9&nbsp;#  Download Fedora Core 4 update from mirror site using rsync. 
  10&nbsp;#  Should also work for newer Fedora Cores -- 5, 6, . . .
  11&nbsp;#  Only download latest package if multiple versions exist,
  12&nbsp;#+ to save space.
  13&nbsp;
  14&nbsp;URL=rsync://distro.ibiblio.org/fedora-linux-core/updates/
  15&nbsp;# URL=rsync://ftp.kddilabs.jp/fedora/core/updates/
  16&nbsp;# URL=rsync://rsync.planetmirror.com/fedora-linux-core/updates/
  17&nbsp;
  18&nbsp;DEST=${1:-/var/www/html/fedora/updates/}
  19&nbsp;LOG=/tmp/repo-update-$(/bin/date +%Y-%m-%d).txt
  20&nbsp;PID_FILE=/var/run/${0##*/}.pid
  21&nbsp;
  22&nbsp;E_RETURN=85        # Something unexpected happened.
  23&nbsp;
  24&nbsp;
  25&nbsp;# General rsync options
  26&nbsp;# -r: recursive download
  27&nbsp;# -t: reserve time
  28&nbsp;# -v: verbose
  29&nbsp;
  30&nbsp;OPTS="-rtv --delete-excluded --delete-after --partial"
  31&nbsp;
  32&nbsp;# rsync include pattern
  33&nbsp;# Leading slash causes absolute path name match.
  34&nbsp;INCLUDE=(
  35&nbsp;    "/4/i386/kde-i18n-Chinese*" 
  36&nbsp;#   ^                         ^
  37&nbsp;# Quoting is necessary to prevent globbing.
  38&nbsp;) 
  39&nbsp;
  40&nbsp;
  41&nbsp;# rsync exclude pattern
  42&nbsp;# Temporarily comment out unwanted pkgs using "#" . . .
  43&nbsp;EXCLUDE=(
  44&nbsp;    /1
  45&nbsp;    /2
  46&nbsp;    /3
  47&nbsp;    /testing
  48&nbsp;    /4/SRPMS
  49&nbsp;    /4/ppc
  50&nbsp;    /4/x86_64
  51&nbsp;    /4/i386/debug
  52&nbsp;   "/4/i386/kde-i18n-*"
  53&nbsp;   "/4/i386/openoffice.org-langpack-*"
  54&nbsp;   "/4/i386/*i586.rpm"
  55&nbsp;   "/4/i386/GFS-*"
  56&nbsp;   "/4/i386/cman-*"
  57&nbsp;   "/4/i386/dlm-*"
  58&nbsp;   "/4/i386/gnbd-*"
  59&nbsp;   "/4/i386/kernel-smp*"
  60&nbsp;#  "/4/i386/kernel-xen*" 
  61&nbsp;#  "/4/i386/xen-*" 
  62&nbsp;)
  63&nbsp;
  64&nbsp;
  65&nbsp;init () {
  66&nbsp;    # Let pipe command return possible rsync error, e.g., stalled network.
  67&nbsp;    set -o pipefail                  # Newly introduced in Bash, version 3.
  68&nbsp;
  69&nbsp;    TMP=${TMPDIR:-/tmp}/${0##*/}.$$  # Store refined download list.
  70&nbsp;    trap "{
  71&nbsp;        rm -f $TMP 2&#62;/dev/null
  72&nbsp;    }" EXIT                          # Clear temporary file on exit.
  73&nbsp;}
  74&nbsp;
  75&nbsp;
  76&nbsp;check_pid () {
  77&nbsp;# Check if process exists.
  78&nbsp;    if [ -s "$PID_FILE" ]; then
  79&nbsp;        echo "PID file exists. Checking ..."
  80&nbsp;        PID=$(/bin/egrep -o "^[[:digit:]]+" $PID_FILE)
  81&nbsp;        if /bin/ps --pid $PID &#38;&#62;/dev/null; then
  82&nbsp;            echo "Process $PID found. ${0##*/} seems to be running!"
  83&nbsp;           /usr/bin/logger -t ${0##*/} \
  84&nbsp;                 "Process $PID found. ${0##*/} seems to be running!"
  85&nbsp;            exit $E_RETURN
  86&nbsp;        fi
  87&nbsp;        echo "Process $PID not found. Start new process . . ."
  88&nbsp;    fi
  89&nbsp;}
  90&nbsp;
  91&nbsp;
  92&nbsp;#  Set overall file update range starting from root or $URL,
  93&nbsp;#+ according to above patterns.
  94&nbsp;set_range () {
  95&nbsp;    include=
  96&nbsp;    exclude=
  97&nbsp;    for p in "${INCLUDE[@]}"; do
  98&nbsp;        include="$include --include \"$p\""
  99&nbsp;    done
 100&nbsp;
 101&nbsp;    for p in "${EXCLUDE[@]}"; do
 102&nbsp;        exclude="$exclude --exclude \"$p\""
 103&nbsp;    done
 104&nbsp;}
 105&nbsp;
 106&nbsp;
 107&nbsp;# Retrieve and refine rsync update list.
 108&nbsp;get_list () {
 109&nbsp;    echo $$ &#62; $PID_FILE || {
 110&nbsp;        echo "Can't write to pid file $PID_FILE"
 111&nbsp;        exit $E_RETURN
 112&nbsp;    }
 113&nbsp;
 114&nbsp;    echo -n "Retrieving and refining update list . . ."
 115&nbsp;
 116&nbsp;    # Retrieve list -- 'eval' is needed to run rsync as a single command.
 117&nbsp;    # $3 and $4 is the date and time of file creation.
 118&nbsp;    # $5 is the full package name.
 119&nbsp;    previous=
 120&nbsp;    pre_file=
 121&nbsp;    pre_date=0
 122&nbsp;    eval /bin/nice /usr/bin/rsync \
 123&nbsp;        -r $include $exclude $URL | \
 124&nbsp;        egrep '^dr.x|^-r' | \
 125&nbsp;        awk '{print $3, $4, $5}' | \
 126&nbsp;        sort -k3 | \
 127&nbsp;        { while read line; do
 128&nbsp;            # Get seconds since epoch, to filter out obsolete pkgs.
 129&nbsp;            cur_date=$(date -d "$(echo $line | awk '{print $1, $2}')" +%s)
 130&nbsp;            #  echo $cur_date
 131&nbsp;
 132&nbsp;            # Get file name.
 133&nbsp;            cur_file=$(echo $line | awk '{print $3}')
 134&nbsp;            #  echo $cur_file
 135&nbsp;
 136&nbsp;            # Get rpm pkg name from file name, if possible.
 137&nbsp;            if [[ $cur_file == *rpm ]]; then
 138&nbsp;                pkg_name=$(echo $cur_file | sed -r -e \
 139&nbsp;                    's/(^([^_-]+[_-])+)[[:digit:]]+\..*[_-].*$/\1/')
 140&nbsp;            else
 141&nbsp;                pkg_name=
 142&nbsp;            fi
 143&nbsp;            # echo $pkg_name
 144&nbsp;
 145&nbsp;            if [ -z "$pkg_name" ]; then   #  If not a rpm file,
 146&nbsp;                echo $cur_file &#62;&#62; $TMP    #+ then append to download list.
 147&nbsp;            elif [ "$pkg_name" != "$previous" ]; then   # A new pkg found.
 148&nbsp;                echo $pre_file &#62;&#62; $TMP                  # Output latest file.
 149&nbsp;                previous=$pkg_name                      # Save current.
 150&nbsp;                pre_date=$cur_date
 151&nbsp;                pre_file=$cur_file
 152&nbsp;            elif [ "$cur_date" -gt "$pre_date" ]; then
 153&nbsp;                                                #  If same pkg, but newer,
 154&nbsp;                pre_date=$cur_date              #+ then update latest pointer.
 155&nbsp;                pre_file=$cur_file
 156&nbsp;            fi
 157&nbsp;            done
 158&nbsp;            echo $pre_file &#62;&#62; $TMP              #  TMP contains ALL
 159&nbsp;                                                #+ of refined list now.
 160&nbsp;            # echo "subshell=$BASH_SUBSHELL"
 161&nbsp;
 162&nbsp;    }       # Bracket required here to let final "echo $pre_file &#62;&#62; $TMP" 
 163&nbsp;            # Remained in the same subshell ( 1 ) with the entire loop.
 164&nbsp;
 165&nbsp;    RET=$?  # Get return code of the pipe command.
 166&nbsp;
 167&nbsp;    [ "$RET" -ne 0 ] &#38;&#38; {
 168&nbsp;        echo "List retrieving failed with code $RET"
 169&nbsp;        exit $E_RETURN
 170&nbsp;    }
 171&nbsp;
 172&nbsp;    echo "done"; echo
 173&nbsp;}
 174&nbsp;
 175&nbsp;# Real rsync download part.
 176&nbsp;get_file () {
 177&nbsp;
 178&nbsp;    echo "Downloading..."
 179&nbsp;    /bin/nice /usr/bin/rsync \
 180&nbsp;        $OPTS \
 181&nbsp;        --filter "merge,+/ $TMP" \
 182&nbsp;        --exclude '*'  \
 183&nbsp;        $URL $DEST     \
 184&nbsp;        | /usr/bin/tee $LOG
 185&nbsp;
 186&nbsp;    RET=$?
 187&nbsp;
 188&nbsp;   #  --filter merge,+/ is crucial for the intention. 
 189&nbsp;   #  + modifier means include and / means absolute path.
 190&nbsp;   #  Then sorted list in $TMP will contain ascending dir name and 
 191&nbsp;   #+ prevent the following --exclude '*' from "shortcutting the circuit." 
 192&nbsp;
 193&nbsp;    echo "Done"
 194&nbsp;
 195&nbsp;    rm -f $PID_FILE 2&#62;/dev/null
 196&nbsp;
 197&nbsp;    return $RET
 198&nbsp;}
 199&nbsp;
 200&nbsp;# -------
 201&nbsp;# Main
 202&nbsp;init
 203&nbsp;check_pid
 204&nbsp;set_range
 205&nbsp;get_list
 206&nbsp;get_file
 207&nbsp;RET=$?
 208&nbsp;# -------
 209&nbsp;
 210&nbsp;if [ "$RET" -eq 0 ]; then
 211&nbsp;    /usr/bin/logger -t ${0##*/} "Fedora update mirrored successfully."
 212&nbsp;else
 213&nbsp;    /usr/bin/logger -t ${0##*/} \
 214&nbsp;    "Fedora update mirrored with failure code: $RET"
 215&nbsp;fi
 216&nbsp;
 217&nbsp;exit $RET</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>See also <A
HREF="contributed-scripts.html#NIGHTLYBACKUP"
>Example A-32</A
>.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Using <A
HREF="communications.html#RCPREF"
>rcp</A
>, <A
HREF="communications.html#RSYNCREF"
>rsync</A
>, and similar
	      utilities with security implications in a shell
	      script may not be advisable. Consider, instead, using
	      <B
CLASS="COMMAND"
>ssh</B
>, <A
HREF="communications.html#SCPREF"
>scp</A
>,
	      or an <B
CLASS="COMMAND"
>expect</B
> script.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="SSHREF"
></A
><B
CLASS="COMMAND"
>ssh</B
></DT
><DD
><P
><TT
CLASS="REPLACEABLE"
><I
>Secure shell</I
></TT
>, logs onto
	      a remote host and executes commands there. This
	      secure replacement for <B
CLASS="COMMAND"
>telnet</B
>,
	      <B
CLASS="COMMAND"
>rlogin</B
>, <B
CLASS="COMMAND"
>rcp</B
>, and
	      <B
CLASS="COMMAND"
>rsh</B
> uses identity authentication
	      and encryption. See its <A
HREF="external.html#MANREF"
>manpage</A
>
	      for details.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REMOTE"
></A
><P
><B
>Example 16-44. Using <I
CLASS="FIRSTTERM"
>ssh</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# remote.bash: Using ssh.
   3&nbsp;
   4&nbsp;# This example by Michael Zick.
   5&nbsp;# Used with permission.
   6&nbsp;
   7&nbsp;
   8&nbsp;#   Presumptions:
   9&nbsp;#   ------------
  10&nbsp;#   fd-2 isn't being captured ( '2&#62;/dev/null' ).
  11&nbsp;#   ssh/sshd presumes stderr ('2') will display to user.
  12&nbsp;#
  13&nbsp;#   sshd is running on your machine.
  14&nbsp;#   For any 'standard' distribution, it probably is,
  15&nbsp;#+  and without any funky ssh-keygen having been done.
  16&nbsp;
  17&nbsp;# Try ssh to your machine from the command-line:
  18&nbsp;#
  19&nbsp;# $ ssh $HOSTNAME
  20&nbsp;# Without extra set-up you'll be asked for your password.
  21&nbsp;#   enter password
  22&nbsp;#   when done,  $ exit
  23&nbsp;#
  24&nbsp;# Did that work? If so, you're ready for more fun.
  25&nbsp;
  26&nbsp;# Try ssh to your machine as 'root':
  27&nbsp;#
  28&nbsp;#   $  ssh -l root $HOSTNAME
  29&nbsp;#   When asked for password, enter root's, not yours.
  30&nbsp;#          Last login: Tue Aug 10 20:25:49 2004 from localhost.localdomain
  31&nbsp;#   Enter 'exit' when done.
  32&nbsp;
  33&nbsp;#  The above gives you an interactive shell.
  34&nbsp;#  It is possible for sshd to be set up in a 'single command' mode,
  35&nbsp;#+ but that is beyond the scope of this example.
  36&nbsp;#  The only thing to note is that the following will work in
  37&nbsp;#+ 'single command' mode.
  38&nbsp;
  39&nbsp;
  40&nbsp;# A basic, write stdout (local) command.
  41&nbsp;
  42&nbsp;ls -l
  43&nbsp;
  44&nbsp;# Now the same basic command on a remote machine.
  45&nbsp;# Pass a different 'USERNAME' 'HOSTNAME' if desired:
  46&nbsp;USER=${USERNAME:-$(whoami)}
  47&nbsp;HOST=${HOSTNAME:-$(hostname)}
  48&nbsp;
  49&nbsp;#  Now excute the above command-line on the remote host,
  50&nbsp;#+ with all transmissions encrypted.
  51&nbsp;
  52&nbsp;ssh -l ${USER} ${HOST} " ls -l "
  53&nbsp;
  54&nbsp;#  The expected result is a listing of your username's home
  55&nbsp;#+ directory on the remote machine.
  56&nbsp;#  To see any difference, run this script from somewhere
  57&nbsp;#+ other than your home directory.
  58&nbsp;
  59&nbsp;#  In other words, the Bash command is passed as a quoted line
  60&nbsp;#+ to the remote shell, which executes it on the remote machine.
  61&nbsp;#  In this case, sshd does  ' bash -c "ls -l" '   on your behalf.
  62&nbsp;
  63&nbsp;#  For information on topics such as not having to enter a
  64&nbsp;#+ password/passphrase for every command-line, see
  65&nbsp;#+    man ssh
  66&nbsp;#+    man ssh-keygen
  67&nbsp;#+    man sshd_config.
  68&nbsp;
  69&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Within a loop, <B
CLASS="COMMAND"
>ssh</B
> may cause
		unexpected behavior. According to a <A
HREF="http://groups-beta.google.com/group/comp.unix.shell/msg/dcb446b5fff7d230"
TARGET="_top"
>		Usenet post</A
> in the comp.unix shell archives,
		<B
CLASS="COMMAND"
>ssh</B
> inherits the loop's
		<TT
CLASS="FILENAME"
>stdin</TT
>. To remedy this, pass
		<B
CLASS="COMMAND"
>ssh</B
> either the <TT
CLASS="OPTION"
>-n</TT
>
		or <TT
CLASS="OPTION"
>-f</TT
> option.</P
><P
>Thanks, Jason Bechtel, for pointing this out.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="SCPREF"
></A
><B
CLASS="COMMAND"
>scp</B
></DT
><DD
><P
><TT
CLASS="REPLACEABLE"
><I
>Secure copy</I
></TT
>, similar in
	      function to <B
CLASS="COMMAND"
>rcp</B
>, copies files between
	      two different networked machines, but does so using
	      authentication, and with a security level similar to
	      <B
CLASS="COMMAND"
>ssh</B
>.</P
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="COMMLOCAL1"
></A
>Local Network</B
></P
><DL
><DT
><A
NAME="WRITEREF"
></A
><B
CLASS="COMMAND"
>write</B
></DT
><DD
><P
>This is a utility for terminal-to-terminal communication.
	      It allows sending lines from your terminal (console or
	      <I
CLASS="FIRSTTERM"
>xterm</I
>) to that of another user. The
	      <A
HREF="system.html#MESGREF"
>mesg</A
> command may, of course,
	      be used to disable write access to a terminal</P
><P
>Since <B
CLASS="COMMAND"
>write</B
> is interactive, it
	      would not normally find use in a script.</P
></DD
><DT
><A
NAME="NETCONFIGREF"
></A
><B
CLASS="COMMAND"
>netconfig</B
></DT
><DD
><P
>A command-line utility for configuring a network adapter
	      (using <I
CLASS="FIRSTTERM"
>DHCP</I
>). This command is native
	      to Red Hat centric Linux distros.</P
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="COMMMAIL1"
></A
>Mail</B
></P
><DL
><DT
><B
CLASS="COMMAND"
>mail</B
></DT
><DD
><P
>Send or read e-mail messages.</P
><P
>This stripped-down command-line mail client
	      works fine as a command embedded in a script.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SELFMAILER"
></A
><P
><B
>Example 16-45. A script that mails itself</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/sh
   2&nbsp;# self-mailer.sh: Self-mailing script
   3&nbsp;
   4&nbsp;adr=${1:-`whoami`}     # Default to current user, if not specified.
   5&nbsp;#  Typing 'self-mailer.sh wiseguy@superdupergenius.com'
   6&nbsp;#+ sends this script to that addressee.
   7&nbsp;#  Just 'self-mailer.sh' (no argument) sends the script
   8&nbsp;#+ to the person invoking it, for example, bozo@localhost.localdomain.
   9&nbsp;#
  10&nbsp;#  For more on the ${parameter:-default} construct,
  11&nbsp;#+ see the "Parameter Substitution" section
  12&nbsp;#+ of the "Variables Revisited" chapter.
  13&nbsp;
  14&nbsp;# ============================================================================
  15&nbsp;  cat $0 | mail -s "Script \"`basename $0`\" has mailed itself to you." "$adr"
  16&nbsp;# ============================================================================
  17&nbsp;
  18&nbsp;# --------------------------------------------
  19&nbsp;#  Greetings from the self-mailing script.
  20&nbsp;#  A mischievous person has run this script,
  21&nbsp;#+ which has caused it to mail itself to you.
  22&nbsp;#  Apparently, some people have nothing better
  23&nbsp;#+ to do with their time.
  24&nbsp;# --------------------------------------------
  25&nbsp;
  26&nbsp;echo "At `date`, script \"`basename $0`\" mailed to "$adr"."
  27&nbsp;
  28&nbsp;exit 0
  29&nbsp;
  30&nbsp;#  Note that the "mailx" command (in "send" mode) may be substituted
  31&nbsp;#+ for "mail" ... but with somewhat different options.</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="MAILTOREF"
></A
><B
CLASS="COMMAND"
>mailto</B
></DT
><DD
><P
>Similar to the <B
CLASS="COMMAND"
>mail</B
> command,
	      <B
CLASS="COMMAND"
>mailto</B
> sends e-mail messages
	      from the command-line or in a script. However,
	      <B
CLASS="COMMAND"
>mailto</B
> also permits sending MIME
	      (multimedia) messages.</P
></DD
><DT
><A
NAME="MAILSTATSREF"
></A
><B
CLASS="COMMAND"
>mailstats</B
></DT
><DD
><P
>Show <I
CLASS="FIRSTTERM"
>mail statistics</I
>. This command
	      may be invoked only by <I
CLASS="FIRSTTERM"
>root</I
>.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>root# </TT
><TT
CLASS="USERINPUT"
><B
>mailstats</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>Statistics from Tue Jan  1 20:32:08 2008
  M   msgsfr  bytes_from   msgsto    bytes_to  msgsrej msgsdis msgsqur  Mailer
  4     1682      24118K        0          0K        0       0       0  esmtp
  9      212        640K     1894      25131K        0       0       0  local
 =====================================================================
  T     1894      24758K     1894      25131K        0       0       0
  C      414                    0</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="VACATIONREF"
></A
><B
CLASS="COMMAND"
>vacation</B
></DT
><DD
><P
>This utility automatically replies to e-mails that
	      the intended recipient is on vacation and temporarily
	      unavailable. It runs on a network, in conjunction with
	      <B
CLASS="COMMAND"
>sendmail</B
>, and is not applicable to a
	      dial-up POPmail account.</P
></DD
></DL
></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.AEN13320"
HREF="communications.html#AEN13320"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><A
NAME="DAEMONREF"
></A
></P
><P
>A <I
CLASS="FIRSTTERM"
>daemon</I
> is a background
		    process not attached to a terminal session. Daemons
		    perform designated services either at specified times
		    or explicitly triggered by certain events.</P
><P
>The word <SPAN
CLASS="QUOTE"
>"daemon"</SPAN
> means ghost in
		    Greek, and there is certainly something mysterious,
		    almost supernatural, about the way UNIX daemons
		    wander about behind the scenes, silently carrying
		    out their appointed tasks.</P
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