Sometimes when you try to execute a long command, it scrolls beyond the screen. Then if you want to modify the command and re-execute it, there is an easy way for it. Just type “fc” which will load the command in your default editor; in my case vi. Now you can modify the command in the editor and exit the editor, and your modified command is executed automatically.

For example try typing the following command in the bash shell and type “fc“.

$ find /etc -iname '*.conf' -exec grep -H 'log' {} \;

$ fc

“fc” will bring the last command typed into an editor, “vi” if that’s the default editor. Of course you can specify a different editor by using the -e switch as follows:

$ fc -e emacs

To list last few commands, type:

$ fc -l

For the last 10 commands it will be:

$ fc -l -10

To seach for a command, type “CTRL+r” at the shell prompt for starting a search as you type prompt. Once you found your command, press enter to execute it.

If you want to transpose two characters say you typed ‘sl’ instead of ‘ls’. Then move the cursor between ‘sl’ and type “CTRL+t“.

Thank you,

Ravi Bhure

UMASK is a UNIX environment variable which automatically sets file permissions on newly created files.

The UMASK variable can be confusing to use, because it does work as a mask. In other words, you set the permissions that you do not want in the UMASK.

To calculate permissions which will result from specific UMASK values, subtract the UMASK from 666 for files and from 777 for directories.

If you want all files created with permissions of 666, set your UMASK to 000. Alternatively, if you want all files created with permissions of 000, set your UMASK to 666.

A reasonable value for UMASK is 022, which will cause files to be created with permissions of 644 (rw-r–r–) and directories to be created with permissions of 755 (rwxr-xr-x).

A more secure value for UMASK is 066, which will cause files to be created with permissions of 600 (rw——-) and directories to be created with permissions of 700 (rwx——).

UMASK is nomally defined in the .profile or .login user startup files.

Thank you,

Ravi Bhure

Introduction-

There are many different to find out the number of hosts (machine) within network specially LAN.

Method(1)– Simplest way to use ping command with -b option… and specify broadcast address. Use ifconfig command to find the broadcast address….

root@arunbagul:/home/arun# ping -b 192.168.0.255
WARNING: pinging broadcast address
64 bytes from 192.168.0.100: icmp_seq=1 ttl=64 time=0.512 ms
64 bytes from 192.168.0.190: icmp_seq=1 ttl=64 time=0.531 ms
…
64 bytes from 192.168.0.232: icmp_seq=1 ttl=64 time=0.557 ms
…….

** If the number of Host within network is less (class C network), then you can below script (ping command)…

root@arunbagul:~# cat arun.sh

#!/bin/bash
for ip in $(seq 1 254);
do
ping -c 1 192.168.0.${ip} >/dev/null
if [ $? -eq 0 ]; then
echo “192.168.0.$ip is UP”
fi
done
echo “done”

root@arunbagul:~#

root@arunbagul:~# sh arun.sh
192.168.0.1 is UP
192.168.0.2 is UP
…
192.168.0.13 is UP
192.168.0.14 is UP
192.168.0.15 is UP
…..
…….
done

root@arunbagul:~#

Method(2) fping command –

fping send ICMP ECHO_REQUEST packets to network host. fping uses the Internet Control Message Protocol (ICMP) echo request to determine if a target host is responding. fping differs from ping in that you can specify any number of targets on the command line, or specify a file containing the lists of targets to ping. Instead of sending to one target until it times out or replies, fping will send out a ping packet and move on to the next target in a round-robin fashion.

In the default mode, if a target replies, it is noted and removed from the list of targets to check; if a target does not respond within a certain
time limit and/or retry limit it is designated as unreachable. fping also supports sending a specified number of pings to a target, or looping indefinitely (as in ping ).

Options-

-a Show systems that are alive.

-A Display targets by address rather than DNS name.

-bn Number of bytes of ping data to send.

-c Number of request packets to send to each target.

-d Use DNS to lookup address of return ping packet. This allows you to give fping a list of IP addresses as input and print hostnames in the
output.

-e Show elapsed (round-trip) time of packets.

-g Generate a target list from a supplied IP netmask, or a starting and ending IP

-l Loop sending packets to each target indefinitely.

root@arunbagul:~# fping -g 192.168.0.0 192.168.0.255

ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.2
ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.3
ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.4
ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.5
ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.6
…

root@arunbagul:~#

OR

root@arunbagul:~# fping -g 192.168.0.0/24

ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.2
ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.3
ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.4
ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.5
ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.6
ICMP Host Unreachable from 192.168.0.1 for ICMP Echo sent to 192.168.0.7
….

root@arunbagul:~#

Method(3) nmap command –

Nmap (“Network Mapper“) is an open source tool for network exploration and security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what
services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. While Nmap is commonly used for security audits, many systems and network
administrators find it useful for routine tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime.

Options –

-sL (List Scan)
The list scan is a degenerate form of host discovery that simply lists each host of the network(s) specified, without sending any packets
to the target hosts. By default, Nmap still does reverse-DNS resolution on the hosts to learn their names.

-sP (Ping Scan)
This option tells Nmap to only perform a ping scan (host discovery), This is one step more intrusive than the list scan.

-P0 (No ping)
This option skips the Nmap discovery stage altogether. Normally, Nmap uses this stage to determine active machines for heavier scanning. By
default, Nmap only performs heavy probing such as port scans, version detection, or OS detection against hosts that are found to be up.

-PS [portlist] (TCP SYN Ping)
This option sends an empty TCP packet with the SYN flag set. The default destination port is 80. A comma separated list of ports can even be
specified (e.g. -PS22,23,25,80,113,1050,35000), in which case probes will be attempted against each port in parallel.

-PU [portlist] (UDP Ping)
Another host discovery option is the UDP ping, which sends an empty (unless –data-length is specified) UDP packet to the given ports. The
portlist takes the same format as with the previously discussed -PS and -PA options

-n (No DNS resolution)
Tells Nmap to never do reverse DNS resolution on the active IP addresses it finds

-R (DNS resolution for all targets)
Tells Nmap to always do reverse DNS resolution on the target IP addresses. Normally reverse DNS is only performed against responsive (online) hosts.

root@arunbagul:~# nmap -sP 192.168.0.0/24

Starting Nmap 4.20 ( http://insecure.org ) at 2008-04-06 10:52 IST
Host arunbagul.laptop (192.168.0.1) appears to be up.
Host shekar.com (192.168.0.2) appears to be up.
Host tsantosh.desktop (192.168.0.3) appears to be up.
…
…….

Nmap finished: 256 IP addresses (1 host up) scanned in 5.670 seconds
root@arunbagul:~#

Method(4) NAST (Network Analyzer Sniffer Tool) –

Nast is a packet sniffer and a LAN analyzer based on Libnet and Libpcap. It can sniff in normal mode or in promiscuous mode. It dumps the headers of packets and the payload in ascii or ascii-hex format. You can apply a filter. The sniffed data can be saved in a separated file.

Options-

-i, –interface
Select the Interface, if not specified will be auto-detected.

-p, –promisc
Disable promiscuous mode on NIC.

-d, –ascii-data
Print data in ascii format.

-x, –ascii-hex-data
Print data in ascii-hex format.

-f, –filter <“filter”>
Apply <“filter”> to sniffer (see “FILTER SYNTAX” section below for syntax)

–ld <filename>
Log captured data to <filename> (only payload). Use -l to log all packet instead, useful with -B

-m, –host-list
Map the LAN by performing a series of ARP request to sequential subnet IP addresses.

-s, –tcp-stream
Follow a TCP/IP connection printing all data in payload. You must specify the IP addresses of the ends.

-g, –find-gateway
Try to find possible Internet-gateways.

-r, –reset-connection
Destroy an established connection. You must specify the IP addresses of the ends and at least one port.(?)

-S, –port-scanner
Performs a half-open port scanning on the selected host. It tries also to determine some firewall (just iptables) rules.

-M, –multi-port-scanner
Same as above (-S) but done on all hosts of the lan.

root@arunbagul:~# nast
Nast V. 0.2.0
Sniffing on:

– Device: eth0
– MAC address: XX:1X:3X:XX:0X:XX
– IP address: 192.168.XX.XX
– Netmask: 255.255.255.0
– Promisc mode: Set
– Filter: None
– Logging: None

—[ ARP ]———————————————————–
XX:XX:XX:0X:C5:FA -> XX:XX:FF:00:XX:XX
Type: ARP request: Who has 192.XX.XX.? Tell XX.XX.XX.XX
Hardware size: 6 – Protocol size: 4
Packet Number: 1

—[ UDP ]———————————————————–
192.168.X.X:1985(unknown) -> X.X.X.X:1985(unknown)
Version: 4 Total Lenght: 48 TTL: 1
Packet Number: 2

Packets Received: 4
Packets Dropped by kernel: 0
root@arunbagul:~#

root@arunbagul:~# nast -m
Nast V. 0.2.0
Mapping the Lan for 255.255.254.0 subnet … please wait

MAC address Ip address (hostname)
===========================================================
XX:XX:XX:XX:XX:XX 192.168.X.X (mygateway.com)
XX:XX:XX:XX:XX:XX 192.168.X.X (ravi.desktop)
….
……

root@arunbagul:~#

Method(5) nbtscan command –

nbtscan is a program for scanning networks for NetBIOS name information. It sends NetBIOS status query to each address in supplied range and lists received information in human readable form. For each responded host it lists IP address, NetBIOS computer name, logged-in user name and MAC address.

Options-

-v Verbose output. Print all names received from each host.

-d Dump packets. Print whole packet contents. Cannot be used with -v, -s or -h options.

-e Format output in /etc/hosts format.

-t timeout
Wait timeout seconds for response. Default 1.

-m retransmits
Number of retransmits. Default 0.

-f filename
Take IP addresses to scan from file “filename”

scan_range
What to scan. Can either be single IP or range of addresses in one of two forms: X.X.X.X/24 or X.X.X.X-24

root@arunbagul:~# nbtscan 192.18.0.0/24
Doing NBT name scan for addresses from 192.18.0.0/24

IP address NetBIOS Name Server User MAC address
——————————————————————————
192.168.0.X MINDTOOKPUG-DX <server> XXX XX:XX:XX:XX:XX:XX
192.168.0.X MUCHTHESE-DV <server> <unknown> XX:XX:XX:XX:XX:XX
…
……

root@arunbagul:~#

Thank you,
Arun Bagul

1] cat the file arun.txt which has several blank lines

root@indianGNU.org:/home# cat arun.txt
This is first line of file

this is second line

ok this script will help you to delete all blank links from file
How to start shell scripting

make sure that you will win

arun bagul

end of file

root@indianGNU.org:/home#

2] Run command for removing lines

root@indianGNU.org:/home# grep -v “^$” arun.txt > no_blank_lines.txt

3] Chcek new file –

root@indianGNU.org:/home# cat no_blank_lines.txt
This is first line of file
this is second line
ok this script will help you to delete all blank links from file
How to start shell scripting
make sure that you will win
arun bagul
end of file
root@indianGNU.org:/home#

1] Introduction:

root is super user with unrestricted access to all system resources and files in Linux OS. The uid and gid of root user is 0 (zero). But many time the system/Linux admin wants to give some restricted access to some user or groups of user. In such case the sudo is the best technique to do this….

sudo utility allows users defined in the /etc/sudoers configuration file to have temporary access to run commands. Normally they would not able to do it, due to file permission restrictions. The commands can be run as user “root” or as any other user defined in the /etc/sudoers configuration file.

All commands run as sudo are logged in the log file /var/log/messages.

2] Sudo configuration file – /etc/sudoers

/etc/sudoers file contains all the configuration and permission parameters needed for sudo to work. There are a number of guidelines that need to be followed when editing it with visudo.

Format of sudo file is as below…

user_names/group_name servername = (usernames command can be run as) command

3] How to open sudo file.. /etc/sudoers

Always use visudo command to open /etc/sudoers file

root@indiangnu.org:~# visudo

4] How to permit few user to start/restart/stop apache server.

* open sudore file by using above command

# access to Apache2 and MySQL to webgroup group with out password

%webgroup ALL= NOPASSWD: /etc/init.d/apache2

%webgroup ALL= NOPASSWD: /etc/init.d/mysql restart

web ALL= NOPASSWD: /etc/init.d/apache2

* In above example the users in group webgroup can only restart MySQL, they can’t stop or start MySQL service.

# access to Apache2 with password to user webuser

webuser ALL= /etc/init.d/apache2

# Members of the myroot group may gain root privileges

%myroot ALL=(ALL) ALL

4] How to use sudo

web@indiangnu.org:~$ sudo /etc/init.d/apache2 restart
* Forcing reload of apache 2.0 web server… [ ok ]
web@indiangnu.org:~$

** Here in below example the webuser need to enter his password to very that ‘webuser’ is authentic user.

webuser@indiangnu.org:~$ sudo /etc/init.d/apache2 restart
Password:
* Forcing reload of apache 2.0 web server… [ ok ]
webuser@indiangnu.org:~$