hello-teacher

#! /usr/bin/env bash
#
# This is a heavily commented template for a clean and modern bash program.
# It also serves as a demo of essential bash features and good practices.
#
# Replace this notice with your own description. Please kindly indicates why
# you had to create this code and impose the burden of its maintenance
# to your successors.
#
# Requires bash >= 4.3. Upgrade if needed.
#

# This enables 3 options that will make the program fail when something's
# wrong. Their use is recommended in all shell scripts.
#
# - 'set -e' makes the program fail when a command fails (otherwise the
#   execution would continue like normally in interactive mode).
#
# - 'set -u' makes the program fail during execution when there's an attempt
#   to dereference an unset variable (otherwise, it expands to nothing).
#
# - '-o pipefail' makes a pipeline 'a | b | c' fail if any of a, b, or c fails
#   (otherwise its exit status is the exit status of c).
#
set -eu -o pipefail

# Here we demonstrate several things:
#
# - heredoc strings, which are multiline text with variable expansions.
# - function scoping. The default_greeting variable needs only be defined
#   at the time the function is called.
# - capture of the output of external commands. Here, 'basename' extracts the
#   last component of the path of this program, identified by $0.
#
usage() {
  cat <<EOF
Usage: $(basename "$0") [OPTIONS]
Print a greeting message.

Options:
  --greeting MSG
      Use another greeting message than '$default_greeting'.
  --help
      Print this error message and exit.
EOF
}

# The double-quotes are not necessary here because the string doesn't contain
# a space. It's ok though, and makes modifications easier in the hands
# of non-experts.
#
default_greeting="Hello"

# Consume the command line arguments. Each call to shift eliminates argument $1
# and the remaining arguments are shifted to the left. $0 remains. The number
# of arguments is counter starting with $1 and is identified by $#. It is
# decremented with each call to shift.
#
greeting="$default_greeting"
while [[ $# -gt 0 ]]; do

  # In doubt, always use double-quotes around variable
  # expansions: "$1" "$whatever". It is necessary in many contexts, and most
  # users don't know when. Without double-quotes, $whatever means
  # 'split the string at delimiters which by default are whitespace'.
  #
  case "$1" in
    --greeting)
      greeting="$2"
      shift
      ;;
    --help)
      usage
      exit 0
      ;;
    *)
      # Take care to write the error message to stderr (the '2' in '>&2')
      # and exit with a non-zero status.
      #
      echo "Unsupported argument '$1'. Try --help." >&2
      exit 1
  esac
  shift
done

set_username() {
  username="stranger"
  # Test if a variable is set. The recommended way to make this work
  # with bash versions before 4.3 is [[ -n ${USER+x} ]].
  #
  if [[ -v USER ]]; then
    username="$USER"
  fi
}

set_username
echo "$greeting, $username."