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How to use the most popular command in Unix - Grep

grep is by far the most popular command that exists in Unix. Though some may argue about that, but once you begin using grep, it would always be present in all your complex commands that you think of executing at the shell prompt. grep stands for 'global regular expression printer' . Which makes no sense to most.. In sensible words grep extracts those lines from a given text that match the conditions set by the user.

Basically grep lets you enter a pattern of text and then it searches for this pattern within then text that you provide it. It would return all the lines that have the given pattern in them. grep can be used in 2 ways - Either on its own or along with pipes

Using grep on its own

$ grep '12.00' /home/david/backup/log.txt

This command basically shows how you can use grep to extract lines containing a particular string from a text file. (Text files need not necessarily end in .txt) The above command searches for the string 12.00 in the text file specified in the command, and displays all the lines that have this string in them.
The above command could be used to find out all the backups that took place at 12.00 (In case you have a log.txt file in that directory with a list of all the timings for the backups that you have made).

$ grep -v '12.00' /home/david/backup/log.txt

The above command would now show you all the lines in the text file except those that have the string 12.00 in them.

$ grep -l 'delay' /code/*.c

The above command searches for those files that end with a '.c' (within the /code directory) and in which the text 'delay' is present. It only returns the names of these files and not the lines where it found the string.

$ grep -w '\' *

The above commands search for text in a more refined way. The first command searches for those lines where any word in that line begins with the letters 'bay' and the second command searches for those lines where any word in that line ends with the letter 'watch'
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Using grep with pipes

$ ls -l | grep rwxrwxrwx

As you must be knowing ls -l displays the directory listing for any directory. The grep rwxrwxrwx part of the command extracts only those lines which display the files having their read,write,execute permissions set for user, group and others also. Thus instead of getting a listing of all the files in the directory, you would only see those files that have their r,w,x permissions set for all everybody.

The output of grep can also be piped to another program as follows

$ du | grep 'mp3' | more

You should be able to figure out what the above command does..

$ grep '^#' /home/david/script1 | more

The above command would display those lines (from the file /home/david/script1) that begin with a '#'. The term '^#' means that # should be present as the first character on a line. The more part of the command should be known to you. If not, more basically displays the output a page at a time incase the output exceeds one page.

$ grep -v '^[0-9]' /home/david/backup/log.txt | more

This command searches for lines having any of the numbers from 0-9 in them as the first character on the line. It then prints all the lines except the ones it found initially.

Important : It's necessary to enclose patterns (as used in the above 2 commands) in single quotes so that the shell understands it correctly.Otherwise, the shell may interpret it in another method.

Some extra options for grep

-v
Reverses the normal behaviour of the grep command - Instead of selecting lines, it rejects the lines that match the given criteria.

-c
It supresses the normal output and only prints the total count of matching lines instead of the actual lines.

-i
Ignores the case of the text when matching the given pattern.

-w
Checks if the given pattern is a word by itself and not a part of another word. Thus if you search for 'bay' and the word 'baywatch' is present in a file, the particular line conatining that word would not be returned in the result.

-l
Only gives the names of the files in which the given pattern was found.

-r
Checks for the given pattern , recursively within the directory that you specify after the -r option

I hope this tutorial helps you get started with grep. grep is defintely one of the tools that gives Linux the advantage over other Operating Systems. Using grep effectively along with other tools gives the user a lot of power in Unix.

Setting the PATH in linux

This article explains how set your PATH variable under Linux. This has the same use as that of setting the PATH variable under DOS. Under Linux too, modifying the PATH would add these new directories to your default search path.
So in case you have a particular executable in a particular directory, then if you add that directory to your PATH, then you would only have to type the name of the executable at the prompt rather than then absolute path for that executable. Got it?? Read the example below to figure out what exactly I am speaking about..

Assumption

Suppose you have a program by the name 'tetris' in a folder called /usr/local/games . So in order to run this program you would have to type the following at the prompt

$/usr/local/games/tetris

The above command would execute your program. But typing this every time you want to play this wonderful game makes it slightly cumbersome. It would be much better if you could only type 'tetris'.

Solution

A solution would be to add the /usr/local/games directory to your PATH, so that next time onwards you would only have to type 'tetris' at the prompt rather than the absolute path.

To add this directory to your PATH you have to edit a file called 'bash_profile' that would be present in your Home directory (in Redhat Linux 6.2). So if there is a user by the name David then this file would mostly be found at /home/David/.bash_profile

Note : The period (.) before the name of the file. This period make this file a hidden file. So remember to view hidden files also while seeing a directory listing (This option would be in some menu in X , at the prompt simply use 'ls -a' to see hidden files).

This file would be having a particular line starting with the string PATH. For e.g. the file that I have on my machine has a line such as

PATH=/optional/bin:$PATH:$HOME/bin

To add the directory /usr/local/games to this I would have to modify this line as follows

PATH=/usr/local/games:optional/bin:$PATH:$HOME/bin

Once you have modified this file, save it and then execute it as follows

. $HOME/.bash_profile

Note : To execute this script basically at the $ prompt type a period ' . ' leave a space and then type $HOME/ Once this is done press key. Doing so would replace what ever you have typed with the path to your home directory. Once this happens all you have to do is append a .bash_profile to what is already present at the prompt and finally press

On executing the script you wouldn't see any messages at the output, but then onwards you could simply type 'tetris' at the prompt to execute the program /usr/local/games/tetris
So now you are on the right PATH

Linux Body Painting

Its, awesome...

Facts About the Linux Kernel

Exactly 16 years ago, on March 1994, Linux kernel version 1.0.0 was humbly released for the world to tinker with. To celebrate the historic moment, I have collected some really interesting facts about the Linux kernel. So if you are a Linux fan, grab a beer and read on.

A 21 year-old Finnish college student created the Linux kernel as a hobby. (Do you know him?)
An asteroid was named after the creator of the Linux kernel.
Thousands of developers/programmers scattered all around the world are continuously contributing to the development of the Linux kernel.
The Linux kernel's official mascot is a penguin named Tux.
According to a study funded by the European Union, the estimated cost to redevelop the most recent kernel versions would be at $1.14 billion USD.
As of today, only 2% of the Linux kernel has been written by Linus Torvalds.
The Linux kernel is written in the version of the C programming language.
Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from handheld computers to mainframe servers.
Linux kernel 1.0.0 was released with 176,250 lines of code. The latest Linux kernel has over 10 million lines of code.
Microsoft Windows and the Linux kernel can run simultaneously in parallel on the same machine using a software called Cooperative Linux (coLinux).
At first, Torvalds wanted to call the kernel he developed Freax (a combination of "free", "freak", and the letter X to indicate that it is a Unix-like system), but his friend Ari Lemmke, who administered the FTP server where the kernel was first hosted for downloading, named Torvalds' directory linux.
A guy name William Della Croce, Jr. trademarked the name Linux and eventually demanded royalties for its use. He later agreed to assign the trademark to Torvalds.
The Linux kernel can be found on more than 87% of systems on the world's Top 500 supercomputers.
A "vanilla kernel" is not an ice cream flavor but an unmodified version of the Linux kernel.
The Linux Kernel is not in any way related to the army rank called ‘Colonel’. (hehe)

Cheers!

Change MAC address in Linux

If you wish to change your MAC address in Linux, all you have to do is bring the interface down then use the hw ether switch:

#ifconfig eth0 down
#ifconfig eth0 hw ether 02:01:02:03:04:08
#ifconfig eth0 up

but if you want your pc to change its MAC address on boot add that to a script in /etc/init.d/ folder, and also add symbolic link(ln) to /etc/rc2.d, /etc/rc3.d, /etc/rc4.d, /etc/rc5.d which refers to the script in /init.d/

#!/bin/bash

ifconfig eth0 down
ifconfig eth0 hw ether 02:01:02:03:04:08
ifconfig eth0 up
/etc/init.d/networking restart

Reset the MySQL password

You forgot your MySQL password. No problem, you can recover it easily. Stop the MySQL service with

# /etc/init.d/mysql stop

…and restart it without a password:

# mysqld_safe –skip-grant-tables &

You’ll get an output similar to [1] 5958. Now press Enter and type the following one after the other:

# mysql -u root
mysql> use mysql;
mysql> update user set password=PASSWORD(”NEW-ROOT-PASSWORD”) where User=’root’;
mysql> flush privileges;
mysql> quit
# /etc/init.d/mysql restart

It’s all done now and you should be able to login with the new password.

What is Slackware?

Slackware was the first Linux distribution to achieve widespread use. It was started by Patrick Volkerding in late 1992. He had gotten introduced to Linux when he needed an inexpensive LISP interpreter for a project. At that time, there were very few distributions, so Patrick went with the distribution from Soft Landing Systems (SLS Linux).

However, SLS had some problems, so Patrick started to fix little bugs as he found them. Eventually, he decided to merge all of those fixes into his own distribution for himself and friends. This private distribution quickly gained popularity, and Patrick made it available to the public under the name of Slackware.

Along the way, Patrick added new things to the distribution like a user-friendly installation program based on a menuing system and the concept of package management. This allows users to easily add, remove, or upgrade software packages from their system.

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