4.1. Invoking AWK Programs

AWK programs can be executed in two primary ways: directly from the command line for simple, one-off tasks, or from a script file for more complex or reusable logic. This section covers both invocation methods and introduces foundational operations like field selection and pattern matching.

4.2. Invocation Methods

Command-Line Invocation

This method is best suited for short scripts. The entire program is provided as a string enclosed in single quotes.

• Syntax: awk [options] ‘program’ file …

For example, to print the entire content of marks.txt, we can use the print action:

[jerry]$ awk ‘{print}’ marks.txt

Output:

1) Amit Physics 80

2) Rahul Maths 90

3) Shyam Biology 87

4) Kedar English 85

5) Hari History 89

Script File Invocation

For longer or reusable scripts, it is better to place the AWK commands in a file. The -f option tells AWK to read the program from this file.

• Syntax: awk [options] -f file …

First, create a file named command.awk with the following content:

{print}

Now, execute it with the -f flag:

[jerry]$ awk -f command.awk marks.txt

Output:

1) Amit Physics 80

2) Rahul Maths 90

3) Shyam Biology 87

4) Kedar English 85

5) Hari History 89

4.3. Core Operations with Examples

Using the marks.txt file, let’s explore some of AWK’s most common operations.

Printing Specific Fields (Columns)

AWK automatically splits each record into fields based on whitespace. These fields are accessed using the syntax $n, where $1 is the first field, $2 is the second, and so on. The following command prints only the third and fourth columns, separated by a tab.

[jerry]$ awk ‘{print $3 “\t” $4}’ marks.txt

Output:

Physics 80

Maths   90

Biology 87

English 85

History 89

Pattern Matching and Record Printing

AWK can filter records, printing only those that match a specified pattern. $0 is a special variable that represents the entire record. The following command prints all lines containing the letter a.

[jerry]$ awk ‘/a/ {print $0}’ marks.txt

When no action block is provided, AWK’s default action is to print the matching line ({print $0}). Therefore, the command can be abbreviated:

[jerry]$ awk ‘/a/’ marks.txt

Output:

2) Rahul Maths 90

3) Shyam Biology 87

4) Kedar English 85

5) Hari History 89

Combining Pattern Matching and Field Selection

You can combine pattern matching with field selection to print specific columns only from the lines that match the pattern.

[jerry]$ awk ‘/a/ {print $3 “\t” $4}’ marks.txt

Output:

Maths   90

Biology 87

English 85

History 89

Performing Simple Calculations

AWK can perform actions based on patterns. The following script counts the number of lines containing the letter a. The counter cnt is incremented in the body for each match, and the final result is printed in the END block.

[jerry]$ awk ‘/a/{++cnt} END {print “Count = “, cnt}’ marks.txt

Output:

Count =  4

Using Built-in Functions for Conditions

Built-in functions can be used to form conditional patterns. The length() function returns the length of a string. The following command prints only lines that are longer than 18 characters.

[jerry]$ awk ‘length($0) > 18’ marks.txt

Output:

3) Shyam Biology 87

4) Kedar English 85

These basic operations demonstrate AWK’s power. To unlock more complex logic, we must explore the built-in variables that AWK provides.