16.6. Communications Commands

Certain of the following commands find use in network data transfer and analysis, as well as in chasing spammers.

Information and Statistics


Searches for information about an Internet host by name or IP address, using DNS.

bash$ host surfacemail.com
surfacemail.com. has address


Displays IP information for a host. With the -h option, ipcalc does a reverse DNS lookup, finding the name of the host (server) from the IP address.

bash$ ipcalc -h


Do an Internet "name server lookup" on a host by IP address. This is essentially equivalent to ipcalc -h or dig -x . The command may be run either interactively or noninteractively, i.e., from within a script.

The nslookup command has allegedly been "deprecated," but it is still useful.

bash$ nslookup -sil
nslookup kuhleersparnis.ch

 Non-authoritative answer:
 Name:   kuhleersparnis.ch


Domain Information Groper. Similar to nslookup, dig does an Internet name server lookup on a host. May be run from the command-line or from within a script.

Some interesting options to dig are +time=N for setting a query timeout to N seconds, +nofail for continuing to query servers until a reply is received, and -x for doing a reverse address lookup.

Compare the output of dig -x with ipcalc -h and nslookup.

bash$ dig -x
;; Got answer:
 ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649
 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

 ;         IN      PTR

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

 ;; Query time: 537 msec
 ;; WHEN: Wed Jun 26 08:35:24 2002
 ;; MSG SIZE  rcvd: 91

Example 16-40. Finding out where to report a spammer

# spam-lookup.sh: Look up abuse contact to report a spammer.
# Thanks, Michael Zick.

# Check for command-line arg.
if [ $# -ne "$ARGCOUNT" ]
  echo "Usage: `basename $0` domain-name"

dig +short $1.contacts.abuse.net -c in -t txt
# Also try:
#     dig +nssearch $1
#     Tries to find "authoritative name servers" and display SOA records.

# The following also works:
#     whois -h whois.abuse.net $1
#           ^^ ^^^^^^^^^^^^^^^  Specify host.  
#     Can even lookup multiple spammers with this, i.e."
#     whois -h whois.abuse.net $spamdomain1 $spamdomain2 . . .

#  Exercise:
#  --------
#  Expand the functionality of this script
#+ so that it automatically e-mails a notification
#+ to the responsible ISP's contact address(es).
#  Hint: use the "mail" command.

exit $?

# spam-lookup.sh chinatietong.com
#                A known spam domain.

# "crnet_mgr@chinatietong.com"
# "crnet_tec@chinatietong.com"
# "postmaster@chinatietong.com"

#  For a more elaborate version of this script,
#+ see the SpamViz home page, http://www.spamviz.net/index.html.

Example 16-41. Analyzing a spam domain

#! /bin/bash
# is-spammer.sh: Identifying spam domains

# $Id: is-spammer, v 1.4 2004/09/01 19:37:52 mszick Exp $
# Above line is RCS ID info.
#  This is a simplified version of the "is_spammer.bash
#+ script in the Contributed Scripts appendix.

# is-spammer <domain.name>

# Uses an external program: 'dig'
# Tested with version: 9.2.4rc5

# Uses functions.
# Uses IFS to parse strings by assignment into arrays.
# And even does something useful: checks e-mail blacklists.

# Use the domain.name(s) from the text body:
# http://www.good_stuff.spammer.biz/just_ignore_everything_else
#                       ^^^^^^^^^^^
# Or the domain.name(s) from any e-mail address:
# Really_Good_Offer@spammer.biz
# as the only argument to this script.
#(PS: have your Inet connection running)
# So, to invoke this script in the above two instances:
#       is-spammer.sh spammer.biz

# Whitespace == :Space:Tab:Line Feed:Carriage Return:

# No Whitespace == Line Feed:Carriage Return

# Field separator for dotted decimal ip addresses

# Get the dns text resource record.
# get_txt <error_code> <list_query>
get_txt() {

    # Parse $1 by assignment at the dots.
    local -a dns
    dns=( $1 )
    if [ "${dns[0]}" == '127' ]
        # See if there is a reason.
        echo $(dig +short $2 -t txt)

# Get the dns address resource record.
# chk_adr <rev_dns> <list_server>
chk_adr() {
    local reply
    local server
    local reason

    reply=$( dig +short ${server} )

    # If reply might be an error code . . .
    if [ ${#reply} -gt 6 ]
        reason=$(get_txt ${reply} ${server} )
    echo ${reason:-' not blacklisted.'}

# Need to get the IP address from the name.
echo 'Get address of: '$1
ip_adr=$(dig +short $1)
dns_reply=${ip_adr:-' no answer '}
echo ' Found address: '${dns_reply}

# A valid reply is at least 4 digits plus 3 dots.
if [ ${#ip_adr} -gt 6 ]
    declare query

    # Parse by assignment at the dots.
    declare -a dns
    dns=( ${ip_adr} )

    # Reorder octets into dns query order.

# See: http://www.spamhaus.org (Conservative, well maintained)
    echo -n 'spamhaus.org says: '
    echo $(chk_adr ${rev_dns} 'sbl-xbl.spamhaus.org')

# See: http://ordb.org (Open mail relays)
    echo -n '   ordb.org  says: '
    echo $(chk_adr ${rev_dns} 'relays.ordb.org')

# See: http://www.spamcop.net/ (You can report spammers here)
    echo -n ' spamcop.net says: '
    echo $(chk_adr ${rev_dns} 'bl.spamcop.net')

# # # other blacklist operations # # #

# See: http://cbl.abuseat.org.
    echo -n ' abuseat.org says: '
    echo $(chk_adr ${rev_dns} 'cbl.abuseat.org')

# See: http://dsbl.org/usage (Various mail relays)
    echo 'Distributed Server Listings'
    echo -n '       list.dsbl.org says: '
    echo $(chk_adr ${rev_dns} 'list.dsbl.org')

    echo -n '   multihop.dsbl.org says: '
    echo $(chk_adr ${rev_dns} 'multihop.dsbl.org')

    echo -n 'unconfirmed.dsbl.org says: '
    echo $(chk_adr ${rev_dns} 'unconfirmed.dsbl.org')

    echo 'Could not use that address.'

exit 0

# Exercises:
# --------

# 1) Check arguments to script,
#    and exit with appropriate error message if necessary.

# 2) Check if on-line at invocation of script,
#    and exit with appropriate error message if necessary.

# 3) Substitute generic variables for "hard-coded" BHL domains.

# 4) Set a time-out for the script using the "+time=" option
     to the 'dig' command.

For a much more elaborate version of the above script, see Example A-28.


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 grep or sed in a pipe.

bash$ traceroute
traceroute to (, 30 hops max, 38 byte packets
 1  tc43.xjbnnbrb.com (  191.303 ms  179.400 ms  179.767 ms
 2  or0.xjbnnbrb.com (  179.536 ms  179.534 ms  169.685 ms
 3 (  189.471 ms  189.556 ms *


Broadcast an ICMP ECHO_REQUEST 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.

bash$ ping localhost
PING localhost.localdomain ( from : 56(84) bytes of data.
 64 bytes from localhost.localdomain ( icmp_seq=0 ttl=255 time=709 usec
 64 bytes from localhost.localdomain ( 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

A successful ping returns an exit status of 0. This can be tested for in a script.

  HNAME=news-15.net  # Notorious spammer.
# HNAME=$HOST     # Debug: test for localhost.
  count=2  # Send only two pings.

if [[ `ping -c $count "$HNAME"` ]]
  echo ""$HNAME" still up and broadcasting spam your way."
  echo ""$HNAME" seems to be down. Pity."


Perform a DNS (Domain Name System) lookup. The -h option permits specifying which particular whois server to query. See Example 4-6 and Example 16-40.


Retrieve information about users on a network. Optionally, this command can display a user's ~/.plan, ~/.project, and ~/.forward files, if present.

bash$ finger
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)

bash$ finger bozo
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.

Out of security considerations, many networks disable finger and its associated daemon. [1]


Change information disclosed by the finger command.


Verify an Internet e-mail address.

This command seems to be missing from newer Linux distros.

Remote Host Access

sx, rx

The sx and rx command set serves to transfer files to and from a remote host using the xmodem protocol. These are generally part of a communications package, such as minicom.

sz, rz

The sz and rz command set serves to transfer files to and from a remote host using the zmodem protocol. Zmodem has certain advantages over xmodem, such as faster transmission rate and resumption of interrupted file transfers. Like sx and rx, these are generally part of a communications package.


Utility and protocol for uploading / downloading files to or from a remote host. An ftp session can be automated in a script (see Example 19-6 and Example A-4).

uucp, uux, cu

uucp: UNIX to UNIX copy. This is a communications package for transferring files between UNIX servers. A shell script is an effective way to handle a uucp command sequence.

Since the advent of the Internet and e-mail, uucp 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 uucp 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.


uux: UNIX to UNIX execute. Execute a command on a remote system. This command is part of the uucp package.


cu: Call Up a remote system and connect as a simple terminal. It is a sort of dumbed-down version of telnet. This command is part of the uucp package.


Utility and protocol for connecting to a remote host.


The telnet protocol contains security holes and should therefore probably be avoided. Its use within a shell script is not recommended.


The wget utility noninteractively retrieves or downloads files from a Web or ftp site. It works well in a script.

wget -p http://www.xyz23.com/file01.html
#  The -p or --page-requisite option causes wget to fetch all files
#+ required to display the specified page.

wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE
#  The -r option recursively follows and retrieves all links
#+ on the specified site.

wget -c ftp://ftp.xyz25.net/bozofiles/filename.tar.bz2
#  The -c option lets wget resume an interrupted download.
#  This works with ftp servers and many HTTP sites.

Example 16-42. Getting a stock quote

# quote-fetch.sh: Download a stock quote.


if [ -z "$1" ]  # Must specify a stock (symbol) to fetch.
  then echo "Usage: `basename $0` stock-symbol"
  exit $E_NOPARAMS


# Fetches an HTML file, so name it appropriately.
# Yahoo finance board, with stock query suffix.

# -----------------------------------------------------------
wget -O ${stock_symbol}${file_suffix} "${URL}${stock_symbol}"
# -----------------------------------------------------------

# To look up stuff on http://search.yahoo.com:
# -----------------------------------------------------------
# URL="http://search.yahoo.com/search?fr=ush-news&p=${query}"
# wget -O "$savefilename" "${URL}"
# -----------------------------------------------------------
# Saves a list of relevant URLs.

exit $?

# Exercises:
# ---------
# 1) Add a test to ensure the user running the script is on-line.
#    (Hint: parse the output of 'ps -ax' for "ppp" or "connect."
# 2) Modify this script to fetch the local weather report,
#+   taking the user's zip code as an argument.

See also Example A-30 and Example A-31.


The lynx Web and file browser can be used inside a script (with the -dump option) to retrieve a file from a Web or ftp site noninteractively.

lynx -dump http://www.xyz23.com/file01.html >$SAVEFILE

With the -traversal option, lynx starts at the HTTP URL specified as an argument, then "crawls" through all links located on that particular server. Used together with the -crawl option, outputs page text to a log file.


Remote login, initates a session on a remote host. This command has security issues, so use ssh instead.


Remote shell, executes command(s) on a remote host. This has security issues, so use ssh instead.


Remote copy, copies files between two different networked machines.


Remote synchronize, updates (synchronizes) files between two different networked machines.

bash$ rsync -a ~/sourcedir/*txt /node1/subdirectory/

Example 16-43. Updating FC4

# fc4upd.sh

# Script author: Frank Wang.
# Slight stylistic modifications by ABS Guide author.
# Used in ABS Guide with permission.

#  Download Fedora Core 4 update from mirror site using rsync. 
#  Should also work for newer Fedora Cores -- 5, 6, . . .
#  Only download latest package if multiple versions exist,
#+ to save space.

# URL=rsync://ftp.kddilabs.jp/fedora/core/updates/
# URL=rsync://rsync.planetmirror.com/fedora-linux-core/updates/

LOG=/tmp/repo-update-$(/bin/date +%Y-%m-%d).txt

E_RETURN=85        # Something unexpected happened.

# General rsync options
# -r: recursive download
# -t: reserve time
# -v: verbose

OPTS="-rtv --delete-excluded --delete-after --partial"

# rsync include pattern
# Leading slash causes absolute path name match.
#   ^                         ^
# Quoting is necessary to prevent globbing.

# rsync exclude pattern
# Temporarily comment out unwanted pkgs using "#" . . .
#  "/4/i386/kernel-xen*" 
#  "/4/i386/xen-*" 

init () {
    # Let pipe command return possible rsync error, e.g., stalled network.
    set -o pipefail                  # Newly introduced in Bash, version 3.

    TMP=${TMPDIR:-/tmp}/${0##*/}.$$  # Store refined download list.
    trap "{
        rm -f $TMP 2>/dev/null
    }" EXIT                          # Clear temporary file on exit.

check_pid () {
# Check if process exists.
    if [ -s "$PID_FILE" ]; then
        echo "PID file exists. Checking ..."
        PID=$(/bin/egrep -o "^[[:digit:]]+" $PID_FILE)
        if /bin/ps --pid $PID &>/dev/null; then
            echo "Process $PID found. ${0##*/} seems to be running!"
           /usr/bin/logger -t ${0##*/} \
                 "Process $PID found. ${0##*/} seems to be running!"
            exit $E_RETURN
        echo "Process $PID not found. Start new process . . ."

#  Set overall file update range starting from root or $URL,
#+ according to above patterns.
set_range () {
    for p in "${INCLUDE[@]}"; do
        include="$include --include \"$p\""

    for p in "${EXCLUDE[@]}"; do
        exclude="$exclude --exclude \"$p\""

# Retrieve and refine rsync update list.
get_list () {
    echo $$ > $PID_FILE || {
        echo "Can't write to pid file $PID_FILE"
        exit $E_RETURN

    echo -n "Retrieving and refining update list . . ."

    # Retrieve list -- 'eval' is needed to run rsync as a single command.
    # $3 and $4 is the date and time of file creation.
    # $5 is the full package name.
    eval /bin/nice /usr/bin/rsync \
        -r $include $exclude $URL | \
        egrep '^dr.x|^-r' | \
        awk '{print $3, $4, $5}' | \
        sort -k3 | \
        { while read line; do
            # Get seconds since epoch, to filter out obsolete pkgs.
            cur_date=$(date -d "$(echo $line | awk '{print $1, $2}')" +%s)
            #  echo $cur_date

            # Get file name.
            cur_file=$(echo $line | awk '{print $3}')
            #  echo $cur_file

            # Get rpm pkg name from file name, if possible.
            if [[ $cur_file == *rpm ]]; then
                pkg_name=$(echo $cur_file | sed -r -e \
            # echo $pkg_name

            if [ -z "$pkg_name" ]; then   #  If not a rpm file,
                echo $cur_file >> $TMP    #+ then append to download list.
            elif [ "$pkg_name" != "$previous" ]; then   # A new pkg found.
                echo $pre_file >> $TMP                  # Output latest file.
                previous=$pkg_name                      # Save current.
            elif [ "$cur_date" -gt "$pre_date" ]; then
                                                #  If same pkg, but newer,
                pre_date=$cur_date              #+ then update latest pointer.
            echo $pre_file >> $TMP              #  TMP contains ALL
                                                #+ of refined list now.
            # echo "subshell=$BASH_SUBSHELL"

    }       # Bracket required here to let final "echo $pre_file >> $TMP" 
            # Remained in the same subshell ( 1 ) with the entire loop.

    RET=$?  # Get return code of the pipe command.

    [ "$RET" -ne 0 ] && {
        echo "List retrieving failed with code $RET"
        exit $E_RETURN

    echo "done"; echo

# Real rsync download part.
get_file () {

    echo "Downloading..."
    /bin/nice /usr/bin/rsync \
        $OPTS \
        --filter "merge,+/ $TMP" \
        --exclude '*'  \
        $URL $DEST     \
        | /usr/bin/tee $LOG


   #  --filter merge,+/ is crucial for the intention. 
   #  + modifier means include and / means absolute path.
   #  Then sorted list in $TMP will contain ascending dir name and 
   #+ prevent the following --exclude '*' from "shortcutting the circuit." 

    echo "Done"

    rm -f $PID_FILE 2>/dev/null

    return $RET

# -------
# Main
# -------

if [ "$RET" -eq 0 ]; then
    /usr/bin/logger -t ${0##*/} "Fedora update mirrored successfully."
    /usr/bin/logger -t ${0##*/} \
    "Fedora update mirrored with failure code: $RET"

exit $RET

See also Example A-32.


Using rcp, rsync, and similar utilities with security implications in a shell script may not be advisable. Consider, instead, using ssh, scp, or an expect script.


Secure shell, logs onto a remote host and executes commands there. This secure replacement for telnet, rlogin, rcp, and rsh uses identity authentication and encryption. See its manpage for details.

Example 16-44. Using ssh

# remote.bash: Using ssh.

# This example by Michael Zick.
# Used with permission.

#   Presumptions:
#   ------------
#   fd-2 isn't being captured ( '2>/dev/null' ).
#   ssh/sshd presumes stderr ('2') will display to user.
#   sshd is running on your machine.
#   For any 'standard' distribution, it probably is,
#+  and without any funky ssh-keygen having been done.

# Try ssh to your machine from the command-line:
# $ ssh $HOSTNAME
# Without extra set-up you'll be asked for your password.
#   enter password
#   when done,  $ exit
# Did that work? If so, you're ready for more fun.

# Try ssh to your machine as 'root':
#   $  ssh -l root $HOSTNAME
#   When asked for password, enter root's, not yours.
#          Last login: Tue Aug 10 20:25:49 2004 from localhost.localdomain
#   Enter 'exit' when done.

#  The above gives you an interactive shell.
#  It is possible for sshd to be set up in a 'single command' mode,
#+ but that is beyond the scope of this example.
#  The only thing to note is that the following will work in
#+ 'single command' mode.

# A basic, write stdout (local) command.

ls -l

# Now the same basic command on a remote machine.
# Pass a different 'USERNAME' 'HOSTNAME' if desired:

#  Now excute the above command-line on the remote host,
#+ with all transmissions encrypted.

ssh -l ${USER} ${HOST} " ls -l "

#  The expected result is a listing of your username's home
#+ directory on the remote machine.
#  To see any difference, run this script from somewhere
#+ other than your home directory.

#  In other words, the Bash command is passed as a quoted line
#+ to the remote shell, which executes it on the remote machine.
#  In this case, sshd does  ' bash -c "ls -l" '   on your behalf.

#  For information on topics such as not having to enter a
#+ password/passphrase for every command-line, see
#+    man ssh
#+    man ssh-keygen
#+    man sshd_config.

exit 0


Within a loop, ssh may cause unexpected behavior. According to a Usenet post in the comp.unix shell archives, ssh inherits the loop's stdin. To remedy this, pass ssh either the -n or -f option.

Thanks, Jason Bechtel, for pointing this out.


Secure copy, similar in function to rcp, copies files between two different networked machines, but does so using authentication, and with a security level similar to ssh.

Local Network


This is a utility for terminal-to-terminal communication. It allows sending lines from your terminal (console or xterm) to that of another user. The mesg command may, of course, be used to disable write access to a terminal

Since write is interactive, it would not normally find use in a script.


A command-line utility for configuring a network adapter (using DHCP). This command is native to Red Hat centric Linux distros.



Send or read e-mail messages.

This stripped-down command-line mail client works fine as a command embedded in a script.

Example 16-45. A script that mails itself

# self-mailer.sh: Self-mailing script

adr=${1:-`whoami`}     # Default to current user, if not specified.
#  Typing 'self-mailer.sh wiseguy@superdupergenius.com'
#+ sends this script to that addressee.
#  Just 'self-mailer.sh' (no argument) sends the script
#+ to the person invoking it, for example, bozo@localhost.localdomain.
#  For more on the ${parameter:-default} construct,
#+ see the "Parameter Substitution" section
#+ of the "Variables Revisited" chapter.

# ============================================================================
  cat $0 | mail -s "Script \"`basename $0`\" has mailed itself to you." "$adr"
# ============================================================================

# --------------------------------------------
#  Greetings from the self-mailing script.
#  A mischievous person has run this script,
#+ which has caused it to mail itself to you.
#  Apparently, some people have nothing better
#+ to do with their time.
# --------------------------------------------

echo "At `date`, script \"`basename $0`\" mailed to "$adr"."

exit 0

#  Note that the "mailx" command (in "send" mode) may be substituted
#+ for "mail" ... but with somewhat different options.

Similar to the mail command, mailto sends e-mail messages from the command-line or in a script. However, mailto also permits sending MIME (multimedia) messages.


Show mail statistics. This command may be invoked only by root.

root# mailstats
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


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 sendmail, and is not applicable to a dial-up POPmail account.



A daemon is a background process not attached to a terminal session. Daemons perform designated services either at specified times or explicitly triggered by certain events.

The word "daemon" 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.