How to imporve efficiency while working in Unix/Linux?

There are various flavors of Unix/Linux currently available in market.Below techniques which would be helpful to you while working in Unix/Linux. 

1. Using history command 

It is a Unix/Linux command used to get list of commands fired on Unix/Linux kernel.

You can use command  as  history | grep "mykeyword" to get desired list of commands which you have used recently.

HISTSIZE - a unix/linux environment variable, which specifies how many command history can store.

2. Using pipe (|) instead of two commands

Here, history and  grep command used to get the output of history command having "myword" word used in earlier command executions. 

3. Use  of ! and !! command

! command is used for to execute last command which is used in Unix/Linux, whether it successfully executed or not.
eg. 
In Unix, After doing ls –lrt myword.txt, I need to open the file myword.txt, you can use command like - vi !$.  Here !$ is the last argument of the shell.
In Linux, if you have installed vim editor , you can use it like  - !vim, 

This method saves your 30-35% time  time at a time of critical conditions.
Depending on Unix/Linux flavor and depends on shell, sometime SSH client like Putty, MultiTabbed Putty, etc. also require some settings, with that you can refer previous command with help of keyboard Up and Down arrows. 

!! command is used to execute the very last command you have executed and extremely fast as in processing. It works on the shells in which Up and Down arrow not working.

4. Use CTRL+R

It is used for repeating the last matching command.
Usage of this method is like that press CTRL+R and type "myword" which i have used in last command.Unix/Linux will find "myword" and once displaying the search result, you just need to press "Enter" to execute that command.

5. Use of aliases

You have to define aliases , in your  bash profile .profile or .bashrc file, which are used by your frequently while working. You can find this file by using ls -a command in your home directory.
eg. 
"l" which finds all files. 
"l." which finds all hidden files. 
"ls" which finds all files of using option - ls -lrt.
"cdh" which finds all files of using option - cd /home/myyword directory.

6. Using regular expression in grep and find and common commands like cd.
  
grep and find are two best commands provided by Unix/Linux mainly to search in a file , a directory , certain words in file e.g. ERROR or Warning.
eg.
grep "error" *.tmp  | grep "warning" *.tmp - Using two grep command for finding error and warning individually.
egrep "error|warning" *.tmp - Instead of  using two grep command for finding error and warning individually, usage of a single command.
cd  - to switch between two directories in Unix/Linux.

7. Use of CTRL+Z, fg and bg commands

Use  CTRL+Z and fg and bg to suspend a process running for a particular task.
Use fg and bg to run a particular command to run in background and foreground.
eg.
fg 1  - last background command or job to bring on foreground. 
fg 2  - second last background command or job to bring it on foreground.

Lastly, You have to Minimize the key strokes or increase the speed of typing, in Unix/Linux to get quicker output. You have to learn more n more commands with different options and usage of them, it will reduce your thinking time and increase your productivity. 

Some techniques might work in one flavor of Linux, but same might be not work for you.

What is the difference between sleep() and wait() in Java?

sleep() 

• It is the blocking operation which keeps a holding on the screen or locking of a shared object properties while a process execution for the specified number of milliseconds.
• It is most commonly used for polling during execution.
• It is used to check for certain results after specific interval during process execution.

wait() 

• It simply stops / pauses the process execution thread until  given number of milliseconds to be elapsed  or after receiving a notification from parent process execution without keeping a holding or locking of the shared object.
• It is generally used in multi threaded applications where as required to interaction with notify() / notifyAll() to achieve synchronization during process execution.

find command in UNIX/LINUX.

Here, are some find Command examples in Unix/Linux.

To run  last executed find command :

!find will repeat the last find command executed by user..

root@testenv ~/java : !find

./emp.java

./dept.java

"." says starts finding from current directory and include all sub directory and

"/" says starts finding from root directory 

To find files names as test.java:

find -name "test.java"

To find files names other then test.java file in test directory:

find /test/ –not -name "test.java"

To find files which has been modified/accessed/created less than a day, minute or hour :

-mtime is used to search files based on last modification time in days.

-atime is used to search files based on last accessed time in days.

-ctime is used to search files based on last changed time in days.

+ sign is used to search for greater than.

- sign is used to search for less than.

without sign is used for exact.

(all the files modified exact 1 day)

find . -mtime 1 

(find all the files modified less than 1 day)

find . -mtime -1

(find all the files modified more than 1 day)

find . -mtime +1

We can use same for –atime and –ctime for last accessed and last changed files.

-mmin is used to search files based on last modification time in minutes.

-amin is used to search files based on last accessed time in minutes.

-cmin is used to search files based on last changed time in minutes.

(find all the files created exact 15 min)

find . -cmin 15

(find all the files created less than 15 min)

find . -cmin -15

(find all the files created more than 15 min – means all other files)

find . -cmin +15

To find all the files and directories which holds the 777 permission:

–perm option is used to find files based upon permissions.

(find all the files with file permission as 777 – rwx on owner, group and others)

find –perm 777

(find all the files with file permission as 750 – rwx on owner, r-x on group and no permission to others)

find –perm  750

(find all the files with file permission as 444 – rrr on owner, group and others)

find –perm 444

(find all the files with file permission as 644 – rw- on owner, r—on group and others)

find –perm 644

To find case insensitive search.

-iname,  by default find searches are case sensitive. It is used for errors and exceptions in log files.

find . –iname "error" –print

To delete temporary files.

–delete option to do a remove specific files from the search criteria.

find . –delete –name "error" –print

use -print0 to avoid problems with white space in the path.

find . -name "*.tmp" –print0 | xargs rm –f

find . -name "*.tmp" -print | xargs -0 rm –f

To find all java file which contains word “Error” with sorted o/p or without sorted o/p.

find . –name "*.java" –print | xargs grep “Error”,

find . –name "*.java" –print | xargs grep “Error” | sort

To find files only in current directory not searching on sub directories:

While using find command I required only files, directories and links that are new, only in the current directory.

find . -type f -newer –name "*.java"

find . -type d –newer –name "*.java"

find . -type l –newer –name "*.java"

While using find command I required only files, directories and links that are new, only in the current directory with searching for two sub directories – specifying the depth of find.

-mindepth  for to find files upto minimum depth levels in directory.

-maxdepth  for to find files upto maximum depth levels in directory.

find . –maxdepth 2 -type f -newer –name "*.java"

find . –maxdepth 2 -type d –newer –name "*.java"

find . –maxdepth 2 -type 1 –newer –name "*.java"

find . –mindepth 2 –maxdepth 10 -type f –name "*.java"

To find files based on size.

–size option to find files based upon certain size. It finds all files in current directory and sub-directory, greater than some size, here size is 1MB = 1048576 bytes:

find . -size +1048576c -exec ls -l {} \;

find . -size +10M -exec ls -l {} \;

-c after the number means specify the size in bytes.

To find files using a range of file sizes, a minus or plus sign can be used before the number.

- minus sign means less than

+ plus sign means "greater than.

If I want to find all the files within a range of 5 MB to 10 MB:

find . -size +5242880c -size -10485760c -print

find . -size +5M -size -10M -print

To find files 30 days older and above 10 MB size.

You can use –mtime and –size option to find files which are 30 days old and greater than 10MB in size

find . -mtime +30 -size +10485760c -exec ls -l {} \;

find . -mtime +30 -size +10M -exec ls -l {} \;

To find all of the symbolic links in your home directory, and print the files your symbolic links points to:

find . -type l -print | xargs ls -ld | awk '{print $10}'

To find all empty files from the server.

find . -empty

To find largest and smallest file in the current directory:

find . -type f -exec ls -s {} \; | sort -n -r | head -1

find . -type f -exec ls -s {} \; | sort -n -r | tail -1

find . -type f -exec ls -s {} \; | sort -n | head -1

To find all hidden files from the directory:

find -type d -name ".*"

find –print and find is same as –print is a default option of find command.

find –print0 should be used to avoid any issue with white space in file name or path while forwarding output to xargs, also use xargs -0 along with find –print0.

find has an option called –delete which can be used in place of  -exec rm {} \;

What is the difference between InnoDB & MYISAM in MYSQL?

InnoDB 1.1 combines more reliability and performance with usability enhancement compare to InnoDB 1.0.

Since MySQL 5.5, InnoDB is the default storage engine, instead of MyISAM.

CREATE TABLE TABLENAME statement without an ENGINE= clause creates an InnoDB table.

•	Its supports DML operations and follow the ACID property and with can do commit and rollback for long transactions and supports for crash-recovery to protect user data.
•	Its supprts Row-level locking and Oracle-style consistent reads to increase multi-user concurrency and performance.
•	InnoDB tables stores your data on disk to optimize queries based on primary keys.
•	InnoDB table has a primary key index called the clustered index to organizes the data to minimize I/O.
•	InnoDB also supports FOREIGN KEY constraints to remove inconsistencies across different tables.
•	You can freely mix InnoDB tables with tables from other MySQL storage engines. We can join data from InnoDB and MEMORY tables in a single query.
•	It is designed for CPU efficiency and maximum performance when processing large data.
•	It use B-tree indexes only. Not supports for T-tree indexes & Hash indexes.
•	It can store up to 64TB in a table and supports row level locking.
•	It can store Compressed data and Encrypted data along with Replication support and Foreign key support.
•	It uses Data caches and Index caches while storing data.

As compare to InnoDB, MyISAM tables have characteristics are as under:

•	MYISAM tables are stored with the low byte first, which makes the data machine and operating system are as independent. Due to that widely used in mainstream machines.
•	All numeric key values are stored with the high byte first for better index compression.
•	It can store (232)2 rows in a MyISAM table and maximum number of indexes per MyISAM table is 64 and max 16 columns can have an index. It stores maximum key length is 1000 bytes, which can be changed by changing the source and recompiling table to default size upto 1024 bytes.
•	It improves space utilization in index tree by rows are inserted in sorted order when you are using AUTO_INCREMENT column in index tree by splitting in a manner that only high node only contains one key.
•	It supports Dynamic-sized rows, due to that it is much less fragmented when mixing deletes with updates and inserts, automatically by combining adjacent deleted blocks and by extending blocks if the next block is deleted.
•	MyISAM supports concurrent inserts, If a table has no free blocks in the middle of the data file, you can INSERT new rows.
•	We can store the data file and index file in different directories on different physical devices to get more speed with the DATA DIRECTORY and INDEX DIRECTORY table options to CREATE TABLE.
•	Large files - up to 63-bit file length and 256 TB in size to support large files.
•	BLOB and TEXT columns can be indexed, NULL values can inserted in indexed columns, which takes 0 to 1 bytes per key.
•	Each character column can have a different character set.
•	It Support for a true VARCHAR, a VARCHAR column starts with a length stored in one or two bytes. MYISAM Tables with VARCHAR columns may have fixed or dynamic row length.
•	It does not supports row level locking and transactions.
•	It use B-tree indexes only. Not supports for T-tree indexes & Hash indexes.
•	It can store Compressed data and Encrypted data along with Replication support, but not for Foreign key support.
•	It use B-tree indexes only. Not supports for T-tree indexes and Hash indexes.
•	It supports Full-text search indexes and not for the Clustered indexes and Hash indexes.
•	It uses Index caches while storing data but not uses Data caches.

PRAGMA RESTRICT_REFERENCES for Restricting Package security.

PRAGMA RESTRICT_REFERENCES is an instruction or a hint to the compiler which is processed at compile time, not at runtime, it used to control the side effects of PL/SQL Subprograms. Every PL/SQL Subprograms must follow some rules in terms of transaction control and security. 

PRAGMA RESTRICT_REFERENCES mainly used in PL/SQL Package and it ensures that a subprogram does not read or write database tables or package variables. 

PRAGMA RESTRICT_REFERENCES can appear only in a package specification or object type specification. Typically, this pragma is specified for functions. If a function calls procedures, specify the pragma for those procedures also.

From Oracle 11g, PRAGMA RESTRICT REFERENCES is deprecated. Oracle recommends using DETERMINISTIC and PARALLEL_ENABLE instead of RESTRICT REFERENCES.

• RNDS – Read No Database State. function not to read or query tables
• RNPS – Read No Package State. function not to read or reference package variables
• WNDS – Write No Database State. function not modify database tables
• WNPS – Write No Package State. function not modify package variables
• TRUST – function can be trusted not to violate one or more rules. Used only when C or JAVA routines are called from PL/SQL.

Without using pragma in package, package successfully complied but it may result runtime error, and developers need to change the code.

ORA-14551: cannot perform a DML operation inside a query

Using pragma in package, package will be successfully complied with compile time error, which leads developers need to change the code.

PLS-00452: Subprogram ‘send_emp_alloc’ violates its associated pragma

  

CREATE OR REPLACE PACKAGE pkg_transfergoods

IS

function send_emp_alloc(p_empno integer) return number;    

PRAGMA restrict_references(send_emp_alloc, RNDS, RNPS, WNDS, WNPS);

END pkg_transfergoods;

/

To apply PRAGMA restrict_references functionalities on all package functions – sub programs by using Keyword = DEFAULT to all of the sub programs inside the package. It applies (WNDS – Write No Database State to all package not modify database tables) to all database tables which are used in package.

CREATE OR REPLACE PACKAGE pkg_emp_alloc

IS

function send_emp_alloc(p_empno integer) return number;

function get_emp_name(p_empno integer) return varchar2;

PRAGMA restrict_references(DEFAULT, WNDS);

END pkg_emp_alloc;

/

What are major difference between Traditional Import-Export and Datapump Import-Export?

Traditional EXP/IMP both are separately executing.	Data Pump EXPDP and IMPDP are executed as DATAPUMP jobs. DB server processes access db files and db objects to perform EXP/IMP on respective directory objects of DB. This enforces a security model for DBAs.
Single data stream execution of  Traditional IMP/EXP.	EXPDP/IMPDP can take advantages of server’s parallel process to read or write multiple data streams simultaneously.
EXP/IMP not have very interactive command line mode.	EXPDP/IMPDP have very powerful interactive command line mode, allows user to monitor and control the process.
EXP/IMP cannot have disconnect and reconnect feature. Once started it need to be finish.	DATAPUMP jobs entirely running on server hence we can disconnect, detach and reconnect like, PAUSE and RESUME.
No data transfer between two databases in EXP/IMP.	DATAPUMP has ability to pass data between two databases over a database link, without creating a dump file on disk.
EXP/IMP uses sequential path access method.	DATAPUMP uses Direct Path access method.
EXP/IMP operates on single file.	DATAPUMP operates on group of files.
EXP/IMP can access files of server and client.	DATAPUMP exports/imports files using database directories.
Traditional EXP/IMP supports sequential media like tape drive and others.	DATAPUMP not supports on export/import from/to on Sequential media like tape drives.
EXP/IMP uses Byte by Byte mode.	DUMP Files can be Compressed, Encrypted and uses BLOCK by BLOCK mode.
EXP/IMP not supports for XML Schemas and XML Type data.	DATAPUMP Can support XML schemas and XML Type data.
EXP/IMP works with all oracle versions.	DATAPUMP works with oracle 10g or later versions.

What are Materialized View & Materialized View Logs?

A materialized view is a database object that contains the results of a query same like a table. They are local copies of data located on remote site. Also, used to create summary tables based on aggregations of a table's data, which store data based on remote tables are also, referred as Snapshots.

A materialized view can query tables, views, and other materialized views. Collectively these are called master tables (a replication term) or detail tables (a data warehouse term).

For replication purposes, materialized views allow you to maintain copies of remote data on your local server as read-only data.If we want to update the local copies of data, we have to use the Advanced Replication features in oracle. We can select data from a materialized view same as local table or view.

Materialized Views refer local data from Materialized View Logs (Snapshot Logs), which need to create on Local tables.

CREATE MATERIALIZED VIEW LOG ON emp;

Primary Key Materialized Views which are created with "WITH PRIMARY KEY" clause.

CREATE MATERIALIZED VIEW mv_emp_pk
REFRESH FAST START WITH SYSDATE 
NEXT  SYSDATE + 1/48
WITH PRIMARY KEY 
AS SELECT * FROM emp@remote;

Rowid Materialized Views which are created with "ROWID" clause.

CREATE MATERIALIZED VIEW mv_emp_rowid
REFRESH WITH ROWID 
AS SELECT * FROM emp@remote;

Subquery Materialized Views which are created by using subquery.

CREATE MATERIALIZED VIEW  mv_empdept
AS SELECT * FROM emp@remote e
WHERE EXISTS
     (SELECT * FROM dept@remote d
     WHERE e.deptid = d.deptid)

For data warehousing purposes, materialized views are the aggregate views, single-table aggregate views and join views.

CREATE MATERIALIZED VIEW  mv_empdept_dwh
AS SELECT * FROM emp@remote a, dept@remote b
WHERE a.deptid = b.deptid
and a.deptid < 1000;

COMPLETE refreshes by recalculating the defining query of the materialized view.

FAST refreshes by incrementally applying changes to the materialized view as per materialized view logs. For local materialized views, it chooses the refresh method which is estimated by optimizer to be most efficient.

FORCE Attempts a FAST refresh. If that is not possible, it does a complete refresh. For local materialized views, it chooses the refresh method which is estimated by optimizer to be most efficient. The refresh methods considered are log based FAST and COMPLETE.