Simple bash and sed templating using delimiters.

template.sh

#!/usr/bin/env bash

# Simple bash and sed templating using delimiters.
#
# For example, given a template /template-a with the contents
#
#     # This is template A
#     #
#     ### EXAMPLE_TEMPLATE_BEGIN
#     echo "Hello from template A"
#     ### EXAMPLE_TEMPLATE_END
#     #
#     # Anything before or after the delimiters is ignored
#
# And a file /file-name with the contents
#
#     echo "Some text already on /file-name"
#
# The following command
#
#     template append EXAMPLE /template-a </file-name
#
# will generate the following output
#
#     echo "Some text already on /file-name"
#     ### EXAMPLE_TEMPLATE_BEGIN
#     echo "Hello from template A"
#     ### EXAMPLE_TEMPLATE_END
#
# calling the command `template append EXAMPLE /template-a </file-name` again will
# not append the template again.
# 
# To remove the template call:
#
#     template strip EXAMPLE </file-name
#
# To prepend the template call:
#
#     template prepend EXAMPLE /template-a </file-name
#
# To replace the template call:
#
#     template replace EXAMPLE /template-a </file-name
#
# To inject a template at a '### TEMPLATE_EXAMPLE_PLACEHOLDER' call:
#
#     template inject EXAMPLE /template-a </file-name
#
# Note: anything outside _BEGIN / _END tags will be ignored.
# Limitations:
# - Only one template with the same name per file.
# - No support for evaluating variables. Use envsubst for that purpose.
#
# Author: Bermi Ferrer
# License: MIT
# Code and bugs: https://gist.github.com/bermi/a3f2327e0abd3c722a7194287fe5204d
#

set -eEuo pipefail

shopt -s inherit_errexit
IFS=$'\n\t'

get_template() {
  set -eEuo pipefail;
  local template_name="${1:?You must provide a template name}"
  template_name="$(echo -n "$template_name" | tr '[:lower:]' '[:upper:]' | tr '-' '_')"
  local template
  template="$(cat)"

  if [ "$(echo "$template" | grep -c "### ${template_name}_TEMPLATE_BEGIN")" != "1" ]; then
    echo "Error: template must contain exactly one '### ${template_name}_TEMPLATE_BEGIN' delimiter" >&2
    exit 1
  fi
  if [ "$(echo "$template" | grep -c "### ${template_name}_TEMPLATE_END")" != "1" ]; then
    echo "Error: template must contain exactly one '### ${template_name}_TEMPLATE_END' delimiter" >&2
    exit 1
  fi

  echo "$template" | sed -n "/^[ ]*### ${template_name}_TEMPLATE_BEGIN/,\${p;/^[ ]*### ${template_name}_TEMPLATE_END/q}"
}

strip_template() {
  set -eEuo pipefail;
  local template_name="${1:?You must provide a template name}"
  template_name="$(echo -n "$template_name" | tr '[:lower:]' '[:upper:]' | tr '-' '_')"
  local template
  template="$(cat)"

  echo "$template" | sed "/^[ ]*### ${template_name}_TEMPLATE_BEGIN/,/^[ ]*### ${template_name}_TEMPLATE_END/d"
}

append_template() {
  set -eEuo pipefail;
  local template_name="${1:?You must provide a template name}"
  local template_path="${2:?You must provide a path to the template file}"
  strip_template "$1" | cat - <(echo "") <(get_template "$template_name" <"$template_path")
}

prepend_template() {
  set -eEuo pipefail;
  local template_name="${1:?You must provide a template name}"
  local template_path="${2:?You must provide a path to the template file}"
  get_template "$template_name" <"$template_path";
  strip_template "$1";
}

replace_template() {
  set -eEuo pipefail;
  local template_name="${1:?You must provide a template name}"
  template_name="$(echo -n "$template_name" | tr '[:lower:]' '[:upper:]' | tr '-' '_')"
  local template_path="${2:?You must provide a path to the template file}"
  # get_template "$template_name" <"$template_path";
  # strip_template "$1";
  sed -E "s/([ ]*### ${template_name}_TEMPLATE_BEGIN)/### TEMPLATE_${template_name}_PLACEHOLDER\n\1/" | \
  sed "/^[ ]*### ${template_name}_TEMPLATE_BEGIN/,/^[ ]*### ${template_name}_TEMPLATE_END/d"
}

inject_template() {
  set -eEuo pipefail;
  local template_name="${1:?You must provide a template name}"
  template_name="$(echo -n "$template_name" | tr '[:lower:]' '[:upper:]' | tr '-' '_')"
  local template_path="${2:?You must provide a path to the template file}"
  local placeholder="### TEMPLATE_${template_name}_PLACEHOLDER"
  local rendered_template_path
  rendered_template_path=$(mktemp)
  get_template "$template_name" <"$template_path" >"$rendered_template_path"
  sed "/${placeholder}/e cat '$rendered_template_path'" | sed "/${placeholder}/d"
  rm -rf "$rendered_template_path"
}

if [ "${1:-false}" != "false" ]; then
  if [ "$1" = "append" ]; then
    shift
    append_template "${1:?You must provide a template name}" "${2:?You must provide a path to the template file}"
  elif [ "$1" = "prepend" ]; then
    shift
    prepend_template "${1:?You must provide a template name}" "${2:?You must provide a path to the template file}"
  elif [ "$1" = "inject" ]; then
    shift
    inject_template "${1:?You must provide a template name}" "${2:?You must provide a path to the template file}"
  elif [ "$1" = "replace" ]; then
    shift
    replace_template "${1:?You must provide a template name}" "${2:?You must provide a path to the template file}"
  elif [ "$1" = "strip" ]; then
    shift
    strip_template "${1:?You must provide a template name}"
  elif [ "$1" = "get" ]; then
    shift
    get_template "${1:?You must provide a template name}"
  else
    echo "Error: Invalid method $1. Valid method are: append, strip and get" >&2
    exit 1
  fi
fi