How to change the hostname in ubuntu without restart server

sudo vi /etc/hostname

change the existing old_hostname with new_hostname

sudo vi /etc/hosts

change the existing old_hostname with new_hostname

127.0.1.1 old_hostname -> 127.0.1.1 new_hostname

Issues following command without reset the system

sudo hostname new_hostname

Using curl to download file

curl http://some.url --output some.file

which --output flag denotes the filename (some.file) of the downloaded URL (http://some.url)

Regular Expression Concept

Regular Expression concepts

A regular expression, often called a pattern, is an expression used to specify a set of strings required for a particular purpose. A simple way to specify a finite set of strings is to list its elements or members.

Boolean "or"
A vertical bar separates alternatives. For example, abc|abb can match "abc" or "abb".

Grouping
Parentheses are used to define the scope and precedence of the operators (among other uses). For example, abc|abb and ab(c|b) are equivalent patterns which both describe the set of "abc" or "abb".

Quantification
    ?     The question mark indicates zero or one occurrences of the preceding element. For example, colou?r matches both "color" and "colour".

    *     The asterisk indicates zero or more occurrences of the preceding element. For example, ab*c matches "ac", "abc", "abbc", "abbbc", and so on.

    +     The plus sign indicates one or more occurrences of the preceding element. For example, ab+c matches "abc", "abbc", "abbbc", and so on, but not "ac".

    {n}     The preceding item is matched exactly n times.

    {min,}     The preceding item is matched min or more times.

    {min,max}     The preceding item is matched at least min times, but not more than max times.

      .      Dot Matches any single character (many applications exclude newlines, and exactly which characters are considered newlines is flavor-, character-encoding-, and platform-specific, but it is safe to assume that the line feed character is included). Within POSIX bracket expressions, the dot character matches a literal dot. For example, a.c matches "abc", etc., but [a.c] matches only "a", ".", or "c".

  [ ]  A bracket expression. Matches a single character that is contained within the brackets. For example, [abc] matches "a", "b", or "c". [a-z] specifies a range which matches any lowercase letter from "a" to "z". These forms can be mixed: [abcx-z] matches "a", "b", "c", "x", "y", or "z", as does [a-cx-z].
The - character is treated as a literal character if it is the last or the first (after the ^, if present) character within the brackets: [abc-], [-abc]. Note that backslash escapes are not allowed. The ] character can be included in a bracket expression if it is the first (after the ^) character: []abc].

    [^ ]  Matches a single character that is not contained within the brackets. For example, [^abc] matches any character other than "a", "b", or "c". [^a-z] matches any single character that is not a lowercase letter from "a" to "z". Likewise, literal characters and ranges can be mixed.

     ^         Matches the starting position within the string. In line-based tools, it matches the starting position of any line.

      $        Matches the ending position of the string or the position just before a string-ending newline. In line-based tools, it matches the ending position of any line.

Keyboard short cut for windows stickynotes

Ctrl+U     Underlined text Ctrl+T      StrikeThrough Ctrl+Shift+L     Bulleted (press once) or Numbered (press twice) list Ctrl+Shift+>     Increased text size Ctrl+Shift+<     Decreased text size Ctrl+A     Select all Ctrl+Shift+A     Toggles all caps Ctrl+L     Left aligns text Ctrl+R     Right aligns text Ctrl+E     Centers text Ctrl+Shift+L     Small Alpha list (3rd), Capital Alpha list (4th), small roman (5th), Capital roman (6th) Ctrl+Scroll Wheel     Increase/Decrease text size Ctrl+1     Single-space lines Ctrl+2     Double-space lines Ctrl+5     Set 1.5-line spacing Ctrl+=     Subscript Ctrl+Shift++     Superscript Ctrl+Z     Undo Ctrl+Y     Redo Ctrl+X     Cut Ctrl+C     Copy Ctrl+V     Paste Ctrl+N     New sticky note

Open Command Prompt from windows explorer

This tutorial share out how to open command from windows explorer for windows 8, windows 7 and windows XP

• Windows 8 

1. Select the folder you want to open in the command prompt
2. Go to File -> Open Command Prompt

• Windows 7
1. Select the folder you want to open in the command prompt
2. Right-click on it while holding the Shift key
3. Select Open command window here

• Windows XP

1. Open regedit
2. Navigate to HKEY_LOCAL_MACHINE/Software/Classes/Folder/Shell
3. Create a new key called Command Prompt
4. Your default value should be Command Prompt Here
5. Create a new key called Command under the Command Prompt key
6. Your default value should be cmd.exe /k pushd %

Your popup menu when you right click on a folder will now contain an option called Command Prompt Here which will open a command prompt in the selected folder.

Use ls Command to display file size unit

'ls' command to display the file size with the UNIT of KiloBytes, MegaBytes

ls -lah

Disable HyberV

HyperV may  cause incompatible for virtual box, We can disable HyperV with following command:

bcdedit /set hypervisorlaunchtype off

To Reenable it back with

bcdedit /set hypervisorlaunchtype auto

  And restart the system

Mount Virtual Box share folder

Mount Virtual Box share folder command
1. Make sure you add a share folder in Share Folder Setting
2. go to /etc/passwd to check the uid and gid
3. open the terminal and issue the command

sudo mount -t vboxsf -o uid=[uid],gid=[gid] [share_folder_name] [mount_path]    

Sometimes we may need to mount Virtual Box Guess Addition ISO file, following command use to mount VBoxGuessAdditions.ISO

sudo mount -t iso9660 -o loop  [VBoxGussAddition.ISO path] [mount_path]

Upgrade jdk in centos 5 from 1.6 to 1.7 with alternatives command

1. Download latest java archive from oracle official website
2. Extract the the archieve with tar

# tar xzf jdk-7u79-linux-i586.tar.gz

3. After extracting Java archive file, set up to use newer version of Java using alternatives

# cd /opt/jdk1.7.0_79/
# /usr/sbin/alternatives --install /usr/bin/java java /opt/jdk1.7.0_79/bin/java 2
# /usr/sbin/alternatives --install /usr/bin/jar jar /opt/jdk1.7.0_79/bin/jar 2
# /usr/sbin/alternatives --install /usr/bin/javac javac /opt/jdk1.7.0_79/bin/javac 2
# /usr/sbin/alternatives --set java  /opt/jdk1.7.0_79/bin/java
# /usr/sbin/alternatives --set jar /opt/jdk1.7.0_79/bin/jar
# /usr/sbin/alternatives --set javac /opt/jdk1.7.0_79/bin/javac  

4. After complete step 3, check Java version, make sure the version is what we expected
# java -version

Allocate a 512M swap file in digital ocean linux droplet

By default Digital Ocean VPS instance is not configured to use swap space
login as root user, perform the following:
dd if=/dev/zero of=/swap.dat bs=1024 count=512K 
mkswap /swap.dat 
swapon /swap.dat 
#Edit the /etc/fstab, add the following entry. 
/swap.dat none swap sw 0 0