Eighth session of Some of basic UNIX commands that are useful very frequently

Today we will discuss about some more useful commands in UNIX/LINUX based operating system.

If you want to see quickly what is the content of a file, then you can use below command to see the content of file in the same terminal you are working currently.

cat filename

Above command will display content of file named 'filename' present in current working directory in the terminal.

cat filename > /x/y.txt

Above command will copies content of file named 'filename' present in current working directory into file named 'y.txt' present in path '/x'.

cat filename1 filename2 > /x/y.txt

Above command will concatenate content of files named 'filename1'and 'filename2' present in current working directory into file named 'y.txt' present in path '/x'.

cat filename1 filename2

Above command will concatenate content of files and displays in the terminal. It doesn't modify original files instead it display both files content into terminal by concatenating.

One more important command that we are going to discuss today is 'ps'.

ps

Above command will display current processes which are running. This command is some what like a 'Task Manager' in windows where as the difference is in task manager we can kill process which we don't want to run but, here we can know process IDs by which we can delete the process using 'kill' command that we can discuss in our next post. This command will display lot of information about processes that are currently running.

ps -u

Above command will display current processes that are running in current log-in.

Please feel free to post your comments/queries. We can discuss some more useful UNIX/LINUX commands in our next post.

Seventh session of Some of basic UNIX commands that are useful very frequently

Today we are going to discuss about one more useful command which will be very important in security aspect i.e. When ever you have file/folder which is very important and you don't want others to read your content then this post is very very helpful in making your content private.

chmod is a command by which you can modify permission of a file/folder according to your wish.

chmod -R 777 /home/x/y.java

Above command will give permissions to read, write and execute for the file 'y.java' present in path '/home/x' so that 'y.java' can be accessed (read, write and execute) by anyone.

In the above command main important part is 3 '7's which decide up to what level one can access corresponding file.

First digit: First '7' is used to give permissions to corresponding user. And one can assign '0-7' We can discuss what is the meaning of each digit from '0-7'.

Second digit: Second '7'is used to give permissions to group to which corresponding user belongs too. And one can assign '0-7' for this digit too.

Third digit: Third '7'is used to give permissions to others. Similar to above 2 digits one can assign '0-7' for this digit too. And you can also refer previous post which contains same explanation.

Regarding assigning '0-7' to each user/group/others please read below table for clear explanation.

#	Permission	rwx
7	full	111
6	read and write	110
5	read and execute	101
4	read only	100
3	write and execute	011
2	write only	010
1	execute only	001
0	none	000

So, as per above table if you don't want to give neither of read,write and execute permission to user, group and others then you can use '000' to assign permissions to corresponding file.

chmod -R 700 /home/x/y.java

Yes, as per your guess, above command will give read,write and execute permission to user and gives no permission to group and others. So, that we can restrict 'y.java' file to corresponding user only.

Like this you can use different combinations in the above table to give/restrict permissions to corresponding user, group and others.

One more important point to remember to use this command is, file to which you are going to modify permissions should be closed and you should be owner of the file. You can also get more information about this command by hitting 'man chmod' in the terminal.

Please feel free to post any help/suggestion. Will discuss some more in next posts.....

Sixth session of Some of basic UNIX commands that are useful very frequently

Today we will discuss about some more useful commands that are used very frequently when you work with UNIX/LINUX based Operating Systems.

mkdir is a command which created new directory in your file system.

mkdir NewDirectory

Above command will create new folder named 'NewDirectory' in present working directory. You can fetch your present working directory by hitting 'pwd' in your terminal. For suppose you are in directory '/home/x/y' then new directory named 'NewDirectory' will be created in ' /home/x/y'.

mkdir /x/y/z NewDirecotory

Above command will create new folder named 'NewDirecotry' in the path '/x/y/z'.

Similarly you can remove a folder and its contents by using command rmdir.

rmdir NewDirectory

Above command will deletes/removes a folder named 'NewDirectory' from current working directory. If that directory contains any sub folders/files then will confirm from user before deleting each file/folder with 'Y/N ?'. So, you can remove/delete each file/folder by pressing 'y'.

rmdir -f NewDirectory

Above command will deletes/removes 'NewDirectory' and its contents without asking confirmation to delete.

rmdir -f /home/x/y/*

Above command will deletes/removes content (files/sub folders) in the path '/home/x/y' without asking confirmation to delete. Like '*', we can use patterns to delete files. Below is small example:

rmdir -f /home/x/y/a*.java

Above command will deletes/removes files whose names starts with 'a' and extension '.java' in the path '/home/x/y/'.

Don't hesitate to try these when you are free.. If you already had a chance to try these, then don't hesitate to write your valuable comments. Will discuss some more in next post....

Fifth session of Some of basic UNIX commands that are useful very frequently

Today we will discuss about some more useful commands that are used frequently when you use UNIX/LINUX based system.

Whenever you are running your application in UNIX/LINUX based server, then you may get to view log files for debugging purpose. In that case below command will be useful.

more filename

Above command will display content in the file named 'filename' that fits according to screen. If you want next content in that file to display, then you can simply press 'space' to scroll file content. If you want to quit to view the file then you can simply press 'q' to quit that view. In the displayed content of file if you want to search for some string, then you can use '/pattern' for searching that string.

For example, above file name displayed below content:

Hi My Name is John.

And in that if you want to search for 'John' then you can simply press '/John' and enter will highlight the word 'John'. If there are more 'John's in the displayed content then all will be highlighted. You can traverse one by one.

There is one more way to view file content but not whole content instead just last few lines in that file, then you can use below command.

tail -f filename

Above command will display last few lines of file according to screen fit. If that file is getting modified then it will display accordingly.

For example you are viewing 'catalina.log' file in the tomcat server, then it will be modified frequently based up on the hits to your application. In that case this command will display last few lines accordingly. 

If you want to rename one file to another, then you can use below command.

mv filename1 filename2

Above command renames 'filename1' to 'filename2' . I.e. moves content from 'filename1' to 'filename2'. If you want to move 'filename1' to different directory then also you can use same command but in different manner.

mv filename1 dir2

Above command will move 'filename1'from current directory to 'dir2'. If your file is not present in current directory, then you need to give 'filename1'with full path.

As specified in my previous post, you can get more uses of this command by hitting 'man mv' in your terminal.

Like 'mv' command that we use for renaming or moving files, there is one more command that you may used very frequently in windows if you are get familiar with command prompt. i.e. copy command. Below is its usage:

cp filename1 filename2

Above command will copy 'filename1'into 'filename2'. As specified above with 'mv' command, you can use all similar patterns with this command.

To delete a file you can use below command.

rm filename

Above command will removes file named 'filename' from the system. You can use option ' -i  ' for asking confirmation to delete. 

For all these three commands, you can use patterns for handling set of files. Below is one small example.

mv /dir1/* /dir2

Above command will moves all files present in '/dir1' to '/dir2'. Here '*' is one patten that we discussed in our previous post.

Will discuss some more useful commands in next post...

Fourth session of Some of basic UNIX commands that are useful very frequently

Today we are going to have some useful information on very useful command 'find' in UNIX systems. find is a command used to search any set of directories that you specified for searching files that matched your search criteria. It is useful to locate the path of files on a UNIX/LINUX system. Below is sample syntax:

 find where-to-look criteria  

  find . -name "*.java" 

Above command will display all the files that have file name extension as '.java'in the current directory.  

find . -mtime 1 

Above command will display all the files that are modified exactly one day in the current directory.  

find .. -mtime -1 

Above command will display all the files that are modified less than one day in the parent directory i.e. suppose if you are in directory '/x/y' then above command will display all files that are modified less than one day in 'x' directory. It will also include 'y' directory in searching.

 find . -perm 444 'perm' 

sub-option is used to search files based up on permissions. I.e. Above command will display all the files that have permission to read for owner,group and others in the current directory. Here in '444', first '4'indicates read permission to current user. Second '4'indicates read permission to current group that current user belongs to. Third '4'indicates read permission to other users that are in current system.

 find . -iname "string" 

If you want to search with case insensitive then you can use option '-i' preceding 'name'. I.e. Above command will display all files that have word 'string' irrespective of case. 

As specified in my previous post, you can also get more information about this command by hitting 'man find' in your terminal.

Third session of Some of basic UNIX commands that are useful very frequently

Hey Folks,

Am back for continuing on posting some more of UNIX commands that are useful whenever you are using any UNIX based operating System.

Grep is a command which is useful for searching any word/name of file/partial name of file in a working directory or any path mentioned. This will display with both case sensitive and case-insensitive results based on the command specified..

grep -i "searchword" FILE

Above command will search for 'searchword' in file named 'FILE' and display matching lines in that file as a result.

We can also use regular expressions to search as per our requirements.

grep "string.*string1" file

Above command will display any line that has 'string' starting with and 'string1'ending with and anything in between... in the file named 'file'.

A regular expression may be followed with one of the following operators.

? The preceding item is matched with at most one time.
* The preceding item is matched with zero or more times.
+ The preceding item is matched with one or more times.
{n,m} The preceding item is matched with minimum 'n' times and maximum 'm' times.

Like this you can use of regular expression for searching your patterns.

In this way 'grep' is useful for searching purpose. There are many more uses of this command that you can know by hitting command 'man grep' in your unix/linux based operating system's terminal(you can call it as cmd prmpt in terms of windows).

shutdown is a command in unix that is useful for letting your system down. You can shutdown your machine immediately or after a specified time..

shutdown -t 2

Above command will shutdown the system securely after 2 seconds.

shutdown -r

Above command will reboot your machine securely..

shutdown -c

Above command will cancel shutdown job that is already running..

As specified above there are many more uses of this command that u can get to know by hitting 'man shutdown' in your terminal..

Will specify some more in next post...

Second session of Some of basic UNIX commands that are useful very frequently

Hi Folks,

In this i want to share some other UNIX/LINUX command which is useful for us very frequently.

To display list of files,directories that are available in current directly please run below command.

'ls'

If you want to see more options that are available for this command then you can run below command

ls -a - This option is used to display hidden files along with normal files.

ls -l - This option displays full details of all files and folders present in current directory. For example see below sample output of this command.

drwxr-xr-x 2 user group 4096 2011-01-06 12:06 Public
In this output, highlighted part consists total 10 fields which includes of below information:
d - means its a directory.(If it is file then this will be blank).
r - read permission for user,group,others.
w - write permission for user,group,others.
x - execute permission for user,group,others.
So, first 'rwx' means this file has all permissions for user. Next 'r-x' means this file has only read and execute permissions for group. Next 'r-x' means this file has only read and execute permissions for others.

ls -d */ - This option shows only folders present in current directory.

ls -s - This option shows files and folders present in current directory along with their sizes in blocks.

ls -lt - This option shows files in the order of their last modified time.

pwd - This option shows present working directory.

Please try these when you are free. In our next session we can discuss some other commands.

First session of Some of basic UNIX commands that are useful very frequently

Hi Folks,

In this blog i want to discuss some of basic unix/linux commands that are useful very frequently.

If you want to display total,used and free RAM of your machine in size of MB then run below command.

free -m

If you want to see more options that are available for this command then you can run below command

man free

Some of useful options in free command:

free -t - This option will also displays data containing the totals for physical memory and swap space along with RAM.

free -ms 5 - This option i.e. '-s' followed by an integer will display commands output i.e. total,used and free RAM in MB available in your machine for every 5 seconds.

watch free - Usage of this command is same as above but in an interval of 2 seconds. When you time this command and then press enter then a new screen will be opened and automatically refresh total,used and free available of RAM in KBs. If you press 'CTRL + C' then newly opened screen will be closed and you will be returned to your command ran screen.

watch -n 1 -d free - This is can be taken as advanced version of above command. Here '-d' option is for highlighting the changes in memory usage and '-n' is for interval. For example in this command '1' is followed by '-n' so, for every one second its going to display RAM usage in KB as we didn't mention '-m' option.

Please try these when you are free. In our next session we can discuss some other commands.

Some Of Basic FAQ about Core JAVA

Hi Friends,

In this blog i want to share some of basic FAQ about Core JAVA that may help you have some basic information.

1. Can multiple inheritance be achieved in Java. If yes, how?
   Answer – Yes, through implementing interfaces as we can implemente multile interfaces in a single class.
   
2. What is the basic difference between String and StringBuffer object?
   Answer - String is an immutable object. StringBuffer is a mutable object.
   
3. What is the base class for Error and Exception?
   Answer – Throwable is base class for Exception.
   
4. What is a static block?
   Answer – this is the block executed before the class initialization.
   
5. Can static methods call directly non static methods of the same class?
   Answer – No. They cannot directly call. If they need, they will have to create object.
   
6. From an overridden method of the sub class, how can you call the overridden base class method ?
   Answer – call the method by using 'super' keyword.
   
7. Can protected methods be accessed by classes in different package.
   Answer – Yes, provided they are subclasses. Otherwise no.
   
8. Can a subclass reference hold a superclass object.
   Eg: Class A, Class B.
   If B extends A. Can we say B b = new A()
   Answer – No. Viceversa is true (if A a = new B() is fine).
   
9. If a super class method is throwing an IOException, what should the overriding sub class method throw?
   Answer – It should either
   Throw the same IOException
   Throw a sub class of IOException.
   Throw nothing
   
10. What are the two ways of making a class non-extensible?
    Make the class Final
    Make the constructor private
    

Brief uses and advantages of java.util.Arrays

Hi Folks,

In this post i want to introduce you guys about one common utility class that is very helpful in sorting and searching of values in an array of any type like int,long,String etc. Simply we can state it as java.util.Arrays which is very useful in manipulating arrays such as searching,sorting and viewing all values as a list at a time without any iteration etc.

This class contains various methods to manipulate arrays of any type. For example, you have an array of string values like below

String s[] = {"hi", "ramu", "cris"};

and you want to sort these values then you don't need to write/implement any sorting algorithm instead you can simply use statement

Arrays.sort(s[]);

to sort those values. This java.util.Arrays.sort(Object[] a) method is useful to sort array of objects. Here our s[] is String type of object so we can simply use this method to sort our array.

If you want to print all the values using at a time then also you don'e need to iterate instead you can use

System.out.println(Arrays.asList(s[]);

This method java.util.Arrays.asList(T... a) is used to back the values to a List.

Like these there are many more methods by which you can easily manipulate arrays.