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This is the first article in the new awk tutorial series. We will publish several articles on awk in the coming time, which will explain all the features of awk with practical examples.

In this article, let's review the basic awk working methods and 7 practical awk printing examples.

awk introduction and printing operations

awk is a programming language that can easily manipulate structured data and generate formatted reports. Awk stands for the names of its authors, " Aho , Weinberger , and Kernighan ."

Awk is mainly used for pattern scanning and processing. It searches one or more files to see if they contain lines that match the specified pattern, and then performs related operations.

Some key features of awk are:

• awk treats text files as records and fields.
• Like common programming languages, Awk has variables, conditionals, and loops.
• awk has arithmetic and string operators.
• awk can generate formatted reports

awk reads data from a file or from its standard input and outputs to its standard output. awk cannot process non-text files.

语法

awk '/search pattern1/ {Actions}
     /search pattern2/ {Actions}' file

In the above awk syntax:

• The search pattern is a regular expression.
• Actions - statements to be executed.
• There are many modes and operations that can be used in awk.
• file——input file.
• The single quotes around the program are to avoid any special characters in it being interpreted by the shell.

How awk works

awk reads the input file one line at a time.

• For each line, it matches the given patterns in the given order and if there is a match performs the corresponding action.
• If no pattern matches, no action is performed.
• In the above syntax, either the search pattern or the operation is optional, but not both.
• If no search pattern is given, Awk performs the given action on each line of input.
• If no action is given, all lines matching the given pattern are printed, which is the default action.
• Empty brackets without any action do nothing. It does not perform the default print action.
• Each statement in Actions should be separated by a semicolon.

Let us create employee.txt file with the following contents , which will be used in the examples mentioned below.

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. awk's default behavior

By default, Awk prints each line in a file.

$ awk '{print;}' employee.txt

In the above examples no pattern is given. Hence, these operations are applied to all rows.

By default, the action print without any arguments prints the entire line. So it successfully prints all the lines of the file. Action must be enclosed in curly braces.

awk example 2. Print lines matching a pattern

$ awk '/Thomas/
> /Nisha/' employee.txt

In the above example, it prints all the lines that match "Thomas" or "Nisha". It has two patterns. awk accepts any number of patterns, but each set (pattern and its corresponding action) must be separated by a newline character.

awk example 3. Print only specific fields

awk has many built-in variables. For each record, i.e. line, it by default splits the record separated by whitespace characters and stores it in $nvariables. If the line has 4 words, it will be stored in $1 , $2 , $3, and $4 . $0 represents the entire line. NF is a built-in variable that represents the total number of fields in a record.

$ awk '{print $2,$5;}' employee.txt

In the above example, $2and $5represent Name and Salary respectively . We can also use $NFto get the salary, where $NFrepresents the last field. In printthe statement, ,is a connector.

awk example 4. Initialization and final operations

There are two important modes in awk, designated by the keywords BEGIN and END .

语法

BEGIN { Actions}
{ACTION} # Action for everyline in a file
END { Actions }

# 这里是Awk注释

The actions specified in the BEGIN section will be executed before starting to read lines from the input.

The END action will be executed after the input line has been read and processed.

$ awk 'BEGIN {print "Name\tDesignation\tDepartment\tSalary";}
{print $2,"\t",$3,"\t",$4,"\t",$NF;}
END{print "Report Generated\n--------------";
}' employee.txt

In the above example, it prints the title and the last file of the report.

Awk Example 5. Find employees whose employee id is 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 example above, the first field $1is the employee ID. So if $1it is greater than 200, then just do the default print action and print the entire row.

awk Example 6. Printing a list of employees in the technical department

Now the department name is available as the fourth field, so we need to check $4if it matches the string " Technology " and print the line if so.

$ awk '$4 ~/Technology/' employee.txt

operator is ~used to compare with the regular expression. If it matches the default action i.e. print the entire line will be executed.

awk Example 7. Print the number of employees in the technical department

The following example checks if the department is Technology and if so, increments the variable in Action countthat was initialized to zero in the BEGIN section.

$ awk 'BEGIN { count=0;}
$4 ~ /Technology/ { count++; }
END { print "Number of employees in Technology Dept =",count;}' employee.txt

Then at the end of the process, we simply print countthe value of , which gives us the number of employees in the Technology department.