Skeleton of a shell script.

example.sh

#! /bin/sh

readonly -p VERSION=0.1.0

##############################################################################
# This is an example shell script. I do my best to avoid bashisms when it is
# not too much of a pain to do so. I also try to stick to a 80 char line limit
# and indent with tabs since tabs are easier to work with in `sh` than spaces.

# If your script contains calculations or complex logic, please consider
# writting it in a modern scripting language such as python.

# Inspired by https://dev.to/thiht/shell-scripts-matter and
# http://redsymbol.net/articles/unofficial-bash-strict-mode/#issues-and-solutions

# Make sure to lint with `shfmt -w example.sh && shellcheck example.sh`
# You learn more about shellcheck rules on the wiki.
# https://github.com/koalaman/shellcheck/wiki

# Lint with `$ shfmt -w example.sh && shellcheck example.sh`
# Test scripts with shunit2 and put in version control.
##############################################################################

# Debug with `$ sh -x example.sh`

set -euo pipefail # stop if any failures

# Do not accept spaces as Internal Field Separator.
# Equivalent to IFS=$'\t\n' in Bash.
# Also, use `$@` instead of `$*`

safe_IFS="$(printf '%b_' '\t\n')"
IFS="${safe_IFS%_}" # protect trailing \n

##############################################################################

# Helper functions.

info() {
	if [ "$VERBOSE" -gt 0 ]; then
		echo "[INFO] " "$@" | tee -a "$LOG_FILE" >&2
	else
		echo "[INFO] " "$@" >>"$LOG_FILE"
	fi
}

error() {
	echo "[ERROR] " "$@" | tee -a "$LOG_FILE" >&2
	exit 1
}

heredoc() {
	cat <<-EOM
	Usage:
	    This is an example template of a shell script. It reads
	    parameters from the command line and prints them when run verbosely.
	Example:
	    ./example.sh -p $PWD/file.txt
	
	EOM
}

##############################################################################

# Define variables here.
# Make sure to source secrets from a `.env` instead of passing secrets though
# you may need allow unbound variables when sourcing other people's scripts.
# `$ . example_secrets.env # OK!`
# `$ set +u; . functions.sh; set -u # allow badness in sourc'ed scripts.`

tmp_dir=$(mktemp -d)
export tmp_dir

# Make sure to release any resources we acquired.
tear_down() {
	# sudo systemctl restart important.service
	rm -rf "$tmp_dir"
}

##############################################################################

# Parse command line arguments.
# Switch statement (slightly) adapted from
# https://stackoverflow.com/questions/192249/how-do-i-parse-command-line-arguments-in-bash/14203146#14203146

VERBOSE=0
LOG_FILE="/tmp/$(basename "$0").log"
while [ $# -gt 0 ]; do
	key="$1"
	case $key in
	-V | --version)
		echo Version: "$VERSION"
		shift
		exit 0
		;;
	-h | --help)
		heredoc
		exit 0
		shift
		;;
	-p | --positional)
		POSITIONAL="$2"
		shift # past argument
		shift # past value
		;;
	-o | --optional)
		OPTIONAL="$2"
		shift # past argument
		shift # past value
		;;
	--flag)
		FLAG='YES'
		shift # past argument
		;;

	--nolog)
		LOG_FILE=/dev/null
		shift
		;;
	-v | --verbose)
		VERBOSE=1
		shift
		;;
	*)
		error "$1 is not a valid option."
		shift
		;;
	esac
done

readonly -p POSITIONAL
readonly -p OPTIONAL="${OPTIONAL:-default value}" # set default
readonly -p FLAG="${FLAG:-NO}"
readonly -p LOG_FILE
readonly -p VERBOSE

# Write your functions here.
main() {
	info The positional value is = "${POSITIONAL}"
	info The optional argument is = "${OPTIONAL}"
	info The flag is = "${FLAG}"
}

##############################################################################

date >>"$LOG_FILE"
date +"%Y-%m-%dT%H:%M:%S%z" >>"$LOG_FILE"
main
trap tear_down EXIT # Always call tear_down