Make dir if not exist in bash

Posted on January 3, 2015June 19, 2015 by coder

if [ ! -d directory ]; then
  mkdir directory
fi


if you dont have sub directories you need to  use    "-p"  option

mkdir -p  /dxx/xx/xx/xx/e/3/3/5/DIRECTORY


if [[ ! -e $dir ]]; then
    mkdir $dir
elif [[ ! -d $dir ]]; then
    echo "$dir already exists but is not a directory"
fi

Vpn over TCP or UDP

Posted on January 2, 2015June 27, 2015 by coder

A VPN is for wrapping raw IP packets into some kind of “tunnel” between two sites.

– Connection speed is going slow down cause of Encryption.

Performance : UDP is better

Security : For Experts , it is The same . UDP is also secure, because VPN service adding EXTRA header inside of UDP

Connectionless > TCP is a protocol which sits on top of IP, (which are “unreliable“: Packet lost, duplicated, reordered)

to provide a reliable two-directional channel for data bytes, where bytes always reach the receiver in the order they were sent. TCP does that by using a complex assortment of metadata with explicit acknowledges and reemissions. Thus, TCP incurs a slight network overhead.

If VPN uses TCP, it is paying the TCP overhead twice. An UDP-based VPN thus has the potential for slightly better performance. On the other hand, the cryptographic protection of the VPN requires some state management, which may be harder for the VPN implementation when using UDP, hence it is possible that the UDP-based VPN has an extra overhead to contend with.

Performance is tested by me, and UDP connection performance obviously better than TCP.

combine string variables on bash. concatenate string bash

Posted on January 1, 2015June 19, 2015 by coder

mystring="${arg5}12${arg_someload}endoffile"

# ------------------------------------------
$ arg1=foo
$ arg2=bar
$ mystring="$arg1"
$ mystring+="12"
$ mystring+="$arg2"
$ mystring+="endoffile"
$ echo "$mystring"
foo12barendoffile

# ------------------------------------------
mystring="$string1$string2"

# ------------------------------------------

# ------------------------------------------

# ------------------------------------------

mystring="${arg5}12${arg_someload}endoffile"

# ------------------------------------------

$ arg1=foo

$ arg2=bar

$ mystring="$arg1"

$ mystring+="12"

$ mystring+="$arg2"

$ mystring+="endoffile"

$ echo "$mystring"

foo12barendoffile

# ------------------------------------------

mystring="$string1$string2"

# ------------------------------------------

foo="Hello"
foo="$foo World"
echo $foo
> Hello World

# ------------------------------------------

a='hello'
b='world'
c=$a$b
echo $c
> helloworld

foo="Hello"

foo="$foo World"

echo $foo

> Hello World

# ------------------------------------------

a='hello'

b='world'

c=$a$b

echo $c

> helloworld