Command Line Arguments in Shell Script

Command Line Arguments in Shell Scripts

Command line arguments allow users to pass input directly to a script when executing it. This makes scripts dynamic and flexible, enabling them to handle varying input without modifying the code.

Accessing Command Line Arguments

In Bash, arguments passed to a script can be accessed using special variables:

Variable	Description
$0	Name of the script
$1, $2, ...	Positional arguments (first, second, etc.)
$#	Total number of arguments passed
$@	All arguments as a single string
$*	All arguments as a single string (slightly different from $@)
$?	Exit status of the last executed command
$$	Process ID (PID) of the current script

Example: Simple Argument Handling

#!/bin/bash
echo "Script Name: $0"
echo "First Argument: $1"
echo "Second Argument: $2"
echo "Total Arguments: $#"

Execution:

bash script.sh Hello World

Output:

Script Name: script.sh
First Argument: Hello
Second Argument: World
Total Arguments: 2

Using All Arguments ($@ vs $*)

Example:

#!/bin/bash
echo "Using \$@: $@"
echo "Using \$*: $*"

Execution:

bash script.sh "Hello World" Bash

Output:

Using $@: Hello World Bash
Using $*: Hello World Bash

Key Difference:

• $@: Treats each argument as a separate string.
• $*: Treats all arguments as a single string.

Looping Through Arguments

Use a for loop to process all arguments dynamically.

Example:

#!/bin/bash
echo "Arguments passed:"
for arg in "$@"; do
    echo "$arg"
done

Execution:

bash script.sh apple banana cherry

Output:

Arguments passed:
apple
banana
cherry

Checking Argument Count

You can ensure the script has the required number of arguments before proceeding.

Example:

#!/bin/bash
if [ $# -lt 2 ]; then
    echo "Usage: $0 arg1 arg2"
    exit 1
fi

echo "Arguments are: $1 and $2"

Execution:

bash script.sh Hello

Output:

Usage: script.sh arg1 arg2

Handling Flags and Options

Flags or options are common in command-line tools (e.g., -h, --help). These can be implemented in Bash using a while loop.

Example:

#!/bin/bash
while [[ "$#" -gt 0 ]]; do
    case $1 in
        -h|--help) 
            echo "Usage: $0 [options]"
            echo "-h, --help  Show this help message"
            exit 0
            ;;
        -n|--name) 
            name="$2"
            shift 
            ;;
        -a|--age) 
            age="$2"
            shift 
            ;;
        *)
            echo "Unknown option: $1"
            exit 1
            ;;
    esac
    shift
done

echo "Name: $name"
echo "Age: $age"

Execution:

bash script.sh -n John -a 30

Output:

Name: John
Age: 30

Combining Arguments with Input Validation

Example: A Calculator Script

#!/bin/bash
if [ $# -ne 3 ]; then
    echo "Usage: $0 num1 operator num2"
    exit 1
fi

num1=$1
operator=$2
num2=$3

case $operator in
    +) echo "$num1 + $num2 = $((num1 + num2))" ;;
    -) echo "$num1 - $num2 = $((num1 - num2))" ;;
    \*) echo "$num1 * $num2 = $((num1 * num2))" ;;
    /) echo "$num1 / $num2 = $((num1 / num2))" ;;
    *) echo "Invalid operator. Use +, -, *, or /." ;;
esac

Execution:

bash calculator.sh 5 + 3

Output:

5 + 3 = 8

Debugging Argument Handling

• Print All Arguments:

  echo "All arguments: $@"
  

• Exit on Error:

  set -e
  

• Enable Debugging:

  set -x
  

Conclusion

Command line arguments make Bash scripts flexible and powerful. By using positional parameters, validation, and option handling, you can create scripts that adapt to a wide range of use cases.

Let me know if you'd like additional advanced examples!