Unix - AWK Command by examples. Part 2

In this part, we will learn on AWK Built in Variables:

FS - Input field separator variable:

By default, AWK assumes that fields in a file are separated by space.
If the fields in the file are separated by any other column separator, we can use the FS variable for that.

Example 5. awk 'BEGIN {FS=":"} {print $2}' test_file
OR
awk -F: '{print $2} test_file

output will be as below.

39 AAAA
15 BBBB
39 EEEE
39 GGGG

We can use -F option for to change column separator in command to display output.

awk -F: '{if ($2 == "") print $1 ": no password!"}'

We can easily move from one column separator to another column separator using awk.

#!/bin/awk -f
{
if ($1 == "#START") {
FS=":";
} else if ($1 == "#STOP") {
FS=" ";
} else {
#print the Roman number in column 3
print $3
}

}


OFS - Output field separator variable: 

Example 6. awk '{print $4,$5}' test_file

When we use print statement in awk, it uses default column seperator while displaying result.

The output of this command will be
unixrun 0
unixrun 175
unixrun 430
unixrun 428

We can change this default column seperator by using the OFS variable as

awk 'BEGIN {OFS=":"} {print $4,$5}' test_file

output will be as below.

unixrun:0
unixrun:175
unixrun:430
unixrun:428

If I run same command in this way, it will not work same as earlier, Here no column separator showing in output.

awk 'BEGIN {OFS=":"} {print $4$5}' test_file

unixrun0
unixrun175
unixrun430
unixrun428

NF - Number of fields variable:

The NF used to get the number of fields in line of a file.

Example 7. awk '{print NF}' test_file

This will display the number of columns in each row, output as below.

9
9
9
9

NR - number of records variable: 

The NR used to get the line number or no of lines in the file.

Example 8. awk '{print NR}' test_file

This will display the line numbers from 1.

1

Example 9. awk 'END {print NR}' test_file

This will display the total number of lines in the file, here output will be.

4

FILENAME - The Current Filename Variable

The last variable known to regular AWK is "FILENAME", which tells you the name of the file.

#!/bin/awk -f
# reports which file is being read
BEGIN {
f="";
}
{ if (f != FILENAME) {
print "reading", FILENAME;
f=FILENAME;
}
print;
}

Unix - AWK Command by examples. Part 1

AWK command is one of the most powerful tools in Unix used for processing the rows and columns in a file. AWK has built in string functions and associative arrays and supports all functionalities of C language. We can also convert AWK scripts into Perl scripts using a2p utility, an added advantage of AWK.

awk 'BEGIN {start_action} {action} END {stop_action}' file

The actions in the begin block are performed before processing the file.
The actions in the end block are performed after processing the file.
The rest of the actions are performed while processing the file.

Create a file test_file with the following data. This file can be easily created using the output of ls -l.
-rw-rw-rw- 1 unixrun unixrun   0 Nov  26 21:39 AAAA
-rw-rw-rw- 1 unixrun unixrun 175 Nov  26 21:15 BBBB
-rw-rw-rw- 1 unixrun unixrun 430 Nov  26 21:39 EEEE
-rw-rw-rw- 1 unixrun unixrun 428 Nov  26 21:39 GGGG

Example 1. awk '{print $1}' test_file

$1, $2, $3... represents the first, second, third column,.. in a row respectively.
Here, awk command will print the first column in each row as shown below.

The variable "$0" refers to the entire line that AWK reads in. That is, if you had nine fields in a line,
print $0
is similar to
print $1, $2, $3, $4, $5, $6, $7, $8, $9

-rw-rw-rw-
-rw-rw-rw-
-rw-rw-rw-
-rw-rw-rw-

To print the 2th and 5th columns in a file use awk '{print $2,$5}' test_file

Example 2. awk 'BEGIN {cnt=0} {cnt=cnt+5+$3} END {print cnt}' test_file

This will prints the sum of the value as  (value of 3th column + 5 ).
In the Begin block the cnt variable is assigned with value 0.
In the next block the v (value of 3rd column + 5) is added to cnt variable.
In addition of this, same scenario is repeated for no of rows in a file.
When all the rows are processed the cnt variable will hold the sum of the values as per our formula (value of 3rd column + 5).
Finally, the output printed in the End block.

We can perform the above steps in another way also - by creating command file.

vi summation.awk
#!/usr/bin/awk -f
BEGIN {cnt=0}
{cnt=cnt+$3+5}
END {print cnt}
:wq

This will create a file named as summation.awk, it will be executed by using awk command as below:

awk -f summation.awk test_file.

This will run the script in summation.awk file and displays the sum as per our formula (sum of the 3rd column + 5 ) in the test_file.

Example 3. awk '{ if($5 == "time") print $0;}' test_file

Above command is used for to find a particular string in a file in a specific column.
Here, it searches the string "time" in the 5th column  of the file and prints entire line in output.

-rw-r--r-- 1 unixrun unixrun 43 Nov  26 21:39 time

Example 4. awk 'BEGIN { for(i=1;i<=4;i++) print "Cube of", i, "is",i*i*i; }'

This will print the Cube of numbers from 1 to 4, The output of the command is

Cube of 1 is 1
Cube of 2 is 8
Cube of 3 is 27
Cube of 4 is 64

Constructors in Java.

Constructor in java is a special type of method that is used to initialize the object. Java constructor is invoked at the time of object creation. It constructs the values i.e. provides data for the object that is why it is known as constructor. 

Constructor name must be same as its class name and must have no explicit return type.

There are two types of constructors:
1. Default constructor (Non Parameterized constructor)
Default constructor provides the default values to the object like 0, null etc. depending on the type.

2. Parameterized constructor
A constructor that have parameters is known as parameterized constructor.
Parameterized constructor is used to provide different values to the distinct objects.  

Constructor overloading used in Java in which a class can have no. of constructors that differ in parameter lists.The compiler differentiates these constructors by taking into account the number of parameters in the list and their type. 

class Car

{

    int id;

    String carname;

    String brand;

    int qty;

    Car(int i,String n){

    id = i;

    carname = n;

    }

    Car(int i,String n,Str a){

    id = i;

    carname = n;

    brand =a;

    }

    Car(int i,String n,Str a,int t){

    id = i;

    carname = n;

    brand =a;

    qty = t;

    }

    void display()

    {

    System.out.println(id+" "+carname+" "+brand+" "+qty);

    }

    public static void main(String args[]){

    Car s1 = new Student5(101,"Alto");

    Car s2 = new Student5(102,"Swift","Maruti",25);

    Car s2 = new Student5(102,"Swift","Maruti");    

    s1.display();

    s2.display();

    s3.display();

   }

}

Output:

101 Alto 0

102 Swift Maruti 25

102 Swift Maruti 0

Operator Precedence in Java.

Java has well-defined rules for execution order in which the operators in an expression are evaluated when the expression has several operators.
Below are the execution orders i.e. precedence in Java Coding.

Level	Operator	Description	Associativity
1	= += -= *= /= %=	Assignment Addition assignment Subtraction assignment Multiplication assignment Division assignment Modulus assignment	Right
2	? :	Ternary conditional	Right
3	||	Logical OR	Left
4	&&	Logical AND	Left
5	|	Bitwise inclusive OR	Left
6	^	Bitwise exclusive OR	Left
7	&	Bitwise AND	Left
8	== !=	Relational is equal to Relational is not equal to	Left
9	< <= > >= instanceof	Relational less than Relational less than or equal Relational greater than Relational greater than or equal Type comparison (objects only)	Left
10	<< >> >>>	Bitwise left shift Bitwise right shift with sign extension Bitwise right shift with zero extension	Left
11	+ -	Addition Subtraction	Left
12	* / %	Multiplication Division Modulus	Left
13	++ -- + - ! ~ ( type )	Unary pre-increment Unary pre-decrement Unary plus Unary minus Unary logical negation Unary bitwise complement Unary type cast	Right
14	++ --	Unary post-increment Unary post-decrement	Right
15	() [] ·	Parentheses Array subscript Member selection	Left

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 {} \;