Arguments

Chapter 17: Command Line Arguments

Overview

Command line arguments allow scripts to accept input from users at runtime. This chapter covers positional parameters, flags, options, and best practices for parsing command-line arguments in bash scripts.

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Positional Parameters

Basic Access

args_basic.sh

#!/usr/bin/env bash
# Access arguments
echo "Script: $0"
echo "First: $1"
echo "Second: $2"
echo "Third: $3"
echo "All: $@"
echo "Count: $#"

Accessing All Arguments

args_all.sh

#!/usr/bin/env bash
# $@ - all arguments as separate
for arg in "$@"; do
    echo "Arg: $arg"
done

# $* - all arguments as single string
echo "All: $*"

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Argument Parsing

While Loop Parsing

parse_args.sh

#!/usr/bin/env bash
VERBOSE=false
FORCE=false
OUTPUT=""

while [ $# -gt 0 ]; do
    case "$1" in
        -v|--verbose)
            VERBOSE=true
            shift
            ;;
        -f|--force)
            FORCE=true
            shift
            ;;
        -o|--output)
            OUTPUT="$2"
            shift 2
            ;;
        -h|--help)
            echo "Usage: $0 [OPTIONS]"
            exit 0
            ;;
        *)
            echo "Unknown: $1"
            shift
            ;;
    esac
done

echo "Verbose: $VERBOSE"
echo "Force: $FORCE"
echo "Output: $OUTPUT"

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Practical Examples

Example 1: Deployment Script

deploy.sh

#!/usr/bin/env bash
set -euo pipefail

ENVIRONMENT=""
VERSION="latest"
DRY_RUN=false

show_help() {
    cat << EOF
Usage: $0 [OPTIONS] ENVIRONMENT VERSION

Arguments:
  ENVIRONMENT    Target environment (staging, production)
  VERSION        Application version

Options:
  -h, --help     Show this help
  -d, --dry-run  Dry run mode
EOF
}

while [[ $# -gt 0 ]]; do
    case $1 in
        -h|--help)
            show_help
            exit 0
            ;;
        -d|--dry-run)
            DRY_RUN=true
            shift
            ;;
        -*)
            echo "Unknown option: $1"
            exit 1
            ;;
        *)
            if [ -z "$ENVIRONMENT" ]; then
                ENVIRONMENT="$1"
            elif [ "$VERSION" = "latest" ]; then
                VERSION="$1"
            fi
            shift
            ;;
    esac
done

if [ -z "$ENVIRONMENT" ] || [ "$VERSION" = "latest" ]; then
    show_help
    exit 1
fi

echo "Deploying $VERSION to $ENVIRONMENT (dry-run: $DRY_RUN)"

Example 2: Getopts

getopts_example.sh

#!/usr/bin/env bash
# Using getopts for option parsing
while getopts "vhf:o:" opt; do
    case $opt in
        v) VERBOSE=true ;;
        h) echo "Help"; exit 0 ;;
        f) FILE="$OPTARG" ;;
        o) OUTPUT="$OPTARG" ;;
        \?) exit 1 ;;
    esac
done
shift $((OPTIND -1))

echo "Verbose: ${VERBOSE:-false}"
echo "File: ${FILE:-none}"
echo "Output: ${OUTPUT:-none}"
echo "Remaining: $@"

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Summary

In this chapter, you learned about:

• ✅ Positional parameters
• ✅ $@ vs $*
• ✅ Argument parsing with while loop
• ✅ getopts for option parsing
• ✅ Help message creation
• ✅ Practical examples

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Exercises

Level 1: Basics

1. Create script that prints all arguments
2. Parse simple flags

Level 2: Intermediate

3. Create deployment script with argument parsing
4. Implement help message

Level 3: Advanced

5. Build complete CLI with getopts
6. Add argument validation

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Next Steps

Continue to the next chapter to learn about file operations.

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Previous Chapter: Scope and Variable Visibility Next Chapter: File Operations