print-marker-sections

#!/bin/bash
#
# Prints the contents of marker sections from a file.
#
# If the target file doesn’t contain marker sections, the script doesn’t output
# anything.
# If the target file contains unmatched/orphaned markers, this script fails.

# We don’t use `set -x`, because it would output every single iteration of the
# while loop when iterating through the lines of the target file, and hence
# generate a lot of noise.

# Exit on first failure.
set -e

# Exit on unset variable.
set -u

SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
readonly SCRIPT_DIR
# shellcheck source=lib/markers.sh
. "${SCRIPT_DIR}/lib/markers.sh"

print_help() {
  cat << EOF
Usage: ${0##*/} [--help] TARGET_FILE
Prints the contents of marker sections from a file.
  TARGET_FILE   Path to file with marker sections.
  --help        Display this help and exit.
EOF
}

# Parse command-line arguments.
TARGET_FILE=''
while (( "$#" > 0 )); do
  case "$1" in
    --help)
      print_help
      exit
      ;;
    -*)
      >&2 echo "Unknown flag: $1"
      >&2 echo "Use the '--help' flag for more information"
      exit 1
      ;;
    *)
      TARGET_FILE="$1"
      shift
      ;;
  esac
done
readonly TARGET_FILE

# Ensure target file is specified.
if [[ -z "${TARGET_FILE}" ]]; then
  >&2 echo 'Input parameter missing: TARGET_FILE'
  >&2 echo "Use the '--help' flag for more information"
  exit 1
fi

# Ensure target file exists and is a file.
if [[ ! -f "${TARGET_FILE}" ]]; then
  >&2 echo "Not a file: ${TARGET_FILE}"
  >&2 echo "Use the '--help' flag for more information"
  exit 1
fi

# Read the original file line by line, and preserve all lines that reside
# between the start and end markers (i.e., the section contents).
is_in_marker_section='false'
section_contents=()
while IFS='' read -r line; do
  if [[ "${line}" == "${MARKER_END}" ]]; then
    if ! "${is_in_marker_section}"; then
      >&2 echo 'Unmatched end marker'
      exit 1
    fi
    is_in_marker_section='false'
    continue
  fi
  if [[ "${line}" == "${MARKER_START}" ]]; then
    is_in_marker_section='true'
    continue
  fi
  if "${is_in_marker_section}"; then
    section_contents+=("${line}")
  fi
done < "${TARGET_FILE}"

if "${is_in_marker_section}"; then
  >&2 echo 'Unmatched start marker'
  exit 1
fi

# Print all lines of the section contents.
printf "%s\n" "${section_contents[@]}"