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Shell scripting

UNIX Command Line And The History Of The Shell

Mastering shell prompt can save tons of time. From the article:

The way you interface with a computer is changing constantly. Operating systems that once started as a command line-only interface have moved to a graphical front end. But moving away from what made the operating system great isn’t always a step in the right direction. The IBM® AIX® operating system has kept to what’s important: stability, functionality, robustness. And it has done it by keeping a strong command-line interface (CLI). If you never learned to use the CLI or need a refresher on its basics, read on.

After reading this article, you should now be able to use the Korn shell in ways you may not have known before. Mastering the command line can simplify your work and help you better understand how to make the shell and command line work for you rather than you working harder for it.

Goosh.org Unix-like Shell For Google

Neat idea:

goosh.org – the unofficial google shell. This google-interface behaves similar to a unix-shell.
You type commands and the results are shown on this page.

=> goosh.org

An Interview With Chet Ramey – Maintainer Of Bash Shell

BASH shell is default on many UNIX / Linux systems. There is an interview with Chat Ramney, maintainer of BASH, the Bourne Again Shell. He talke about his experience maintaining Bash and few other things. From the page:

Bash, or the Bourne-Again Shell is a Unix shell created in 1987 by Brian Fox. According to Wikipedia, the name is a pun on an earlier Unix shell by Stephen Bourne (called the Bourne shell), which was distributed with Version 7 Unix in 1978.

In 1990, Chet Ramey, Manager of the Network Engineering and Security Group in Technology Infrastructure Services at Case Western Reserve University, became the primary maintainer of the language.

Computerworld tracked down Ramey to find out more.

=> The A-Z of Programming Languages: BASH/Bourne-Again Shell

How To Become a UNIX command-line Wizard

Some good tips about becoming a command-line wizard…

It’s easy to keep doing things the same way simply because you’re used to it. Expanding your command-line resources can provide a big increase in your productivity and propel you toward becoming a UNIX command line wizard!

As a follow-up to Michael Stutz’s excellent article, this article provides 10 more good habits to adopt that will improve your UNIX command-line efficiency. Learn about common errors and how to overcome them, and discover exactly why these 10 UNIX habits are worth picking up!

=> Learn 10 more good UNIX usage habits

mplayer: Play All Mp3 Files In Reverse Order

mplayer lacks an option for selecting files in a reverse order. So here is a quick way to playback all mp3 files in reverse order:
$ ls -1 -r *.mp3 > mp3.rev
$ mplayer -playlist mp3.rev


  • -r reverse order while sorting
  • -1 list one file per line
  • -playlist file : Play files according to a playlist file

Let’s hear your shell hack in the comments.

Using Color Effectively At a Shell Prompt / Console

Pixelbeat has published a nice article about setting up colorful console under Linux / UNIX. From the article:

I find it very productive working in a terminal environment, as it’s efficient and flexible to deal with processes and data, especially text, and especially on remote machines.

Now terminals have advanced in capability over time, with some form of “xterm” being the usual terminal of choice. Therefore one should not restrict programs to their usual monochrome defaults, as colour can be used to greatly ease the parsing of text by humans. We have dedicated sensors and portions of the brain specifically for colour, so we should not ignore them.

Related: Setting up themes and colorful bash prompt

Get Detailed Information About Particular IP address Connections Using netstat Command

netstat command and shell pipe feature can be used to dig out more information about particular IP address connection. You can find out total established connections, closing connection, SYN and FIN bits and much more. You can also display summary statistics for each protocol using netstat.

This is useful to find out if your server is under attack or not. You can also list abusive IP address using this method.
# netstat -nat | awk '{print $6}' | sort | uniq -c | sort -n

      1 CLOSE_WAIT
      1 established)
      1 Foreign
      3 FIN_WAIT1
      3 LAST_ACK
     17 LISTEN
    154 FIN_WAIT2
    327 TIME_WAIT

Dig out more information about a specific ip address:
# netstat -nat |grep {IP-address} | awk '{print $6}' | sort | uniq -c | sort -n

      2 LAST_ACK
      2 LISTEN
      4 FIN_WAIT1
     91 TIME_WAIT
    130 FIN_WAIT2

Busy server can give out more information:
# netstat -nat |grep | awk '{print $6}' | sort | uniq -c | sort -n

  64 FIN_WAIT_1
  65 FIN_WAIT_2

Get List Of All Unique IP Address

To print list of all unique IP address connected to server, enter:
# netstat -nat | awk '{ print $5}' | cut -d: -f1 | sed -e '/^$/d' | uniq
To print total of all unique IP address, enter:
# netstat -nat | awk '{ print $5}' | cut -d: -f1 | sed -e '/^$/d' | uniq | wc -l


Find Out If Box is Under DoS Attack or Not

If you think your Linux box is under attack, print out a list of open connections on your box and sorts them by according to IP address, enter:
# netstat -atun | awk '{print $5}' | cut -d: -f1 | sed -e '/^$/d' |sort | uniq -c | sort -n


You can simply block all abusive IPs using iptables or just null route them.

Get Live View of TCP Connections

You can use tcptrack command to display the status of TCP connections that it sees on a given network interface. tcptrack monitors their state and displays information such as state, source/destination addresses and bandwidth usage in a sorted, updated list very much like the top command.

Display Summary Statistics for Each Protocol

Simply use netstat -s:
# netstat -s | less
# netstat -t -s | less
# netstat -u -s | less
# netstat -w -s | less
# netstat -s


    88354557 total packets received
    0 forwarded
    0 incoming packets discarded
    88104061 incoming packets delivered
    96037391 requests sent out
    13 outgoing packets dropped
    66 fragments dropped after timeout
    295 reassemblies required
    106 packets reassembled ok
    66 packet reassembles failed
    34 fragments failed
    18108 ICMP messages received
    58 input ICMP message failed.
    ICMP input histogram:
        destination unreachable: 7173
        timeout in transit: 472
        redirects: 353
        echo requests: 10096
    28977 ICMP messages sent
    0 ICMP messages failed
    ICMP output histogram:
        destination unreachable: 18881
        echo replies: 10096
    1202226 active connections openings
    2706802 passive connection openings
    7394 failed connection attempts
    47018 connection resets received
    23 connections established
    87975383 segments received
    95235730 segments send out
    681174 segments retransmited
    2044 bad segments received.
    80805 resets sent
    92689 packets received
    14611 packets to unknown port received.
    0 packet receive errors
    96755 packets sent
    48452 invalid SYN cookies received
    7357 resets received for embryonic SYN_RECV sockets
    43 ICMP packets dropped because they were out-of-window
    5 ICMP packets dropped because socket was locked
    2672073 TCP sockets finished time wait in fast timer
    441 time wait sockets recycled by time stamp
    368562 delayed acks sent
    430 delayed acks further delayed because of locked socket
    Quick ack mode was activated 36127 times
    32318597 packets directly queued to recvmsg prequeue.
    741479256 packets directly received from backlog
    1502338990 packets directly received from prequeue
    18343750 packets header predicted
    10220683 packets header predicted and directly queued to user
    17516622 acknowledgments not containing data received
    36549771 predicted acknowledgments
    102672 times recovered from packet loss due to fast retransmit
    Detected reordering 1596 times using reno fast retransmit
    Detected reordering 1 times using time stamp
    8 congestion windows fully recovered
    32 congestion windows partially recovered using Hoe heuristic
    19 congestion windows recovered after partial ack
    0 TCP data loss events
    39951 timeouts after reno fast retransmit
    29653 timeouts in loss state
    197005 fast retransmits
    186937 retransmits in slow start
    131433 other TCP timeouts
    TCPRenoRecoveryFail: 20217
    147 times receiver scheduled too late for direct processing
    29010 connections reset due to unexpected data
    365 connections reset due to early user close
    6979 connections aborted due to timeout

Display Interface Table

You can easily display dropped and total transmitted packets with netstat for eth0:
# netstat --interfaces eth0

Kernel Interface table
eth0       1500   0  2040929      0      0      0  3850539      0      0      0 BMRU

Other netstat related articles / tips:

  1. Get Information about All Running Services Remotely
  2. Linux / UNIX Find Out What Program / Service is Listening on a Specific TCP Port

Read following man pages for the details:
$ man netstat
$ man cut
$ man awk
$ man sed
$ man grep

Updated for accuracy.