LWM-Linux/07 - Shell Scripting Basics/User Iput and Arguments in Bash.md

User Input and Arguments in Bash

1. Command-line Arguments

Bash scripts can accept arguments when executed from the command line. These arguments are accessible within the script using special variables.

Example:

#!/bin/bash

echo "The script name is: $0"
echo "The first argument is: $1"
echo "The second argument is: $2"
echo "All arguments: $@"

Usage:

./script.sh arg1 arg2 arg3

2. The read Command

The read command allows interactive user input during script execution.

Basic syntax:

read variable_name

Example:

#!/bin/bash

echo "What's your name?"
read name
echo "Hello, $name!"

# Reading multiple variables
echo "Enter your first and last name:"
read first_name last_name
echo "Your full name is $first_name $last_name"

# Using a prompt
read -p "Enter your age: " age
echo "You are $age years old"

# Reading with a timeout
read -t 5 -p "Quick! Enter a number: " number
echo "You entered: $number"

3. Special Variables for Arguments

Bash provides special variables for working with command-line arguments:

• $0: Script name
• $1, $2, $3, etc.: Individual arguments
• $#: Number of arguments
• $@: All arguments as separate words
• $*: All arguments as a single word

Example:

#!/bin/bash

echo "Number of arguments: $#"
echo "All arguments: $@"

# Loop through all arguments
for arg in "$@"
do
    echo "Argument: $arg"
done

4. Handling Options and Flags

You can create scripts that accept options (flags) to modify behavior.

Example:

#!/bin/bash

verbose=false

while [[ $# -gt 0 ]]
do
    key="$1"
    case $key in
        -v|--verbose)
        verbose=true
        shift
        ;;
        -n|--name)
        name="$2"
        shift
        shift
        ;;
        *)
        echo "Unknown option: $1"
        exit 1
        ;;
    esac
done

if [ "$verbose" = true ] ; then
    echo "Verbose mode on"
fi

if [ ! -z "$name" ] ; then
    echo "Hello, $name!"
fi

Usage:

./script.sh -v --name John

5. Validating Input

It's important to validate user input to ensure your script behaves correctly.

Example:

#!/bin/bash

read -p "Enter a number between 1 and 10: " number

if [[ ! $number =~ ^[0-9]+$ ]] ; then
    echo "Error: Not a number"
    exit 1
fi

if (( number < 1 || number > 10 )) ; then
    echo "Error: Number out of range"
    exit 1
fi

echo "You entered a valid number: $number"

6. Using getopts for Advanced Option Parsing

For more complex option parsing, Bash provides the getopts built-in command.

Example:

#!/bin/bash

usage() {
    echo "Usage: $0 [-h] [-v] [-n name]"
    echo "  -h: Display this help message"
    echo "  -v: Enable verbose mode"
    echo "  -n name: Specify a name"
}

verbose=false
name=""

while getopts ":hvn:" opt; do
    case ${opt} in
        h )
            usage
            exit 0
            ;;
        v )
            verbose=true
            ;;
        n )
            name=$OPTARG
            ;;
        \? )
            echo "Invalid option: $OPTARG" 1>&2
            usage
            exit 1
            ;;
        : )
            echo "Invalid option: $OPTARG requires an argument" 1>&2
            usage
            exit 1
            ;;
    esac
done

if [ "$verbose" = true ] ; then
    echo "Verbose mode enabled"
fi

if [ ! -z "$name" ] ; then
    echo "Hello, $name!"
fi

Usage:

./script.sh -v -n Alice