WEBVTT 00:00.200 --> 00:01.100 Hello friends. 00:01.100 --> 00:05.270 Welcome again to the part two of Fundamentals of Linux Shell Scripting. 00:05.300 --> 00:11.810 In this part we are going to talk about the control statements and functions in Linux shell scripting. 00:11.810 --> 00:20.240 But before actually moving to this video, I want you to practice the exercises which are provided after 00:20.240 --> 00:21.350 the first video. 00:21.380 --> 00:21.860 Okay. 00:21.890 --> 00:27.800 And if you have already practiced the exercises then let's move to this video. 00:27.860 --> 00:28.430 Okay. 00:29.630 --> 00:33.170 So first of all let's see what is the control statement. 00:33.200 --> 00:39.740 Control statements in Linux shell scripting are essential for creating complex and dynamic scripts. 00:39.770 --> 00:47.240 They allow you to control the flow of execution based on conditions, loops, and other criteria. 00:47.270 --> 00:50.240 Okay, we can write conditions. 00:50.270 --> 00:50.750 Okay? 00:50.780 --> 00:55.670 We can perform repetition of script with the help of loops okay. 00:55.700 --> 01:05.200 And by using conditions and loops we can control the flow of The execution of our scripts so we can 01:05.320 --> 01:13.000 write different conditions by combining different operators like string operators, Boolean operators, 01:13.060 --> 01:16.750 relational operators, arithmetic operators. 01:16.780 --> 01:17.290 Okay. 01:17.290 --> 01:20.440 And the last one is file test operators. 01:20.590 --> 01:25.480 So the control statements can be categorized into different types. 01:25.480 --> 01:27.520 So let's see the first type. 01:27.610 --> 01:30.640 The first type is conditional statements. 01:30.820 --> 01:41.320 In conditional statements we use the keyword if else if else if else to control the flow of execution 01:41.350 --> 01:42.010 okay. 01:42.010 --> 01:45.220 And we can write different conditions okay. 01:45.250 --> 01:47.800 With the help of these keywords. 01:47.830 --> 01:52.930 The if else and if else if else is called as the keywords. 01:52.960 --> 01:53.470 Okay. 01:53.500 --> 01:57.550 The meaning of these keywords is already known to our shell. 01:57.580 --> 01:58.000 Okay. 01:58.030 --> 02:03.100 So we can write different conditions by using these keywords. 02:03.130 --> 02:03.550 Okay. 02:03.580 --> 02:06.520 So let's see this first example. 02:06.580 --> 02:12.550 In this example we're going to check the age of a person whether that person is an adult or minor. 02:12.580 --> 02:13.420 Okay. 02:13.420 --> 02:20.860 So to check that we have a right a condition here by using the if else keyword. 02:20.890 --> 02:21.580 Okay. 02:21.610 --> 02:26.110 So to write the conditions you need to first type the keyword. 02:26.110 --> 02:31.600 If then inside the square brackets you need to write your condition. 02:31.600 --> 02:37.060 Then you need to use the semicolon here to close that square bracket. 02:37.060 --> 02:40.210 Then you need to use this then keyword. 02:40.300 --> 02:40.720 Okay. 02:40.750 --> 02:48.580 And after that you need to write your commands which you want to execute when this condition becomes 02:48.580 --> 02:49.270 true. 02:49.300 --> 02:50.020 Okay. 02:50.020 --> 02:58.960 And when this condition becomes false then the next command okay, which is present after this else 02:58.990 --> 03:01.270 keyword will be executed. 03:01.420 --> 03:01.900 Okay. 03:01.900 --> 03:08.760 And this Fi stands for the end of if else block or if else condition. 03:08.790 --> 03:13.530 Okay, you need to use this to show that it is the end. 03:13.560 --> 03:19.530 Okay, so this code will check the edge which is entered via keyboard. 03:19.560 --> 03:20.010 Okay. 03:20.010 --> 03:23.070 So now let's understand how this condition works. 03:23.100 --> 03:23.580 Okay. 03:23.580 --> 03:29.910 So in this condition we have used the variable age okay. 03:29.910 --> 03:31.440 Which is defined here. 03:31.470 --> 03:31.860 Okay. 03:31.890 --> 03:36.300 We're gonna read the age of the person from keyboard. 03:36.330 --> 03:36.840 Okay. 03:36.870 --> 03:39.360 That value is assigned to this variable. 03:39.360 --> 03:42.210 And we're gonna access that variable here. 03:42.240 --> 03:42.660 Okay. 03:42.690 --> 03:46.860 And we're going to compare that variable with the number 18. 03:46.980 --> 03:47.370 Okay. 03:47.400 --> 03:54.660 With the help of this relational operator GE hyphen GE stands for greater than or equal to. 03:54.690 --> 03:56.280 Okay let's scroll up. 03:57.600 --> 04:02.880 And here you can see the meaning of this relational operator is greater than or equal to. 04:02.910 --> 04:12.170 When you use this operator in this if block like this, then this will check the value of h. 04:12.200 --> 04:12.530 Okay. 04:12.560 --> 04:16.280 So in such way we can write different conditions. 04:16.310 --> 04:16.760 Okay. 04:16.790 --> 04:19.400 According to our program needs. 04:19.400 --> 04:23.720 So I hope you understand how this operator works. 04:23.750 --> 04:29.180 Okay, so suppose the person entered the number 12. 04:29.210 --> 04:31.910 Then 12 is less than 18. 04:31.910 --> 04:41.150 So the shell will skip this part because the condition here becomes false and will execute the else 04:41.150 --> 04:41.870 block. 04:41.900 --> 04:42.680 Okay. 04:42.680 --> 04:52.490 And if the person enters number 44 then in that case this condition becomes true and this part will 04:52.520 --> 04:53.780 be executed. 04:53.810 --> 04:54.170 Okay. 04:54.200 --> 04:57.500 So this is how this condition works. 04:57.500 --> 05:00.560 So let's see it practically in our shell. 05:01.790 --> 05:06.220 So to save our time I have already typed the examples here. 05:06.490 --> 05:10.450 Okay, so let's execute this script here. 05:12.100 --> 05:14.380 If else dot dash. 05:14.920 --> 05:15.430 Okay. 05:15.460 --> 05:17.740 Now it is asking to enter the age. 05:17.740 --> 05:21.100 So let's enter the age 12. 05:22.090 --> 05:22.630 Okay. 05:22.630 --> 05:24.220 It says you are a minor. 05:24.400 --> 05:26.290 Again execute this script. 05:26.320 --> 05:31.570 Now this time enter the age 55 and it says you are an adult. 05:31.600 --> 05:31.930 Okay. 05:31.960 --> 05:37.150 You can check the script here with cat if else dot bash. 05:37.180 --> 05:37.720 Okay. 05:37.750 --> 05:41.920 This is the same example present in our theoretical part. 05:41.950 --> 05:42.580 Okay. 05:42.610 --> 05:45.490 So this is how the script works. 05:45.520 --> 05:46.150 Okay. 05:46.180 --> 05:49.120 First it receives the value from keyboard. 05:49.150 --> 05:56.710 Then check the value with the help of this relational operator with the number 18 and execute the command 05:56.710 --> 05:58.840 based upon the condition. 05:58.840 --> 06:00.250 Become true or false. 06:00.280 --> 06:00.880 Okay. 06:00.910 --> 06:05.320 After this, the next control statement is if Elif else. 06:05.350 --> 06:11.620 With the help of this, you can write multiple conditions or you can handle multiple conditions. 06:11.740 --> 06:14.500 So let's understand how this script works. 06:14.530 --> 06:22.000 First, it receives a number from keyboard, and here that number is tested with zero okay. 06:22.030 --> 06:25.180 Here hyphen GT stands for greater than. 06:25.180 --> 06:30.250 So if the number is greater than zero that means the number is positive okay. 06:30.280 --> 06:35.350 And if the number is less than zero then the number is negative. 06:35.350 --> 06:40.840 And if you enter zero then it will print this third block. 06:40.870 --> 06:41.470 Okay. 06:41.470 --> 06:49.180 So this example written here is to check whether the entered number is positive or negative or zero 06:49.210 --> 06:49.720 okay. 06:49.750 --> 06:53.710 After this the next type of control statement is case statement. 06:53.710 --> 06:59.650 With case statements you can match variable against multiple patterns. 06:59.680 --> 07:00.370 Okay. 07:00.370 --> 07:07.260 So in this example we're going to check whether the entered number is lowercase or uppercase, or a 07:07.260 --> 07:09.540 digit, or a special character. 07:09.630 --> 07:16.980 Okay, so first of all, it will receive the letter from keyboard okay to the variable letter. 07:16.980 --> 07:21.330 And here we have used the keyword case okay. 07:21.360 --> 07:24.330 And after that we are going to access the variable letter. 07:24.330 --> 07:29.460 And after that the in keyword will create these patterns. 07:29.460 --> 07:33.390 And we can check those patterns one by one okay. 07:33.390 --> 07:36.660 So let's say we have entered small a from keyboard. 07:36.660 --> 07:40.800 Then here the first pattern will be matched. 07:40.800 --> 07:46.860 And it will print the command and shows us that you entered lowercase letter. 07:47.190 --> 07:47.700 Okay. 07:47.940 --> 07:55.260 As you can see here we have set the condition here a hyphen z inside this square bracket. 07:55.260 --> 08:02.490 And this parenthesis is the closing of our first pattern okay. 08:02.490 --> 08:05.430 First pattern of this case okay. 08:05.460 --> 08:12.230 You can write multiple patterns, and these two semicolons are used to close the first block. 08:12.260 --> 08:12.740 Okay. 08:12.980 --> 08:16.820 After this the next pattern is capital A to Z. 08:16.850 --> 08:17.210 Okay. 08:17.240 --> 08:21.050 This pattern is used to match against capital letters. 08:21.230 --> 08:21.410 Okay. 08:21.440 --> 08:25.940 If this condition becomes true, then this command will be printed. 08:25.970 --> 08:29.900 If you enter a number, then this command will be printed. 08:29.900 --> 08:32.900 And if you enter any other value. 08:32.930 --> 08:38.690 That means if you enter a special character, then this star pattern will be printed. 08:38.690 --> 08:41.480 So it is called as the default case. 08:41.510 --> 08:42.200 Okay. 08:42.230 --> 08:50.060 The default pattern is printed only when the rest patterns doesn't match the variable, and it is the 08:50.060 --> 08:53.480 end of this case statement. 08:53.540 --> 08:54.170 Okay. 08:54.200 --> 08:54.980 Esac. 08:55.010 --> 08:57.980 It is very important to write it down. 08:58.010 --> 08:58.610 Okay. 08:58.610 --> 09:01.190 So let's see this example practically. 09:02.450 --> 09:05.120 Now let's execute our script. 09:05.150 --> 09:06.980 Case dot Don't bash. 09:11.300 --> 09:14.510 Spelling mistake is not bash. 09:14.540 --> 09:16.730 Okay, now it is asking to enter the letter. 09:16.730 --> 09:21.530 So let's enter capital A, it says you entered capital letter. 09:21.560 --> 09:27.740 Let's enter a letter capital E it says you entered lower letter. 09:27.860 --> 09:30.740 Let's enter number four. 09:31.160 --> 09:37.790 You entered a digit and let's enter a hash. 09:38.180 --> 09:41.450 You enter a special character okay. 09:41.480 --> 09:45.110 So this is how this code works okay. 09:45.140 --> 09:50.630 You can match your variable against multiple patterns okay. 09:50.630 --> 09:51.830 In this way. 09:52.010 --> 09:58.550 After this let's see the next control statement that is loop with loops. 09:58.550 --> 10:02.540 We can iterate over list of items okay. 10:02.570 --> 10:05.750 So first of all let's see the for loop. 10:05.750 --> 10:09.370 So with the keyword for a variable. 10:09.370 --> 10:17.920 I is created here in this example and with in keyword the list of items okay list of these five items 10:17.920 --> 10:19.270 is created. 10:19.300 --> 10:19.840 Okay? 10:19.870 --> 10:28.090 And when the shell goes to this keyword do it is ready to execute this command echo okay. 10:28.120 --> 10:36.670 The echo will print each item in the list one by one until it reaches to the end of the items in the 10:36.670 --> 10:37.180 list. 10:37.210 --> 10:43.060 Okay, all these numbers from one, two, three, four, five are called as the items in the list. 10:43.090 --> 10:48.430 Okay, which is get created with the help of this in keyword okay. 10:48.460 --> 10:53.080 And this done is used to set the boundary line okay. 10:53.110 --> 10:55.870 The boundary line of our for loop. 10:55.870 --> 10:56.890 It is the end. 10:56.920 --> 10:59.620 It doesn't go to the next line of code. 10:59.650 --> 11:02.500 It goes back to the item list. 11:02.530 --> 11:03.100 Okay. 11:03.100 --> 11:06.730 So let's see how this works practically. 11:09.150 --> 11:09.600 Okay. 11:09.600 --> 11:12.990 So let's execute our example for bash. 11:14.700 --> 11:20.010 And as you can see it executes the number one by one up to five. 11:20.070 --> 11:22.860 So this is called as the loop. 11:22.860 --> 11:27.840 It will continue to execute until the last item in the list. 11:27.870 --> 11:28.650 Okay. 11:28.770 --> 11:31.290 So the same way the while loop works. 11:31.320 --> 11:35.580 The while loop executes the commands as long as the condition is true. 11:35.610 --> 11:38.010 Okay, you need to set the condition here. 11:38.040 --> 11:38.580 Okay. 11:38.610 --> 11:43.410 When the condition becomes false, the shell exits this loop. 11:43.440 --> 11:43.770 Okay. 11:43.800 --> 11:49.590 So in this example counter variable is get created and value one is assigned. 11:49.620 --> 11:57.990 Then we have set the condition for checking whether the entered number is less than or equal to five. 11:58.020 --> 11:58.560 Okay. 11:58.590 --> 12:05.610 Hyphen L stands for as you can see here less than or equal to. 12:05.640 --> 12:06.390 Okay. 12:06.390 --> 12:10.070 So in this example we're gonna No test or check it. 12:10.070 --> 12:16.550 Whether the number is less than or equal to five at the beginning, the condition becomes true because 12:16.550 --> 12:18.320 one is less than five. 12:18.350 --> 12:25.520 So it goes to this block and print the value of variable counter to the screen. 12:25.520 --> 12:28.100 Then it reaches to this next line. 12:28.100 --> 12:37.160 And here the value of counter variable is increased by one because of this arithmetic operator okay. 12:37.190 --> 12:43.490 And after performing the addition operation the result is saved to this counter variable okay. 12:43.520 --> 12:45.950 Now the value of counter becomes two. 12:45.980 --> 12:50.150 Then the shell reaches to this loop again. 12:50.150 --> 12:53.690 So two is less than or equal to five okay. 12:53.720 --> 12:57.590 Then it goes to echo and print the value of counter variable. 12:57.590 --> 12:59.840 That is number two okay. 12:59.870 --> 13:03.500 Again the value of counter is increased by one. 13:03.500 --> 13:05.450 This time it becomes three. 13:05.480 --> 13:08.270 And again it goes to the while loop. 13:08.270 --> 13:11.240 Check it until this condition becomes false. 13:11.240 --> 13:16.430 So when this condition becomes false, when the value becomes six, okay. 13:16.460 --> 13:19.910 Six is not less than or equal to five. 13:19.940 --> 13:25.190 That's why when the value of counter becomes six, the shell exits this loop. 13:25.220 --> 13:31.370 Okay, after this, the next loop statement is until the until loop executes the commands. 13:31.370 --> 13:38.270 As long as the condition is false, then the last part of control statement is loop control. 13:38.450 --> 13:41.630 Here are two keywords break and continue. 13:41.660 --> 13:46.010 The break statement exits the loop immediately or entirely. 13:46.010 --> 13:50.240 There are some conditions where you need to exit the loop. 13:50.270 --> 13:56.150 Okay, so in such conditions you can use the keyword break to exit the loop and continue. 13:56.180 --> 14:03.620 Statement skips the remaining commands in the current loop iteration and moves to the next iteration. 14:03.710 --> 14:07.670 Okay, so this is how the loop works. 14:07.700 --> 14:13.240 You can just execute these examples and it's very easy to understand. 14:13.270 --> 14:13.930 Okay. 14:13.960 --> 14:16.210 And please practice the exercises. 14:16.360 --> 14:16.660 Okay. 14:16.690 --> 14:18.580 Practice will make you perfect. 14:18.580 --> 14:23.890 And if you already know the computer programming then it's so easy to understand. 14:25.240 --> 14:25.660 Okay. 14:25.660 --> 14:27.910 After this let's talk about the last part. 14:27.910 --> 14:29.230 And that is the function. 14:29.230 --> 14:35.770 So functions in Linux shell scripting are powerful feature that allows you to group commands together 14:35.770 --> 14:39.160 and execute them as a single unit okay. 14:39.190 --> 14:46.000 That means we can group multiple commands with the help of function and execute them again and again 14:46.000 --> 14:48.670 without writing them multiple times. 14:48.700 --> 14:51.700 Okay, so first of all, you need to define the function. 14:51.700 --> 14:53.410 So there is a syntax. 14:53.410 --> 14:56.830 To define the function you need to type the function name. 14:56.830 --> 15:02.380 Then opening closing parenthesis and then opening closing curly bracket. 15:02.380 --> 15:08.680 And in that opening closing curly bracket you need to type your commands which you want to execute. 15:08.680 --> 15:10.380 Or the other syntax is Axes. 15:10.380 --> 15:17.640 First of all, you need to type the keyword function, then function name and inside opening closing 15:17.640 --> 15:23.400 curly bracket you need to type your commands and defining the function is not enough. 15:23.400 --> 15:25.590 Here you need to call the function. 15:25.590 --> 15:28.650 So you just need to type your function name to call. 15:28.650 --> 15:34.650 The function means when you call the function, then the commands which are present in our function 15:34.650 --> 15:35.880 will be executed. 15:35.880 --> 15:39.090 So it is very important to call the function okay. 15:39.090 --> 15:42.450 So as you can see here this is an example greet. 15:42.450 --> 15:44.760 So greet is a function okay. 15:44.790 --> 15:48.960 This is the syntax which is opening and closing parenthesis. 15:48.960 --> 15:54.900 And after that within this opening and closing curly bracket the echo command is present okay. 15:54.930 --> 15:57.690 And here is the call of the function. 15:57.690 --> 16:03.960 So when we call the function greet it will print this command to the computer screen. 16:03.960 --> 16:10.440 And you can call this function many times you don't need to type this command or this code again and 16:10.440 --> 16:10.800 again. 16:10.800 --> 16:15.770 You just need to type it once and you can call it many times. 16:15.800 --> 16:18.110 Okay, so let's see it practically. 16:19.610 --> 16:19.940 Okay. 16:19.940 --> 16:24.710 So let's see here our example function dot dash. 16:24.920 --> 16:31.460 So as you can see here this is our function hello world which will print the message hello world. 16:31.490 --> 16:35.960 Then my name and the message executing my first function. 16:35.960 --> 16:38.780 And we have called this function five times. 16:38.780 --> 16:44.900 So when I execute the script the message will be printed to the computer screen five times. 16:44.930 --> 16:50.420 Okay so let's execute the script function dot bash. 16:51.320 --> 16:58.130 And as you can see the message is printed here five times because we called the function five times. 16:58.160 --> 17:01.070 Okay let's edit our script code. 17:01.160 --> 17:01.940 So. 17:05.360 --> 17:06.770 Function dot bash. 17:09.380 --> 17:13.070 Okay I want to call this function only two times. 17:15.410 --> 17:15.950 Okay. 17:18.020 --> 17:22.160 Now let's save this script and execute it again. 17:22.160 --> 17:27.080 And as you can see, the function is get executed only two times. 17:27.110 --> 17:27.440 Okay. 17:27.470 --> 17:29.330 So this is how function works. 17:29.330 --> 17:34.700 You just need to type your commands within these opening and closing curly brackets. 17:34.700 --> 17:37.730 And you can call them as long as you want. 17:37.760 --> 17:38.450 Okay. 17:38.450 --> 17:43.040 So this is how control statements and function works in Linux shell scripting. 17:44.240 --> 17:44.720 Okay. 17:44.750 --> 17:46.910 After this let's talk about the final part. 17:46.910 --> 17:48.260 And that is the project. 17:48.290 --> 17:48.680 Okay. 17:48.710 --> 17:53.270 In the project part we are going to see the real world use of scripting. 17:53.300 --> 17:53.960 Okay. 17:54.140 --> 18:00.110 In this project we are going to create a real world backup script which will backup all the important 18:00.110 --> 18:01.610 files and directories. 18:01.640 --> 18:02.000 Okay. 18:02.030 --> 18:03.110 Which you want. 18:03.140 --> 18:03.800 Okay. 18:04.100 --> 18:07.520 Uh, by reading the script, you will understand how it works. 18:07.520 --> 18:12.740 You just need to enter the source directory and the destination directory. 18:12.770 --> 18:13.550 Okay.