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

• Shell script can accept arguments from the command line.
• An argument is 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.

chmod space plus x space arg.sh

and press Enter

./arg.sh

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.

Press the uparrow key and press Enter

But the first, second and third arguments are blank.

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

sunday monday and tuesday.

Press Enter

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.

Switch to the terminal.

Now press Enter

And type the line as shown here.

Let us execute.

Press the uparrow key and press Enter.

Press Enter and type the lines as shown here.

for arg in “$*”

do

echo “$arg”

done

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

Switch to the terminal.

Let me clear the prompt.

Press Enter

We see that all the arguments on a single line.

for arg in “$@”

do

echo “$arg”

done

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

Switch to the terminal.

Press Enter

You can see the difference now.

$@ prints each argument on separate 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

• Double quote substitutes the value of variables and commands
• Example: echo “Username is $USER”
• It displays the username on the system.

Let me clear the prompt.

echo space within double quotes Username space is dollarUSER in capitals.

Press Enter

Highlight

“Username is lavitha”

The output will vary according to your system.

<<PAUSE>>

Now move back to slides.

Single Quote

• Single quotes preserves the literal meaning of each character of the given string.
• It is used to turn off special meaning of all characters.

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

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

• Backslash removes the special meaning from a single character
• It is used as an escape character in BASH

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

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 our slides.

Let us summarize.

Summary

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

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Acknowledgement

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And this is Ashwini Patil from IIT Bombay signing off.

Thank you for joining.