Awesome AWK

Awk 

Introduction and Printing Operations

Awk is a programming language which allows easy manipulation of structured data and the generation of formatted reports. Awk stands for the names of its authors “Aho, Weinberger, and Kernighan”
The Awk is mostly used for pattern scanning and processing. It searches one or more files to see if they contain lines that matches with the specified patterns and then perform associated actions.
Some of the key features of Awk are:
  • Awk views a text file as records and fields.
  • Like common programming language, Awk has variables, conditionals and loops
  • Awk has arithmetic and string operators.
  • Awk can generate formatted reports
Awk reads from a file or from its standard input, and outputs to its standard output. Awk does not get along with non-text files.

Syntax:

awk '/search pattern1/ {Actions}
     /search pattern2/ {Actions}' file
In the above awk syntax:
  • search pattern is a regular expression.
  • Actions – statement(s) to be performed.
  • several patterns and actions are possible in Awk.
  • file – Input file.
  • Single quotes around program is to avoid shell not to interpret any of its special characters.

Awk Working Methodology

  1. Awk reads the input files one line at a time.
  2. For each line, it matches with given pattern in the given order, if matches performs the corresponding action.
  3. If no pattern matches, no action will be performed.
  4. In the above syntax, either search pattern or action are optional, But not both.
  5. If the search pattern is not given, then Awk performs the given actions for each line of the input.
  6. If the action is not given, print all that lines that matches with the given patterns which is the default action.
  7. Empty braces with out any action does nothing. It wont perform default printing operation.
  8. Each statement in Actions should be delimited by semicolon.
  9. Let us create employee.txt file which has the following content, which will be used in the
    examples mentioned below.
    $cat employee.txt
    100  Thomas  Manager    Sales       $5,000
    200  Jason   Developer  Technology  $5,500
    300  Sanjay  Sysadmin   Technology  $7,000
    400  Nisha   Manager    Marketing   $9,500
    500  Randy   DBA        Technology  $6,000

    Awk Example 1. Default behavior of Awk

    By default Awk prints every line from the file.
    $ awk '{print;}' employee.txt
    100  Thomas  Manager    Sales       $5,000
    200  Jason   Developer  Technology  $5,500
    300  Sanjay  Sysadmin   Technology  $7,000
    400  Nisha   Manager    Marketing   $9,500
    500  Randy   DBA        Technology  $6,000
    In the above example pattern is not given. So the actions are applicable to all the lines.
    Action print with out any argument prints the whole line by default. So it prints all the
    lines of the file with out fail. Actions has to be enclosed with in the braces.

    Awk Example 2. Print the lines which matches with the pattern.

    $ awk '/Thomas/
    > /Nisha/' employee.txt
    100  Thomas  Manager    Sales       $5,000
    400  Nisha   Manager    Marketing   $9,500
    In the above example it prints all the line which matches with the ‘Thomas’ or ‘Nisha’. It has two patterns. Awk accepts any number of patterns, but each set (patterns and its corresponding actions) has to be separated by newline.

    Awk Example 3. Print only specific field.

    Awk has number of built in variables. For each record i.e line, it splits the record delimited by whitespace character by default and stores it in the $n variables. If the line has 4 words, it will be stored in $1, $2, $3 and $4. $0 represents whole line. NF is a built in variable which represents total number of fields in a record.
    $ awk '{print $2,$5;}' employee.txt
    Thomas $5,000
    Jason $5,500
    Sanjay $7,000
    Nisha $9,500
    Randy $6,000
    
    $ awk '{print $2,$NF;}' employee.txt
    Thomas $5,000
    Jason $5,500
    Sanjay $7,000
    Nisha $9,500
    Randy $6,000
    In the above example $2 and $5 represents Name and Salary respectively. We can get the Salary using  $NF also, where $NF represents last field. In the print statement ‘,’ is a concatenator.

    Awk Example 4. Initialization and Final Action

    Awk has two important patterns which are specified by the keyword called BEGIN and END.
    Syntax: 
    
    BEGIN { Actions}
    {ACTION} # Action for everyline in a file
    END { Actions }
    
    # is for comments in Awk
    Actions specified in the BEGIN section will be executed before starts reading the lines from the input.
    END actions will be performed after completing the reading and processing the lines from the input.
    $ awk 'BEGIN {print "Name\tDesignation\tDepartment\tSalary";}
    > {print $2,"\t",$3,"\t",$4,"\t",$NF;}
    > END{print "Report Generated\n--------------";
    > }' employee.txt
    Name Designation Department Salary
    Thomas   Manager   Sales           $5,000
    Jason   Developer   Technology   $5,500
    Sanjay   Sysadmin   Technology   $7,000
    Nisha   Manager   Marketing   $9,500
    Randy   DBA     Technology   $6,000
    Report Generated
    --------------
    In the above example, it prints headline and last file for the reports.

    Awk Example 5. Find the employees who has employee id greater than 200

    $ awk '$1 >200' employee.txt
    300  Sanjay  Sysadmin   Technology  $7,000
    400  Nisha   Manager    Marketing   $9,500
    500  Randy   DBA        Technology  $6,000
    In the above example, first field ($1) is employee id. So if $1 is greater than 200, then just do the default print action to print the whole line.

    Awk Example 6. Print the list of employees in Technology department

    Now department name is available as a fourth field, so need to check if $4 matches with the string “Technology”, if yes print the line.
    $ awk '$4 ~/Technology/' employee.txt
    200  Jason   Developer  Technology  $5,500
    300  Sanjay  Sysadmin   Technology  $7,000
    500  Randy   DBA        Technology  $6,000
    Operator ~ is for comparing with the regular expressions. If it matches the default action i.e print whole line will be  performed.

    Awk Example 7. Print number of employees in Technology department

    The below example, checks if the department is Technology, if it is yes, in the Action, just increment the count variable, which was initialized with zero in the BEGIN section.
    $ awk 'BEGIN { count=0;}
    $4 ~ /Technology/ { count++; }
    END { print "Number of employees in Technology Dept =",count;}' employee.txt
    Number of employees in Tehcnology Dept = 3
    Then at the end of the process, just print the value of count which gives you the number of employees in Technology department.

    Courtesy:TheGeekstuff.com

Popular posts from this blog

Grsecurity: Configure RHEL5/6 Kernel for Grsecurity

Linux Command line tips and Bash stuff

How to check Perl Module version in Linux