UNIX Command: AWK

Why AWK ?

• The name AWK comes from the initials of its designers. Alfred V. Aho, Peter J. Weinberger. and Brian W. Kernighan. The Original version  of AWK was written in 1977.
• UNIX has many utilities and AWK is one of them.
• AWK is an excellent tool for processing rows and columns of data, and is easier to use  AWK than most conventional programing languages.
• AWK can be considered to be a pseudo-C interpreter, as it understands the same arithmetic operators as C.
• AWK also has string manipulation functions, so it can search for particular strings and modify the output.

Syntax of AWK:

                   awk '/search pattern1/ { Actions}  
                   awk '/search pattern1/ { Actions}' file

In The above syntax search pattern is a regular expression.
Actions - Statements to be performed. Several patterns and actions are possible in AWK.
File - Input File.

Example 1:  awk  '{print;}' employee.txt

Here pattern is not there so actions are applicable to all the lines.

Working Methodology of AWK :

• AWK reads the input files one line at a time.
• For each line, it matches with given pattern in the given order, if matches performs the corresponding action.
• If no pattern matches, no action will be performed.
• In the above syntax either search patterns or actions are optional, but not both.
• If  the search pattern is not given, then AWK performs the given actions for each line of the input.
• If the action is not given, print all the lines that matches with the given patterns which is the default action.
• Each statement in actions should be delimited by semicolon.
• Each word in a line is a field $1, $2,..... $NF. 

 Example 2: Print the line which matches the pattern 

awk '/Thomas/Nisha/' employee.txt

It will print the lines that contain Thomas and lines contains Nisha.

Example 3: Print only selective fields.

AWK treats a line as a record and columns as a field.

awk '{print $1,$5;}' employee.txt

It will print only the 2nd and 5th filed of employee.txt

Example 4: Initialization and final action

Syntax: BEGIN {Actions}
 {ACTION}
END {Actions}


The Built in Variables of AWK: 

1. FS (Input Filed Separator): By default AWK assumes that fields in a file are separated by space character. If the fields are separated by any other character, we can use the FS  variable to tell about the delimiter.

$ awk 'BEGIN {FS="|"} {print $2}' input_file  

It will print the second word of the input_file provided the field separator is "|".