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Welcome to the spoken tutorial on Command line arguments and Quoting in BASH

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In this tutorial, we will learn about

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* Command line Arguments and

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* Quoting

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To follow this tutorial, you should be familiar with the Linux Operating System.

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If not, for relevant tutorials please visit our website which as shown

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For this tutorial I am using

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* Ubuntu Linux 12.04 OS

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* GNU Bash version 4.1.10

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GNU Bash version 4 or above is recommended for practice.

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* Shell script can accept arguments from the command line.

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* An argument is passed to a program being called.

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* Any number of arguments can be passed to a program.

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Let us open the terminal by pressing Ctrl Alt andT keys simultaneously on your keyboard.

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I have already written the code in the file named arg.sh

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On the terminal, let me open this file by typing,

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gedit space arg.sh space ampersand sign

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We use the ampersand to free up the prompt.

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Now, Press Enter.

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The text editor is opened.

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Let me explain the code now.

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This is the shenbang line.

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This line will print the Zeroth argument.

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Here, $0 (Dollar zero) will print the name of the shell script.

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This in turn means that, the zeroth  argument is the name of the program itself.

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Let us execute the program and see.

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Switch to the terminal.

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First make the file executable by typing,

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chmod space plus x space arg.sh

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Press Enter

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Now type dot slash arg.sh

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Press Enter

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The output is displayed as: Zeroth argument is arg.sh

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Now come back to our editor and type the three lines as shown here.

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$1 (Dollar one) represents the first argument passed to the program from the command line.

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$2 (Dollar two) represents the second argument passed to the program.

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And $3 (Dollar three) represents the third argument

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Now click on Save

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Let us execute the program and see.

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Press the uparrow key  press Enter

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We see that the zeroeth argument is printed.

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But the first, second and third arguments are blank.

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This is because the command line arguments are given during execution.

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Hence press the uparrow key and type: sunday monday and tuesday.

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Press Enter

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You can see that the first second and third arguments are Sunday Monday and Tuesday

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Now switch back to our editor. Press Enter

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Now type the line as shown here.

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$12 (Dollar twelve) represents the twelveth argument.

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To write an argument greater than 9, we need to use curly brackets.

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Else bash will only take the argument of the integer in the ten's place.

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And you will not get expected output.

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Now click on Save.

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Let us execute the program.

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Switch to the terminal.

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Let me clear the prompt.

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Now we need to give 12 or 13 arguments to the program.

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Hence Type dot slash arg.sh space 1 to 13 Now press enter

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You can see that the 12th argument is 12.

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Come back to our editor.

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And type the line as shown here.

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$# (Dollar hash) gives the total number of arguments that have been passed to a program.

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Now click on Save.

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Let us execute.

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Switch to the terminal.

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Let us execute. Press the uparrow key and press Enter.

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We can see that the total arguments are 13.

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Now switch to the editor.

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Press Enter and type the lines as shown here.

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$* (Dollar asterix) will print all the arguments on a single line.

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We will test this with the help of a simple for loop.

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We will analyse this for loop at the time of execution.

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Now click on Save.Switch to the terminal.

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Let me clear the prompt.

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Now let us type, dot slash arg.sh space sunday monday and tuesday

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Press Enter

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You can see that the total number of arguments are 3 as we have passed 3 arguments to our program.

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As already said $* will print all the arguments on a single line.

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And this is the output for the for loop.

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We see that all the arguments are printed on the single line.

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Now move back to our program and type the lines as shown here.

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$@ (Dollar at) will also print all the arguments.

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However, this time each argument will be printed on  separate line.

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This is another for loop, which will print each argument in a separate line.

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Let us see how. Click on Save

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Switch to the terminal.

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Press the uparrow key.

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Press Enter You can see the difference now.

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These are the arguments printed by $@.

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$@ prints each argument on separate line.

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This is the output for the 2nd for loop.

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Now lets move on to quoting in BASH

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Switch to the slides.

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There are three types of quotes

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Double quote

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Single quote

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Backslash

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* Double quote substitutes the value of variables and commands

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* Example echo “Username is $USER”

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* It displays your username  of the system.

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Switch to the Terminal.

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Let me clear the prompt.

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Now type  echo space within double quotes Username space  is dollar USER  in capitals.

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Press Enter

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The username of the system is printed.

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The output will vary according to your system.

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Now move back to slides.

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* Single quotes preserves the literal meaning of each character of the given string.

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* It is used to turn off special meaning of all characters.

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Switch to the Terminal.

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Type echo space within single quote Username  is dollar USER in capital

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Press Enter

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The output is Username is $USER

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In this example, it prints all the characters which appear within the single quotes.

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It does not substitute the value of variable $USER

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Switch back to our slides.

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* Backslash removes the special meaning from a single character

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* It is used as an escape character in BASH

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Switch to the Terminal.

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Now Type echo space within double quote Username is backslash dollar USER  (in capital)

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Since we have given double quotes, we expect the echo command to display the username.

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Let's try this command so press Enter.

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The output is Username is $USER

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In this example the backslash removes the special meaning of (Dollar) $ symbol.

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$USER is just treated as a string without any special functionality.

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This brings us to the end of this tutorial.

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Switch back to our slides.

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Let us summarize.

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In this tutorial we learnt,

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* Command line arguments

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* Functionality of  Double quote, Single quote and Backslash

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Watch the video available at the link shown below

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And this is Ashwini Patil from IIT Bombay signing off. Thank you for joining.