[Day 7] 3. Back to Shell Scripting

Positional Parameters

Positional parameters are arguments passed to a script from the command line when you execute it. They allow you to pass data directly to the script.

Example Command:

./script27.sh 10 A sunbeam 3.142

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  $1

  ,
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  $2

  ,
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  $3

  , etc.: These refer to individual positional parameters.
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    $1

    =
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    10

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    $2

    =
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    A

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    $3

    =
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    sunbeam

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    $4

    =
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    3.142

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  $0

  : This is the name of the script itself (
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  /script27.sh

  ).
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  $*

  : All positional parameters as a single string.
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  $@

  : All positional parameters as individual strings.
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  $$

  : The process ID (PID) of the current shell.
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  $#

  : The number of positional parameters.

Functions:

Functions allow you to organize and reuse code blocks inside your script. You can define a function using the

function

()

Example Syntax:

function my_function() {     echo "This is a function." }

• Calling the function: You simply call it by name.
• Arguments: Inside functions, arguments are accessed just like positional parameters, using
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  $1

  ,
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  $2

  , etc.

Example:

my_function() {     echo "First argument: $1"     echo "Second argument: $2" } my_function "Hello" "World"

This would output:

First argument: Hello Second argument: World

Arrays

In shell scripting, arrays allow you to store multiple values in a single variable. Arrays are indexed starting from 0.

Declaration:

declare -a arr arr=(5 3 6 8 2 1)

• Accessing array elements: Use

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  ${arr[index]}

  .
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  echo ${arr[0]}   # prints first element (5)

• Printing all elements:

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    ${arr[*]}

    : Prints all elements as a single string.
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    ${arr[@]}

    : Prints each element individually.

• Number of elements: Use

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  ${#arr[@]}

  to get the array length.

Example:

echo "Elements from index 2 onwards: ${arr[*]:2}" # prints from the 3rd element onwards (6 8 2 1)

Strings

In shell scripting, strings are simple variables that hold text. You can compare and manipulate strings using operators.

Common Operators

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  =

  : True if both strings are equal.
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  !=

  : True if both strings are not equal.
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  -n

  : True if the string is not empty.
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  -z

  : True if the string is empty.

String Concatenation

You can concatenate two strings as follows:

str1="Ojas" str2="Dudhabaware" str3=$str1$str2 echo "Concatenated String: $str3"   # Output: OjasDudhabaware

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test

Command

The

test

0

Example of Checking Exit Status:

test -f myfile.txt echo $?    # Outputs 0 if the file exists, 1 otherwise

Using

test

if

if test -f "myfile.txt"; then     echo "File exists" else     echo "File does not exist" fi

By using the

echo $?

0