Linux Introduction

What is Linux?

Linux is, in simplest terms, an operating system. It is the software on a computer that enables applications and the computer operator to access the devices on the computer to perform desired functions.

The operating system (OS) relays instructions from an application to, for instance, the computer’s processor.

The processor performs the instructed task, then sends the results back to the application via the operating system.

What is Kernel mean?

In computing, the kernel is the central component of OS.

It is a bridge between applications and the actual data processing done at the hardware level.

The kernel’s responsibilities include managing the system’s resources (the communication between hardware and software components).

What is CPU ?

It runs the Operating System and applications, constantly reviving input from the user or active software programs.

It processes the data and produces output, which may stored by an application or displayed on the screen.

Here are some CPU types:

• INTEL (x86)
• SPARC  (SUN / Oralce)
• POWER  (IBM)
• AMD

How CPU speed measured?

Clock Speed

• The most common measure of CPU speed is the clock speed, which is measured in MHz or GHz. One GHz equals 1,000 MHz, so a speed of 2.4 GHz could also be expressed as 2,400 MHz. The higher the clock speed, the more operations the CPU can execute per second.
• It’s important to realize that the clock speed of a CPU is not the only factor determining performance. Because of differences in chip architecture, one processor may be able to perform more operations than another over one cycle. Therefore, even if the first processor has a lower clock speed than the second, it may actually be faster.

Multi-Core Technology

• Some processors are “dual core” or “quad core.” These terms indicate the use of multiple CPUs on a single circuit. The idea behind this concept is to improve processing speed by using two or more processors that can run independently or coop

Lets talk about Memory

Computer memory is any physical device capable of storing information temporarily or permanently. For example, Random Access Memory (RAM) is a volatile memory that stores information on an integrated circuit  used by the Operating System, application and hardware.

Front Side Bus and Cache

• With any processor, the cache and front side bus play an important role in speed. The CPU and front side bus work together, so they should ideally run at the same speed so that one cannot slow the other down.
• The cache also plays an important role in processor speed. The cache is memory that is accessed by the CPU to aid in completing certain routine tasks faster, so the larger the cache, the greater the CPU performance.

Volatile vs. non-volatile memory

Memory can be either volatile and non-volatile memory.

Volatile memory is memory that loses its contents when the computer or hardware device loses power.

Computer RAM is an example of a volatile memory and is why if your computer freezes or reboots when working on a program, you lose anything that hasn’t been saved.

Non-volatile memory, sometimes abbreviated as NVRAM, is memory that keeps its contents even if the power is lost. EPROM is an example of a non-volatile memory.

Remember Memory is not a Storage Disk

It is very common for new computer users to be confused by what parts in the computer are memory. Although both the hard drive and RAM are memory, it is more appropriate to refer to RAM as “memory”   and a hard drive as storage.

When a program such as your Internet browser is open, it is loaded from your hard drive and placed into RAM, which allows that program to communicate with the processor at higher speeds. Anything you save to your computer, such as a picture or video, is sent to your hard drive for storage.

UNIX Principles

• Everything in UNIX is a file (including hardware)
• Small, single purpose programs
• Ability to chain programs together to perform complex tasks
• Configuration data stored in text

Logging in to LINUX system

• Two types of login screens available
• Virtual consoles (Text based) & Graphical logins (display managers)
• RHEL use GNOME/KDE display manager
• Run-level-3 is a text based & Run-level-5 is Graphical based.
• Up-to 6 virtual consoles & one graphical console
• Switch among the virtual consoles ( Ctrl-Alt-F[1-6])
• Access the graphical console by typing Ctrl-Alt-F7

Super User here is ROOT user

• Special administrative account
• Also called super user
• Root has complete control over system
• SU creates new shell as root
• SUDO commands runs command as root
• <id> command shows who you are?

Important Directories you must know

1. Home Directory           –    /root , /home/<username>
2. User Executable         –    /bin, /usr/bin,/usr/local/bin
3. System Executable     –    /sbin, /usr/sbin, /usr/local/sbin
4. Other mounts              –    /media , /mnt
5. Configuration              –    /etc
6. Temporary files           –    /tmp
7. Kernels & boot loader  –  /boot
8. Server Data                 –  /var , /srv
9. System information     –  /proc, /sys
10. Shared libraries           –  /lib,/usr/lib, /usr/local/lib

Learn about RUN Levels in LINUX

• A run-level is a system state that is defined by the services listed in /etc/rc.d.
• Configuration file to update run-level
• $ vi /etc/inittab à update line “ id:3:initdefault:
• To check which run level your are now $ who -r
• You can change the run level just by typing # init0 or init6 # init 5

LEVEL	NAME	DESCRIPTION
0	Halt	Immediately shuts down system and powers it off, if it can
1	Single user	Brings system to a bare essentials mode for maintenance
2	User-defined	Custom
3	Multi-user with console only	All services are running but X11
4	User-defined	Custom
5	Multi-user with display and console	All services are running including X11 (or GUI)
6	Reboot	Reboots the machine

What is Open source Linux?

• Software & Source code available to all
• Freedom to distribute software and source code
• Ability to modify and create derived works
• Integrity of author’s code

FEDORA Project  (Open Source)

• community support
• Personal systems

RHEL (Red Hat Enterprise Linux)

• Stable, thoroughly tested software
• Professional support servers
• Centralized Management tools for large networks

How to install apache tomcat as daemon service in Linux environment

Open the terminal and type the below command.
sudo nano /etc/init.d/tomcat

Paste the below script and save the file.

#!/bin/bash
#
# Author : Sree Choudary
# Copyright (c) 2k15
#
# Make kill the tomcat process
#
TOMCAT_HOME=/usr/local/tomcat/apache-tomcat-8.0.23
SHUTDOWN_WAIT=5

tomcat_pid() {
  echo `ps aux | grep org.apache.catalina.startup.Bootstrap | grep -v grep | awk '{ print $2 }'`
}

start() {
  pid=$(tomcat_pid)
  if [ -n "$pid" ] 
  then
    echo "Tomcat is already running (pid: $pid)"
  else
    # Start tomcat
    echo "Starting tomcat"
    /bin/sh $TOMCAT_HOME/bin/startup.sh
  fi

  return 0
}

stop() {
  pid=$(tomcat_pid)
  if [ -n "$pid" ]
  then
    echo "Stoping Tomcat"
    /bin/sh $TOMCAT_HOME/bin/shutdown.sh

    let kwait=$SHUTDOWN_WAIT
    count=0;
    until [ `ps -p $pid | grep -c $pid` = '0' ] || [ $count -gt $kwait ]
    do
      echo -n -e "\nwaiting for processes to exit";
      sleep 1
      let count=$count+1;
    done

    if [ $count -gt $kwait ]; then
      echo -n -e "\nkilling processes which didn't stop after $SHUTDOWN_WAIT seconds"
      kill -9 $pid
      echo  " \nprocess killed manually"
    fi
  else
    echo "Tomcat is not running"
  fi

  return 0
}
pid=$(tomcat_pid)

 if [ -n "$pid" ]
  then
    echo "Tomcat is running with pid: $pid"
    stop
  else
    echo "Tomcat is not running"
    start
  fi
exit 0

sudo chmod 755 /etc/init.d/tomcat
update-rc.d tomcat defaults

Finally you can start and stop tomcat using below commands.

service tomcat start
service tomcat stop

How to add port to firewall in centos

You can just open /etc/sysconfig/iptables file and add the below line and change the port which port you want to allow from firewall.

-A INPUT -m state --state NEW -m tcp -p tcp --dport 8080 -j ACCEPT

Save the file and restart iptables service.

Open the terminal and run the following command /etc/init.d/iptables restart.

Linux Terminal Command Reference

System Info

date – Show the current date and time
cal – Show this month's calendar
uptime – Show current uptime
w – Display who is online
whoami – Who you are logged in as
finger user – Display information about user
uname -a – Show kernel information
cat /proc/cpuinfo – CPU information
cat /proc/meminfo – Memory information
df – Show disk usage
du – Show directory space usage
free – Show memory and swap usage

Keyboard Shortcuts

Enter – Run the command
Up Arrow – Show the previous command
Ctrl + R – Allows you to type a part of the command you're looking for and finds it

Ctrl + Z – Stops the current command, resume with fg in the foreground or bg in the background
Ctrl + C – Halts the current command, cancel the current operation and/or start with a fresh new line
Ctrl + L – Clear the screen

command | less – Allows the scrolling of the bash command window using Shift + Up Arrow and Shift + Down Arrow
!! – Repeats the last command
command  !$ – Repeats the last argument of the previous command
Esc + . (a period) – Insert the last argument of the previous command on the fly, which enables you to edit it before executing the command

Ctrl + A – Return to the start of the command you're typing
Ctrl + E – Go to the end of the command you're typing
Ctrl + U – Cut everything before the cursor to a special clipboard, erases the whole line
Ctrl + K – Cut everything after the cursor to a special clipboard
Ctrl + Y – Paste from the special clipboard that Ctrl + U and Ctrl + K save their data to
Ctrl + T – Swap the two characters before the cursor (you can actually use this to transport a character from the left to the right, try it!)
Ctrl + W – Delete the word / argument left of the cursor in the current line

Ctrl + D – Log out of current session, similar to exit

Learn the Commands

apropos subject – List manual pages for subject
man -k keyword – Display man pages containing keyword
man command – Show the manual for command
man -t man | ps2pdf - > man.pdf  – Make a pdf of a manual page
which command – Show full path name of command
time command – See how long a command takes

whereis app – Show possible locations of app
which app – Show which app will be run by default; it shows the full path

Searching

grep pattern files – Search for pattern in files
grep -r pattern dir – Search recursively for pattern in dir
command | grep pattern – Search for pattern in the output of command
locate file – Find all instances of file
find / -name filename – Starting with the root directory, look for the file called filename
find / -name ”*filename*” – Starting with the root directory, look for the file containing the stringfilename
locate filename – Find a file called filename using the locate command; this assumes you have already used the command updatedb (see next)
updatedb – Create or update the database of files on all file systems attached to the Linux root directory
which filename – Show the subdirectory containing the executable file  called filename
grep TextStringToFind /dir – Starting with the directory called dir, look for and list all files containingTextStringToFind

File Permissions

chmod octal file – Change the permissions of file to octal, which can be found separately for user, group, and world by adding: 4 – read (r), 2 – write (w), 1 – execute (x)
Examples:
chmod 777 – read, write, execute for all
chmod 755 – rwx for owner, rx for group and world
For more options, see man chmod.

File Commands

ls – Directory listing
ls -l – List files in current directory using long format
ls -laC – List all files in current directory in long format and display in columns
ls -F – List files in current directory and indicate the file type
ls -al – Formatted listing with hidden files

cd dir – Change directory to dir
cd – Change to home
mkdir dir – Create a directory dir
pwd – Show current directory

rm name – Remove a file or directory called name
rm -r dir – Delete directory dir
rm -f file – Force remove file
rm -rf dir – Force remove an entire directory dir and all it’s included files and subdirectories (use with extreme caution)

cp file1 file2 – Copy file1 to file2
cp -r dir1 dir2 – Copy dir1 to dir2; create dir2 if it doesn't exist
cp file /home/dirname – Copy the file called filename to the /home/dirname directory

mv file /home/dirname – Move the file called filename to the /home/dirname directory
mv file1 file2 – Rename or move file1 to file2; if file2 is an existing directory, moves file1 into directoryfile2

ln -s file link – Create symbolic link link to file
touch file – Create or update file
cat > file – Places standard input into file
cat file – Display the file called file

more file – Display the file called file one page at a time, proceed to next page using the spacebar
head file – Output the first 10 lines of file
head -20 file – Display the first 20 lines of the file called file
tail file – Output the last 10 lines of file
tail -20 file – Display the last 20 lines of the file called file
tail -f file – Output the contents of file as it grows, starting with the last 10 lines

Compression

tar cf file.tar files – Create a tar named file.tar containing files
tar xf file.tar – Extract the files from file.tar

tar czf file.tar.gz files – Create a tar with Gzip compression
tar xzf file.tar.gz – Extract a tar using Gzip

tar cjf file.tar.bz2 – Create a tar with Bzip2 compression
tar xjf file.tar.bz2 – Extract a tar using Bzip2

gzip file – Compresses file and renames it to file.gz
gzip -d file.gz – Decompresses file.gz back to file

Printing

/etc/rc.d/init.d/lpd start – Start the print daemon
/etc/rc.d/init.d/lpd stop – Stop the print daemon
/etc/rc.d/init.d/lpd status – Display status of the print daemon
lpq – Display jobs in print queue
lprm – Remove jobs from queue
lpr – Print a file
lpc – Printer control tool
man subject | lpr – Print the manual page called subject as plain text
man -t subject | lpr – Print the manual page called subject as Postscript output
printtool – Start X printer setup interface

Network

ifconfig – List IP addresses for all devices on the local machine
ping host – Ping host and output results
whois domain – Get whois information for domain
dig domain – Get DNS information for domain
dig -x host – Reverse lookup host
wget file – Download file
wget -c file – Continue a stopped download

SSH

ssh user@host – Connect to host as user
ssh -p port user@host – Connect to host on port port as user
ssh-copy-id user@host – Add your key to host for user to enable a keyed or passwordless login

User Administration

adduser accountname – Create a new user call accountname
passwd accountname – Give accountname a new password
su – Log in as superuser from current login
exit – Stop being superuser and revert to normal user

Process Management

ps – Display your currently active processes
top – Display all running processes
kill pid – Kill process id pid
killall proc – Kill all processes named proc (use with extreme caution)
bg – Lists stopped or background jobs; resume a stopped job in the background
fg – Brings the most recent job to foreground
fg n – Brings job n to the foreground

Installation from source

./configure
make
make install
dpkg -i pkg.deb – install a DEB package (Debian / Ubuntu / Linux Mint)
rpm -Uvh pkg.rpm – install a RPM package (Red Hat / Fedora)

Stopping & Starting

shutdown -h now – Shutdown the system now and do not reboot
halt – Stop all processes - same as above
shutdown -r 5 – Shutdown the system in 5 minutes and reboot
shutdown -r now – Shutdown the system now and reboot
reboot – Stop all processes and then reboot - same as above
startx – Start the X system

How to install MySQL on Ubuntu/Debian

It may seem easy for some, but for others, installing MySQL on Ubuntu or Debian Linux is not an easy task. This article explains to you how to install the MySQL Server and Client packages on a Ubuntu/Debian system.

First of all, make sure your package management tools are up-to-date. Also make sure you install all the latest software available.

sudo apt-get update
sudo apt-get dist-upgrade
After a few moments (or minutes, depending on the state of your system), you’re ready to install MySQL. ~ By default, recent Ubuntu/Debian systems install a MySQL Server from the 5-branch. This is a good thing, so don’t worry.

First, install the MySQL server and client packages:

sudo apt-get install mysql-server mysql-client
When done, you have a MySQL database read to rock ‘n roll. However, there’s more to do.

You need to set a root password, for starters. MySQL has it’s own user accounts, which are not related to the user accounts on your Linux machine. By default, the root account of the MySQL Server is empty. You need to set it. Please replace ‘mypassword’ with your actual password and myhostname with your actual hostname.

sudo mysqladmin -u root -h localhost password 'mypassword'
sudo mysqladmin -u root -h myhostname password 'mypassword'
Now, you probably don’t want just the MySQL Server. Most likely you have Apache+PHP already installed, and want MySQL to go with that. Here are some libraries you need to install to make MySQL available to PHP:

sudo apt-get install php5-mysql
Or for Ruby:

sudo apt-get install libmysql-ruby
You can now access your MySQL server like this:

mysql -u root -p
Have fun using MySQL Server.

How to install Oracle Java in Linux

Check to see if your Ubuntu Linux operating system architecture is 32-bit or 64-bit, open up a terminal and run the following command below.
getconf LONG_BIT

It will print the exact number 32 or 64.

Download the Oracle Java JDK/JRE for Linux. Make sure you select the correct compressed binaries for your system architecture 32-bit or 64-bit (which end in tar.gz).
For example, if you are on Ubuntu Linux 32-bit operating system download 32-bit Oracle Java binaries.
For example, if you are on Ubuntu Linux 64-bit operating system download 64-bit Oracle Java binaries.

Important Information: 64-bit Oracle Java binaries do not work on 32-bit Ubuntu Linux operating systems, you will receive multiple system error messages, if you attempt to install 64-bit Oracle Java on 32-bit Ubuntu Linux.

Copy the Oracle Java binaries into the /usr/local/java directory. In most cases, the Oracle Java binaries are downloaded to: /home/"your_user_name"/Downloads.

32-bit Oracle Java on 32-bit Ubuntu Linux installation instructions:
Type/Copy/Paste: cd /home/"your_user_name"/Downloads
Type/Copy/Paste: sudo cp -r jdk-7u40-linux-i586.tar.gz /usr/local/java
Type/Copy/Paste: sudo cp -r jre-7u40-linux-i586.tar.gz /usr/local/java
Type/Copy/Paste: cd /usr/local/java

64-bit Oracle Java on 64-bit Ubuntu Linux installation instructions:
Type/Copy/Paste: cd /home/"your_user_name"/Downloads
Type/Copy/Paste: sudo cp -r jdk-7u40-linux-x64.tar.gz /usr/local/java
Type/Copy/Paste: sudo cp -r jre-7u40-linux-x64.tar.gz /usr/local/java
Type/Copy/Paste: cd /usr/local/java

Unpack the compressed Java binaries, in the directory /usr/local/java
32-bit Oracle Java on 32-bit Ubuntu Linux installation instructions:
Type/Copy/Paste: sudo tar xvzf jdk-7u40-linux-i586.tar.gz
Type/Copy/Paste: sudo tar xvzf jre-7u40-linux-i586.tar.gz

64-bit Oracle Java on 64-bit Ubuntu Linux installation instructions:
Type/Copy/Paste: sudo tar xvzf jdk-7u40-linux-x64.tar.gz
Type/Copy/Paste: sudo tar xvzf jre-7u40-linux-x64.tar.gz

Double-check your directories. At this point, you should have two uncompressed binary directories in /usr/local/java for the Java JDK/JRE listed as:
Type/Copy/Paste: ls -a
jdk1.7.0_40
jre1.7.0_40

Edit the system PATH file /etc/profile and add the following system variables to your system path. Use vi, gedit or any other text editor, as root, open up /etc/profile.
Type/Copy/Paste: sudo gedit /etc/profile
or
Type/Copy/Paste: sudo vi /etc/profile

Scroll down to the end of the file using your arrow keys and add the following lines below to the end of your /etc/profile file:
Type/Copy/Paste:

export JAVA_HOME=/usr/local/java/jdk1.7.0_40
export PATH=$PATH:$JAVA_HOME/bin

Save the /etc/profile file and exit.

Reload your system wide PATH /etc/profile by typing the following command:
Type/Copy/Paste: . /etc/profile
Note your system-wide PATH /etc/profile file will reload after reboot of your Ubuntu Linux system

Test to see if Oracle Java was installed correctly on your system. Run the following commands and note the version of Java:

A successful installation of 32-bit Oracle Java will display:
Type/Copy/Paste: java -version
This command displays the version of java running on your system
You should receive a message which displays:
java version "1.7.0_40"
Java(TM) SE Runtime Environment (build 1.7.0_40-b40)
Java HotSpot(TM) Server VM (build 23.1-b03, mixed mode)
Type/Copy/Paste: javac -version
This command lets you know that you are now able to compile Java programs from the terminal.
You should receive a message which displays:
javac 1.7.0_40

A successful installation of Oracle Java 64-bit will display:
Type/Copy/Paste: java -version
This command displays the version of java running on your system
You should receive a message which displays:
java version "1.7.0_40"
Java(TM) SE Runtime Environment (build 1.7.0_40-b40)
Java HotSpot(TM) 64-Bit Server VM (build 23.6-b04, mixed mode)
Type/Copy/Paste: javac -version
This command lets you know that you are now able to compile Java programs from the terminal.
You should receive a message which displays:
javac 1.7.0_40

Congratulations, you just installed Oracle Java on your Linux system. Now reboot your Ubuntu Linux system. Afterwards, your system will be fully configured for running and developing Java programs. Later on you may want to try compiling and running your own Java programs by following this article How to Create your First Java Program on Ubuntu Linux

Quickly Enable Root Login in Ubuntu 12.10

Here’s a quick way to login as root in Ubuntu 12.10. This is not recommended as the root user account is disabled for a reason.

In a perfect world, no one should login with total administrator rights (root user) in Ubuntu. But since we’re not in a perfect world, this brief tutorial is going to show you how to quickly enable root login in Ubuntu 12.10

Objectives:

• Login as Root (Administrator) in Ubuntu 12.10
• Enjoy!

To get started, press Ctrl – Alt – T on your keyboard to open the terminal. When it opens, run the commands below to create a root password.

sudo passwd root

Next, run the commands below to enable root login in Ubuntu 12.10

sudo vi /etc/lightdm/lightdm.conf

add the below line end of the file.

greeter-show-manual-login=true

save the file and restart the system. In the next boot you can see login label in the login screen.

How to create and extract a .tar.gz archive using command line

One of the most common compression formats used in GNU/Linux and variants is tar.gz. A tar.gz file is nothing but a gzipped tar archive. These days users of GNU/Linux system seldom have to use the command line to create or extract tar.gz archives. But it is a useful command to keep in your arsenal if you are a system administrator.

To create a tar.gz archive from a given folder you can use the following command

tar -zcvf tar-archive-name.tar.gz source-folder-name

This will compress the contents of source-folder-name to a tar.gz archive named tar-archive-name.tar.gz

To extract a tar.gz compressed archive you can use the following command

tar -zxvf tar-archive-name.tar.gz

This will extract the archive to the folder tar-archive-name.

Donot forget to give your comments /suggestions and refer to your friends.

Resolve Applying iptables firewall rules failed

Some time we might be get error like Applying iptables firewall rules: iptables-restore: line 1 failed [FAILED].The line Number will be different.That is nothing  while editing editing iptables you may wrongly enter some extra attributes.
For example iptable rule will be like that

-A RH-Firewall-1-INPUT -p tcp -m tcp --dport 21 -j ACCEPT

but you editing rule like

ss-A RH-Firewall-1-INPUT -p tcp -m tcp --dport 21 -j ACCEPT

means you can get like Applying iptables firewall rules: iptables-restore: line 1[this the exact line failed] failed [FAILED].

For that you simply correct the rule and restart your iptables. And you away from that error.

If the iptable rule is correct and you getting same error means add the following line above the rules and add COMMIT statement end of the rule.Go to /etc/init.d/iptables and add

*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:RH-Firewall-1-INPUT - [0:0]
-A INPUT -j RH-Firewall-1-INPUT
-A FORWARD -j RH-Firewall-1-INPUT

-A RH-Firewall-1-INPUT -p tcp -m tcp --dport 21 -j ACCEPT----->rule
-A RH-Firewall-1-INPUT -p tcp -m tcp --dport 25 -j ACCEPT----->rule

COMMIT


and restart your iptables .That  working fine

Donot forget to give your comments /suggestions and refer to your friends.

Linux Basic Commands

1.  Everything in Linux is a file including the hardware and even the directories.

2. # : Denotes the super(root) user

3.  $ : Denotes the normal user

4.  /root: Denotes the super user’s directory

/home: Denotes the normal user’s directory.

5.  Switching between Terminals

§  Ctrl + Alt + F1-F6: Console login

§  Ctrl + Alt + F7: GUI login

6.  The Magic Tab: Instead of typing the whole filename if the unique pattern for a particular file is given then the remaining characters need not be typed and can be obtained automatically using the Tab button.

7.   ~(Tilde): Denotes the current user’s home directory

8.   Ctrl + Z: To stop a command that is working interactively without terminating it.

9.  Ctrl + C: To stop a command that is not responding. (Cancellation).

10.  Ctrl + D: To send the EOF( End of File) signal to a command normally when you see ‘>’.

11.  Ctrl + W: To erase the text you have entered a word at a time.

12.  Up arrow key: To redisplay the last executed command. The Down arrow key can be used to print the next command used after using the Up arrow key previously.

13.  The history command can be cleared using a simple option –c (clear).

14.  cd :   The cd command can be used trickily in the following ways:

cd : To switch to the home user

cd * : To change directory to the first file in the directory (only if the first file is a directory)

cd .. : To move back a folder

cd - : To return to the last directory you were in

15.  Files starting with a dot (.) are a hidden file.

16.   To view hidden files: ls -a

17.   ls: The ls command can be use trickily in the following ways:

ls -lR : To view a long list of all the files (which includes directories) and their subdirectories recursively .

ls *.* : To view a list of all the files with extensions only.

18.   ls -ll: Gives a long list in the following format

drwxr-xr-x 2 root root 4096 2010-04-29 05:17 bin where

drwxr-xr-x : permission where d stands for directory, rwx stands for owner privilege, r-x stands for the group privilege and r-x stands for others permission respectively.

Here r stands for read, w for write and x for executable.

2=> link count

root=>owner

root=>group

4096=> directory size

2010-04-29=>date of creation

05:17=> time of creation

bin=>directory file(in blue)



The color code of the files is as follows:

Blue: Directory file

White: Normal file

Green: Executable file

Yellow: Device file

Magenta: Picture file

Cyan: link file

Red: Compressed file

File Symbol

-(Hyphen) : Normal file

d=directory

l=link file

b=Block device file

c=character device file

19.  Using the rm command: When used without any option the rm command deletes the file or directory ( option -rf) without any warning. A simple mistake like rm / somedir instead of rm /somedir can cause major chaos and delete the entire content of the /(root) directory. Hence it is always advisable to use rm command with the -i(which prompts before removal) option. Also there is no undelete option in Linux.

20.  Copying hidden files: cp .* (copies hidden files only to a new destination)

21. dpkg -l : To get a list of all the installed packages.

23. Use of ‘ > ‘ and ‘ >> ‘ : The ‘ > ‘ symbol ( input redirector sign) can be used to add content to a file when used with the cat command. Whereas ‘ >> ‘ can be used to append to a file. If the ‘ >> ‘ symbol is not used and content is added to a file using only the ‘>’ symbol the previous content of the file is deleted and replaced with the new content.

e.g: $ touch text (creates an empty file)

$ cat >text

This is text’s text. ( Save the changes to the file using Ctrl +D)

$cat >> text

This is a new text. (Ctrl + D)

Output of the file:

This is text’s text.

This is a new text.



23.  To count the number of users logged in : who |wc –l



24.  cat:  The cat command can be used to trickly in the following way:

- To count no. of lines from a file : cat <filename> |wc -l

- To count no. of words from a file : cat <filename> |wc -w

- To count no. of characters from a file : cat <filename> |wc –c



25.  To search a term that returns a pattern: cat <filename> |grep [pattern]



26.  The ‘tr’ command: Used to translate the characters of a file.

tr ‘a-z’ ‘A-Z’ <text >text1 : The command for example is used to translate all the characters from lower case to upper case of the ‘text’ file and save the changes to a new file ‘text1'.

27.  File permission using chmod: ‘chmod’ can be used directly to change the file permission of files in a simple way by giving the permission for root, user and others in a numeric form where the numeric value are as follows:

r(read-only)=>4

w(write)=>2

x(executable)=>1

e.g. chmod 754 text will change the ownership of owner to read, write and executable, that of group to read and executable and that of others to read only of the text file.

28.  more: It is a filter for paging through text one screenful at a time.

Use it with any of the commands after the pipe symbol to increase readability.

e.g. ls -ll |more

29.  cron : Daemon to execute scheduled commands. Cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates.

1 * * * * echo “hi” >/dev/tty1 displays the text “hi” after every 1 minute in tty1

.—————- minute (0 – 59)

| .————- hour (0 – 23)

| | .———- day of month (1 – 31)

| | | .——- month (1 – 12) OR jan,feb,mar,apr …

| | | | .—– day of week (0 – 7) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat

* * * * * command to be executed

Source of example: Wikipedia

30.  fsck: Used for file system checking. On a non-journaling file system the fsck command can take a very long time to complete. Using it with the option -c displays a progress bar which doesn’t increase the speed but lets you know how long you still have to wait for the process to complete.

e.g. fsck -C

31.  To find the path of the command: which command

e.g. which clear

32. Setting up alias: Enables a replacement of a word with another string. It is mainly used for abbreviating a system command, or for adding default arguments to a regularly used command

e.g. alias cls=’clear’ => For buffer alias of clear

33.  The du (disk usage) command can be used with the option -h to print the space occupied in human readable form. More specifically it can be used with the summation option (-s).

e.g. du -sh /home summarizes the total disk usage by the home directory in human readable form.

34.  Two or more commands can be combined with the && operator. However the succeeding command is executed if and only if the previous one is true.

e.g. ls && date lists the contents of the directory first and then gives the system date.

35.  Surfing the net in text only mode from the terminal: elinks [URL]

e.g: elinks www.google.com

Note that the elinks package has to be installed in the system.

36.  The ps command displays a great more deal of information than the kill command does.

37.  To extract a no. of lines from a file:

e.g head -n 4 abc.c is used to extract the first 4 lines of the file abc.c

e.g tail -n 4 abc.c is used to extract the last 4 lines of the file abc.c

38.  Any changes to a file might cause loss of important data unknowingly. Hence    Linux creates a file with the same name followed by ~ (Tilde) sign without the recent changes. This comes in really handy when playing with the configuration files as some sort of a backup is created.

39.   A variable can be defined with an ‘=’ operator. Now a long block of text can be assigned to the variable and brought into use repeatedly by just typing the variable name preceded by a $ sign instead of writing the whole chunk of text again and again.

e.g ldir=/home/my/Desktop/abc

cp abcd $ldir copies the file abcd to /home/my/Desktop/abc.

40. To find all the files in your home directory modified or created today:

e.g. find ~ -type f -mtime 0

41. How to know 32-bit or 64-bit in linux?
A. getconf LONG_BIT
You can type the above command in terminal  press enter it will return 32 or 64 based on OS bit.

Donot forget to give your comments /suggestions and refer to your friends.

Setting Global Environment Variables in CentOS

The easiest way to set an environment variable in CentOS is to use export as in

$> export JAVA_HOME=/usr/java/jdk.1.5.0_12

$> export PATH=$PATH:$JAVA_HOME

However, variables set in such a manner are transient i.e. they will disappear the moment you exit the shell. Obviously this is not helpful when setting environment variables that need to persist even when the system reboots.

In such cases, you need to set the variables within the system wide profile. In CentOS (I’m using v5.2), the folder /etc/profile.d/ is the recommended place to add customizations to the system profile.

For example, when installing the Sun JDK, you might need to set the JAVA_HOME and JRE_HOME environment variables. In this case:

Create a new file called java.sh
vim /etc/profile.d/java.sh

Within this file, initialize the necessary environment variables
export JRE_HOME=/usr/java/jdk1.5.0_12/jre
export PATH=$PATH:$JRE_HOME/bin

export JAVA_HOME=/usr/java/jdk1.5.0_12
export JAVA_PATH=$JAVA_HOME

export PATH=$PATH:$JAVA_HOME/bin

Now when you restart your machine, the environment variables within java.sh will be automatically initialized (checkout /etc/profile if you are curious how the files in /etc/profile.d/ are loaded) .

PS: If you want to load the environment variables within java.sh without having to restart the machine, you can use the source command as in:

$> source java.sh