Linux-AWK/C2/Variables-and-Operators-in-awk/English-timed

Time	Narration
00:01	Hello and welcome to this spoken tutorial on Variables and operators in awk command.
00:07	In this tutorial, we will learn about- User defined variables,
00:12	Operators, BEGIN and END statements.
00:17	We will do this through some examples.
00:20	To record this tutorial, I am using Ubuntu Linux 16.04
00:26	To practice this tutorial, you should have gone through the earlier Linux tutorials on this website.
00:33	You should be familiar with basic operators used in general programming languages like C or C++.
00:41	If not, then please go through the corresponding tutorials on our website.
00:47	awk combines the power of a filter and a programming language.
00:52	So, it supports variables, constants, operators etc.
00:58	Let’s see what a variable in awk is.
01:02	A variable is an identifier that references a value.
01:07	Awk supports both user-defined variables and built-in variables.
01:12	We will learn about user-defined variables in this tutorial.
01:17	For user-defined variables, variable declaration is not required.
01:22	Variables do not have to be initialized explicitly.
01:26	Awk automatically initializes them to zero or null string.
01:32	A variable must begin with a letter and continue with letters, digits and underscores. Variables are case-sensitive.
01:43	So, Salary with capital “S” and salary with small “s” are two different variables.
01:50	Let us look at some examples now.
01:53	Open the terminal by pressing CTRL + ALT and T keys.
01:58	On the terminal, type- 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.
02:25	Type: print x, press Enter.
02:29	print capital X, press Enter.
02:34	print a, press Enter.
02:37	print b, press Enter.
02:40	print a space b, press Enter.
02:44	print small x space b, press Enter.
02:49	print small x plus capital X closing curly brace closing single quote and press Enter.
02:57	Since we have not given a filename, awk would need some input from standard input.
03:03	And hence, we can type any letter, say a and then press Enter.
03:10	This example shows a couple of things. Variables can be initialized with a number.
03:18	It can also be initialized with value as a single character or a string.
03:23	If the value is a character or a string, variable is initialized with value within double quotes.
03:31	We can see the values of the variables.
03:35	Observe that small x and capital X are treated as different variables.
03:41	This proves that variables are case sensitive.
03:45	Also, it shows how two strings can be concatenated.
03:50	Here variables small a and small b are concatenated.
03:55	So, string concatenation operator is simply a space.
04:00	Similarly, when we concatenate small x which is a number and string b, x is auto-converted into string. And the concatenated output becomes 1tutorial.
04:13	Why does the auto-conversion to string happen?
04:16	That's because awk finds a string concatenation operator space here, between x and b.
04:25	Now, look at the output of small x plus capital X. Here, we have the arithmetic operator plus.
04:33	So, X is auto-converted to numeric zero. And the addition output becomes numeric 1.
04:42	Until now, we have seen a couple of operators. Let’s look at what other operators we can use.
04:49	A variety of operators can be used in expressions.
04:53	Please pause the video here and take a look at all the operators mentioned here.
04:58	I assume you are familiar with these basic operators.
05:02	If not, then kindly visit our website for tutorials on operators in C and C++ series.
05:09	I am not going to discuss the working of all these operators in detail.
05:14	Only exception is the string matching operator which may be new to you. Let us understand this with an example.
05:23	A file named awkdemo.txt has been provided in the Code files link. Please download it on your computer.
05:31	Switch to the terminal. Let us end the previous process by pressing Ctrl and D keys.
05:38	Let me clear the terminal.
05:41	Now, go to the folder in which you have saved the awkdemo.txt file using the cd command.
05:48	Let us have a look at this file now.
05:52	Let's say we want to find the students who have passed but have marks less than 80.
05:58	In this case, we need to compare two different fields.
06:02	For such situations, we can use awk's relational operators.
06:07	These operators can compare both strings and numbers.
06:12	So, on the terminal, type: 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 comma dollar 2 comma dollar 4 comma dollar 5 space awkdemo.txt and press Enter.
06:54	This command shows a number of things. One, we compare a string with the fifth field.
07:01	Second, we only compare the fourth field with a number.
07:06	Third, we see that we can join two or more comparisons using ampersand operator.
07:13	Instead of specific numbers or strings, we can also compare regular expressions.
07:19	As we have seen in the slide, we have the tilde and the exclamation tilde operators for this purpose.
07:27	Now suppose, we want to find computer science students who have passed.
07:32	Since computers can have both a small and capital C, we would use a regular expression.
07:40	We would type: 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 small x comma dollar 2 comma dollar 3 comma dollar 5 space awkdemo.txt and press Enter.
08:24	If we want to negate the comparison, we can do so using the exclamation tilde operator.
08:30	Say, now we want a list of all non-computer students who have passed.
08:35	Use the up-arrow to get the previous command.
08:39	Next to dollar 3 add exclamation symbol and press Enter.
08:47	Next, let's count the number of blank lines in the same file.
08:52	Open the file and check how many blank lines are there. So, it has 3 blank lines.
09:00	Now to count the number of empty lines using awk, type: 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.
09:26	We got 3 as our final answer.
09:30	The caret sign signifies start of a line while dollar signifies the end of a line.
09:37	Hence, an empty line would be matched by the regular expression caret-dollar.
09:43	Note, we have not initialized the value of x. Awk has initialized x to the initial value zero.
09:51	This command gives us the running count of blank lines. This is because, every time a blank line is found, x would be incremented and then printed.
10:02	In our last command, we have seen running count of blank lines. But, say we only want to print the total number of blank lines.
10:12	Then we need to print x only once, after the entire file has been traversed.
10:19	We may also want to give a heading saying what the output means.
10:25	For such requirements, awk provides the BEGIN and the END sections.
10:30	The BEGIN section contains procedures for pre-processing.
10:34	This section is executed before the main input loop is executed.
10:40	The END section can contain procedures for post-processing.
10:45	This section is executed after the main input loop has terminated. The BEGIN and END procedures are optional.
10:55	Let's learn how to do this. In the terminal, type: awk space opening single quote BEGIN in caps within curly brace print space within double quotes The number of empty lines in awkdemo are press Enter.
11:14	within front slash caret symbol dollar symbol space within curly braces x equal to x plus 1 press Enter.
11:26	end space within curly braces print space x close single quote space awkdemo.txt and press Enter.
11:39	See, we did not get the desired output! We should get the output as 3 because we have 3 blank lines in the file.
11:48	What do you think happened? Actually, we should have written end as upper case END.
11:54	So, let us modify the command.
11:57	Press the up-arrow key to get the previous executed command on the terminal.
12:03	Now, modify lower case end to upper case END. And press Enter.
12:11	Now the total number of empty lines is displayed in the output.
12:16	Next, let's find the average salary of all the students that we found in the awkdemo.txt file.
12:24	To get that, type the command as shown in the terminal and press Enter. And, we get the desired output.
12:35	This brings us to the end of this tutorial. Let us summarize.
12:40	In this tutorial, we learnt about: User defined variables in awk,
12:45	Operators, BEGIN and END statements.
12:49	As an assignment, print every line where the value of the last field is more than 5000. And, the student belongs to Electrical department.
13:00	Print the average marks of all the students with the heading “Average marks” in the output.
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14:10	The script has been contributed by Antara and this is Praveen from IIT Bombay, signing off. Thank you for joining.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14