Select command in bash

 #!/bin/bash

# Define the menu list here

select brand in Samsung Sony iphone symphony Walton

do

echo "You have chosen $brand"

done

#!/bin/bash

echo "Which Operating System do you like?"

# Operating system names are used here as a data source

select os in Ubuntu LinuxMint Windows8 Windows7 WindowsXP

do

case $os in

# Two case values are declared here for matching

"Ubuntu"|"LinuxMint")

echo "I also use $os."

;;

# Three case values are declared here for matching

"Windows8" | "Windows10" | "WindowsXP")

echo "Why don't you try Linux?"

;;

# Matching with invalid data

*)

echo "Invalid entry."

break

;;

esac

done

find difference between ; and +

Difference between {} \; and {} +

find . -exec grep chrome {} \;

or

find . -exec grep chrome {} +

find will execute grep and will substitute {} with the filename(s) found. The difference between ; and + is that with ; a single grep command for each file is executed whereas with + as many files as possible are given as parameters to grep at once.

find . -exec grep chrome {} +

find will execute grep and will substitute {} with the filename(s) found. The difference between ; and + is that with ; a single grep command for each file is executed whereas with + as many files as possible are given as parameters to grep at once.

Ways to check reachability of remote ports

A port is a logical entity which acts as a endpoint of communication associated with an application or process on an Linux operating system. It is useful to know which ports are open and running services on a target machine before using them.

We can easily list open ports in Linux on a local machine using the netstat or several other Linux commands such NMAP.

In this guide, we will show you how to determine if ports on a remote host are reachable/open using simple netcat (in short nc) command.

netcat (or nc in short) is a powerful and easy-to-use utility that can be employed for just about anything in Linux in relation to TCP, UDP, or UNIX-domain sockets.

# yum install nc                  [On CentOS/RHEL]

# dnf install nc                  [On Fedora 22+]

$ sudo apt-get install netcat     [On Debian/Ubuntu]

We can use it to: open TCP connections, listen on arbitrary TCP and UDP ports, send UDP packets, do port scanning under both IPv4 and IPv6 and beyond.

Using netcat, you can check if a single or multiple or a range of open ports as follows. The command below will help us see if the port 22 is open on the host 192.168.56.10:

$ nc -zv 192.168.1.15 22

In the command above, the flag:

-z – sets nc to simply scan for listening daemons, without actually sending any data to them.

-v – enables verbose mode.

The next command will check if ports 80, 22 and 21 are open on the remote host 192.168.5.10 (we can use the hostname as well):

nc -zv 192.168.56.10 80 22 21

It is also possible to specify a range of ports to be scanned:’

$ nc -zv 192.168.56.10 20-80

builtin(command) bash

builtin command is useful to define a shell function with the same name as a shell builtin, retaining the functionality of the builtin within the function.

 The main use of this command is to define a shell function having the same name as the shell builtin by keeping the functionality of the builtin within the function.

Syntax:

builtin [shell-builtin [arg ..]]

Example: Here, we are creating a function to replace the ‘cd’ command. When you will use cd() function it will change the directory to the desktop directly.