Basic implementation of CLI subcommands/options parsing in Bash

cli.sh

#!/bin/bash

# Private implementation of bar subcommand
_bar() {
  echo "foo_value is $foo_value"
  echo "flag_value is $flag_value"
  echo "1: $1"
  echo "2: $2"
  echo "3: $3"
}

# CLI Subcommands Implementation
subcommands=()

# foo subcommand, simple
subcommands+=("foo")
foo() {
  echo "foo"
  echo "$@"
}

# bar subcommand, with arg parsing and private implementation invocation
subcommands+=("bar")
bar() {

  help_bar() {
    echo "bar subcommand"
    echo "--------------"
    echo
    echo "Example:"
    echo "  $progname bar --foo value --flag <file1> <file2>"
    echo
    echo "Available options:"
    echo "  -f, --foo <value>   Sets a foo value. Required."
    echo "  --flag              Enables the flag."
    echo "  <file1>             Path to file to process. Required."
  }

  bar_args=()
  flag_value=0

  # parse named args
  while [[ $# -gt 0 ]]; do
    case "$1" in
      --help)
        help_bar && exit 0
      ;;
      -f|--foo)
        if [ "$#" -gt 1 ]; then
          shift
          foo_value="$1"
        else
          echo "ERROR: $1 option requires a value." >&2
          help_bar && exit 1
        fi
      ;;
      --flag)
        flag_value=1
      ;;
      *)
        bar_args+=("$1")
      ;;
    esac
    shift
  done

  if [[ ! "$foo_value" ]]; then
    echo "ERROR: --foo option is required." >&2
    help_bar && exit 1
  fi

  if [[ ! "$bar_args[0]" ]]; then
    echo "ERROR: <file1> is required." >&2
    help_bar && exit 1
  fi

  foo_value="$foo_value" flag_value="$flag_value" _bar "${bar_args[@]}"

}

# CLI Main Implementation

progname=$(basename $0)

is_function() { declare -Ff "$1" >/dev/null; }

help_commands() {
  echo "Available Commands:"
  for command in "${subcommands[@]}"; do
    echo "  $command"
  done
  echo ""
  echo "Example:"
  echo "  $progname $subcommands --help"
}

if [ "$#" -eq 0 ]; then
  echo "ERROR: No command specified." >&2
  help_commands && exit 1
fi

case "$1" in
  --help)
    help_commands && exit 1
  ;;
  *)
    subcommand="$1" && shift
    if ! is_function "${subcommand}"; then
      echo "ERROR: Unknown command '${subcommand}'" >&2
      help_commands && exit 1
    fi
    "${subcommand}" "$@"
    exit $?
  ;;
esac