Difference between revisions of "Linux-AWK/C2/Variables-and-Operators-in-awk/English"

• User defined variables
• Operators
• BEGIN and END statements

We will do this through some examples.

You should be familiar with basic operators used in general programming languages like C or C++.

If not, then please go through the corresponding tutorials on our website.

So, it supports variables, constants, operators, etc.

A variable is an identifier that references a value.

Awk supports both user-defined variables and built-in variables.

We will learn about user-defined variables in this tutorial.

For user-defined variables, variable declaration is not required.

Variables do not have to be initialized explicitly.

Awk automatically initializes them to zero or null string.

Variables are case-sensitive.

So, Salary with capital “S” and salary with small “s” are two different variables.

awk '{x=1;X="A";a="awk";b="tutorial"[enter]

print x [enter]

print X [enter]

print a [enter]

print b [enter]

print a b

print x b

print x+X}' [enter]

awk space opening single quote opening curly brace small x equal to 1 semicolon capital X equal to within double quotes capital A semicolon small a equal to within double quotes awk semicolon small b equal to within double quotes tutorial.

Press Enter.

type print x Press Enter

print capital X Press Enter

print a Press Enter

print b Press Enter

print a space b Press Enter

print x space b Press Enter

print small x plus capital X closing curly brace closing single quote and press Enter

And hence, we can type any letter, say a and then press Enter.

Variables can be initialized with a number.

It can also be initialized with value as a single character or a string.

If the value is a character or a string, variable is initialized with value within double quotes.

Observe that small x and capital X are treated as different variables.

This proves that variables are case sensitive.

Here variables small a and small b are concatenated.

So, string concatenation operator is simply a space.

And the concatenated output becomes 1tutorial.

That's because awk finds a string concatenation operator space here between x and b.

Here, we have the arithmetic operator plus.

So, X is auto-converted to numeric zero.

And the addition output becomes numeric 1.

Let’s look at what other operators we can use.

Please pause the video here and take a look at all the operators mentioned here.

I assume you are familiar with these basic operators.

If not, then kindly visit our website for tutorials on operators in C and C++ series.

Only exception is the string matching operator, which may be new to you.

Let understand this with an example.

Pls download it on your computer.

Let us end the previous process by pressing Ctrl and D keys.

Let me clear the terminal.

In this case, we need to compare two different fields.

For such situations, we can use awk's relational operators.

These operators can compare both strings and numbers.

[enter]

awk space hyphen capital F within double quotes vertical bar space Within single quotes dollar 5 equal to equal to within double quotes Pass space ampersand ampersand space dollar 4 less than 80 space within curly braces print space plus plus x coma dollar 2 coma dollar 4 coma dollar 5 space awkdemo.txt and press Enter.

One, we compare a string with the fifth field.

Second, we only compare the fourth field with a number.

Third, we see that we can join two or more comparisons using ampersand operator.

Instead of specific numbers or strings, we can also compare regular expressions.

As we have seen in the slide, we have the tilde and the exclamation tilde operators for this purpose.

Since computers can have both a small and capital C, we would use a regular expression.

awk -F "|" '$5=="Pass" && $3~/[cC]omputers/ {print ++x,$2,$3,$5}' awkdemo.txt

awk space hyphen capital F within double quotes pipe symbol space within single quote dollar 5 equal to equal to within double quotes Pass ampersand ampersand space dollar 3 tilde slash within square brackets small c capital C computers slash space within curly braces print space plus plus ++x coma dollar 2 coma dollar 3 coma dollar 5 space awkdemo.txt and press Enter.

awk -F "|" '$5=="Pass" && $3!~/[cC]omputers/ {print ++x,$2,$3,$5}' awkdemo.txt

Use the up arrow to get the previous command.

Next to dollar 3 add exclamation symbol and press Enter.

So, it has 3 blank lines.

awk '/^$/ {x=x+1; print x}' awkdemo.txt [enter]

awk space within single quote within front slash caret symbol dollar space within curly braces x equal to x plus 1 semicolon space print x space awkdemo.txt

Press Enter.

Awk has initialized x to the initial value zero.

This is because every time a blank line is found, x would be incremented and then printed.

But say we only want to print the total number of blank lines.

Then we need to print x only once, after the entire file has been traversed.

We may also want to give a heading saying what the output means.

For such requirements awk provides the BEGIN and the END sections.

This section is executed before the main input loop is executed.

The END section can contain procedures for

post-processing.

This section is executed after the main input loop has terminated.

The BEGIN and END procedures are optional.

awk 'BEGIN{print "The number of empty lines in awkdemo are"} [press enter]

/^$/ {x=x+1} [press enter]

end {print x}' awkdemo.txt[press enter]

In the terminal type

awk space opening single quote BEGIN incaps within curly brace print space within double quotes The number of empty lines in awkdemo are

press Enter.

within front slash caret symbol dollar symbol space within curly brace x equal to x plus 1

press Enter.

end space within curly braces print space x close single quote space awkdemo.txt and press Enter.

We should get the output as 3, because we have 3 blank lines in the file.

Actually, we should have written end as upper case END.

/^$/ {x=x+1} [press enter]

END {print x}' awkdemo.txt [press enter]

Now modify lower case end to upper case END.

And press Enter.

awk -F"|" 'BEGIN {printf "%-3s %-25s %10s\n", "Sl","Student","Stipend"} [enter] {x=x+1; total = total+$6; printf"%-3d %-25s %-10d\n",x,$2,$6} [enter]

END{avg=total/x; print "The average stipend is" avg}' awkdemo.txt [Enter]

And press Enter.

And we get the desired output.

In this tutorial we learnt about

• User defined variables in awk
• Operators
• BEGIN and END statements

And the student belongs to Electrical department.

Print the average marks of all the students with the heading “Average marks” in the output.

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