Positional Parameters and Argument Handling

#!/usr/bin/env bash
# Called as: ./script.sh one two three four five

echo "$0 = $0"      # ./script.sh  (script name)
echo "$1 = $1"      # one
echo "$2 = $2"      # two
echo "$# = $#"      # 5
echo "$@ = $@"      # one two three four five
echo "$* = $*"      # one two three four five

# Access argument 10+: use braces
# echo "${10}"       # would give the 10th argument

# All args in a loop (preserves spaces in individual args)
for arg in "$@"; do
    echo "  arg: $arg"
done

#!/usr/bin/env bash

function usage {
    cat >&2 <<-EOF
	Usage: $0 [OPTIONS] <source> <destination>

	Options:
	  -v    Verbose
	  -h    Show this help

	Arguments:
	  source       Path to input file
	  destination  Path to output directory
	EOF
    exit 1
}

# Require minimum arguments
if [[ $# -lt 2 ]]; then
    echo "ERROR: Too few arguments" >&2
    usage
fi

# Require exactly N arguments
if [[ $# -ne 2 ]]; then
    echo "ERROR: Expected exactly 2 arguments, got $#" >&2
    usage
fi

#!/usr/bin/env bash

# Set defaults if not provided
host="${1:-localhost}"
port="${2:-8080}"
timeout="${3:-30}"

echo "Connecting to $host:$port (timeout: ${timeout}s)"

# Require argument or exit with message
config="${1:?Usage: $0 <config-file>}"

# Multiple defaults in one go
input_file="${1:-/dev/stdin}"
output_dir="${2:-./output}"
log_level="${3:-info}"

#!/usr/bin/env bash
# Script that accepts any number of files

if [[ $# -eq 0 ]]; then
    echo "Usage: $0 <file> [file...]" >&2
    exit 1
fi

processed=0
failed=0

for file in "$@"; do
    if [[ ! -f "$file" ]]; then
        echo "Skipping (not a file): $file" >&2
        (( failed++ ))
        continue
    fi
    echo "Processing: $file"
    # ... do work ...
    (( processed++ ))
done

echo ""
echo "Done: $processed processed, $failed skipped"

# set -- replaces positional parameters
set -- "new_first" "new_second" "new_third"
echo "$1"    # new_first
echo "$2"    # new_second

# Useful for normalising arguments
# E.g., split "host:port" into $1 and $2
pair="localhost:5432"
IFS=: read -r host port <<< "$pair"
set -- "$host" "$port"
echo "$1"    # localhost
echo "$2"    # 5432