Music file manipulation

transpose.bash

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

##
# Transpose a song from one key to another.
##

version='0.0.1'

# Formatting functions
bold() {
	format_bold='\033[1m'
	format_reset='\033[0m'
	echo -e "${format_bold}$1${format_reset}"
}

error() {
	format_red='\033[0;31m'
	format_reset='\033[0m'
	echo -e "${format_red}$1${format_reset}" >&2
}

panic() {
	error "$1"
	exit 1
}

# Floating point calculations
calculate() {
	# `bc` cannot do fractional exponents but `awk` can
	awk "BEGIN { print $1 }"
}

show_usage() {
	cat <<-EOF
		$(bold transpose) - Transpose a song from one key to another

		$(bold USAGE)

		  $ transpose --help
		  $ transpose --version
		  $ transpose input_file output_file semitones

		  For example, to transpose a song from C to E:

		  $ transpose input_file output_file 4

		  or, to transpose a song from E to C:

		  $ transpose input_file output_file -4

		$(bold OPTIONS)

		  -h, --help
		    Show this help message and exit

		  -v, --version
		    Show the version of this script and exit
	EOF
}

transpose() {
	# Check if usage/version was requested
	if [[ $# -ne 3 ]]; then
		show_usage
		exit 0
	fi

	for arg in "$@"; do
		if [[ $arg == "--help" || $arg == "-h" ]]; then
			show_usage
			exit 0
		elif [[ $arg == "--version" || $arg == "-v" ]]; then
			echo "$version"
			exit 0
		fi
	done

	local input_file="$1"
	local output_file="$2"
	local semitones="$3"

	# Validate arguments
	if [[ ! -r $input_file ]]; then
		panic "Could not read input file"
	fi

	if [[ $semitones -lt -12 || $semitones -gt 12 ]]; then
		panic "Semitones must be between -12 and 12"
	fi

	# Find the difference between the start and end keys, picking the closest one
	local pitch_difference
	pitch_difference=$(calculate "2 ^ ($semitones / 12)")

	# Show progress if pv is installed
	if [[ $(hash pv 2>/dev/null) ]]; then
		pv -i 0.15 "$input_file" | ffmpeg -i - -y -af "rubberband=pitch=$pitch_difference" -v quiet "$output_file"
	else
		ffmpeg -i "$input_file" -y -af "rubberband=pitch=$pitch_difference" -v quiet -stats "$output_file"
	fi
}

transpose "$@"