Shell as a template engine

render.sh

#!/bin/sh

render=true
debug=${debug:-false}
BASENAME=`basename "$0"`

usage() {
  if [ ! "$1" = "usage" ] ; then
    return
  fi

  echo "Usage: Render template shell

Variables default:
  TEMPLATE - Template to render (required)
  OUTPUT   - Directory for output template redering (required)
  debug    - Active logs for debug
  ** All variables in template are required

Example:

$ TEMPLATE=your_template.tpl; cat \$TEMPLATE
User: \$VAR_NAME
Age: \${VAR_AGE}
City: \$VAR_CITY
Address: \\\$Street 1
ZIPCODE: \\\${50001}

$ TEMPLATE=your_template.tpl OUTPUT=output.txt \\
  VAR_NAME=JONH \\
  VAR_AGE=30 \\
  VAR_CITY=\"SAO PAULO\" ./$BASENAME

$ OUTPUT=output.txt; cat \$OUTPUT
User: JONH
Age: 30
City: SAO PAULO
Address: \$Street 1
ZIPCODE: \${50001}
"
  exit 0
}

log_message() {
  local timestamp=""
  if [ $debug = true ] ; then
    timestamp=$(date +"[%Y-%m-%d %H:%M:%S]")
  fi
  echo "$timestamp ${1-}" >&2
  shift
  for message; do
    echo "    $message" >&2
  done
}

validate_variable_template() {
  local var_name=TEMPLATE
  local var_value=$TEMPLATE
  if [ -z $var_value ] ; then
    log_message "!!! ERROR: Variable '${var_name}' is required to render template"
    render=false
    validate_template_render
  fi
  if [ ! -f $var_value ] ; then
    log_message "!!! ERROR: Template '${var_value}' not exists"
    render=false
    validate_template_render
  fi
}

validate_variable_output() {
  local var_name=OUTPUT
  local var_value=$OUTPUT
  if [ -z $var_value ] ; then
    log_message "!!! ERROR: Variable '${var_name}' is required to render template"
    render=false
    validate_template_render
  fi
}

validate_template_variables() {
  validate_template_render
  local tpl=$TEMPLATE
  local tpl_vars=$(grep -oE "[^\\](\\$|\\$\\{)\\w+}?" "${tpl}" | sed -E 's/([[:space:]]|\$|\{|\})//g')

  for tpl_var in $tpl_vars ; do
    var_value=$(eval echo "\$$tpl_var")
    if [ "$var_value" = "" ] ; then
      log_message "!!! ERROR: Template variable ${tpl_var}=NULL"
      render=false
    else
      if [ $debug = true ] ; then
        log_message "Template variable $tpl_var=$var_value"
      fi
    fi
  done
}

validate_template_render() {
  if [ $render = false ] ; then
    log_message "!!! Error rendering template, all variables should be set correctly."
    log_message "Help? ./$BASENAME usage"
    exit 1
  fi
}

prepare_output() {
  local var_name=OUTPUT
  local var_value=$OUTPUT
  parentdir="$(dirname "$var_value")"
  mkdir -p $parentdir 2> /dev/null
  if [ $? = 1 ] ; then
    log_message "!!! Error trying to create output directory: ${parentdir}"
    render=false
  fi
}

render_template() {
  validate_template_render
  local template=$TEMPLATE
  local output=$OUTPUT
  if [ $debug = true ] ; then
    log_message "Processing template: ${template}"
  fi
  eval "echo \"$(cat $template)\"" > $output
  if [ $debug = true ] ; then
    log_message "Output template: ${output}"
  fi
  exit 0
}

usage $@
validate_variable_template
validate_variable_output
validate_template_variables
prepare_output
render_template