Difference between revisions of "BASH/C2/Command-Line-arguments-and-Quoting/English"

• Command line Arguments and
• Quoting

Prerequisites

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System Requirements

• Ubuntu Linux 12.04 OS and
• GNU Bash version 4.1.10

GNU Bash version 4 or above is recommended for practise.

Command Line arguments

• An arguments are passed to the program being called.
• Any number of arguments can be passed to a program.

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

gedit space arg.sh space ampersand sign

We use the ampersand to free up the prompt.

Now,

Press Enter.

Let me explain the code now.

echo "zeroth arg: $0"

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

This in turn means that, the zero^th argument is the name of the program itself.

Switch to the terminal.

First make it executable by typing,

chmod space plus x space arg.sh

and press Enter

Now type,

dot slash arg.sh

Zeroth argument is arg.sh

echo "first arg: $1"

echo "second arg: $2"

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

Let us execute the program and see.

Press the uparrow key and press Enter

But the first, second and third argument are blank.

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

sunday monday and tuesday.

Press Enter

You can see that the first second and third arguments are:

sunday monday and tuesday

Now type the line as shown here.

echo "twelveth arg: ${12}"

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

And you will not get expected output.

Let us execute the program.

Switch to the temrinal.

Let me clear the prompt.

Now press Enter

Come back to our editor.

And type the line as shown here.

Let us execute.

Switch to the temrinal.

Press the uparrow key and press Enter.

We can see that the total arguments are 13.

Now switch to the editor.

Press Enter and type the lines as shown here.

for arg in “$*”

do

echo “$arg”

done

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

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

Switch to the temrinal.

Let me clear the prompt.

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

Press Enter

As we have passed 3 arguments to our program.

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

We see that all the arguments on a single line.

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

for arg in “$@”

do

echo “$arg”

done

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

Switch to the temrinal.

Press the uparrow key.

Press Enter

You can see the difference now.

$@ prints each argument on seperate line.

This is the output for the 2nd for loop.

Switch to the slides.

Quoting

1. Double quote
2. Single quote
3. Backslash

Double Quote

• Example: echo “Username is $USER”
• It displays your username on the system.

Type echo space within double quotes Username space is space dollar sign in capitals USER

and Press Enter

Highlight

“Username is lavitha”

The output wil vary according to your system.

<<PAUSE>>

Now move back to slides.

Single Quote

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

Type echo space within single quote Username space is space dollar sign in capitals USER

and Press Enter

Highlight:

Username is $USER .

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

It does not substitute the value of variable $USER

<<PAUSE>>

Switch back to our slides.

Backslash

• It is used as an escape character in BASH

Type echo space within double quote Username space is space backslash dollar sign in capitals USER

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

Let's try this command; so press Enter.

Highlight

Username is $USER

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

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

<<PAUSE>>

Let us switch back to slides.

Let us summarize.

Summary

• Command line arguments
• Functionality of Double quote, Single quote and Backslash

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Acknowledgement

It is supported by the National Mission on Education through ICT, MHRD, Government of India

More information on this Mission is available at: http://spoken-tutorial.org\NMEICT-Intro

And this is Ashwini Patil from IIT Bombay signing off.

Thank you for joining.