subst

#! /bin/bash
#
# Copyright 2020 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# usage:
#   subst ENV_FILE  INPUT_1 OUTPUT_1  INPUT_2 OUTPUT_2 [...]
#
# Loads environment variable settings from ENV_FILE, and iterates over pairs of
# INPUT/OUTPUT files, copying each INPUT to the corresponding OUTPUT,
# substituting any expressions of the form $VAR or ${VAR} in INPUT with the
# value of VAR from the environment.
#
# If an expression of the form $VAR or ${VAR} appears in INPUT for a VAR that is
# not defined, the empty string is substituted.
#
# Note that any values present in the current environment are used, as well as
# any that are set in ENV_FILE; values in ENV_FILE will override any current
# settings.
#
# Also defines a special variable named D whose value is $; this should be used
# in any INPUT file where a $ is needed.  (There is no way to escape a $; put
# ${D} in the INPUT file instead).
#
# Also provides for a crude conditional inclusion mechanism that works as
# follows:
#
# Sections of content between lines of the form
# __IF${NAME}THEN__
# __ENDIF__
# will be included in the output if there is an environment variable in ENV_FILE
# named NAME with a value that is anything other than the empty string.  If NAME
# is not defined or the empty string, the content between these lines will be
# omitted from the output.  In any case, regardless of the value of NAME, the
# __IF${NAME}THEN__ and __ENDIF__ lines will be omitted from the output.  The
# __IF${NAME}THEN__ and __ENDIF__ expressions must appear on lines by
# themselves, with no leading or trailing whitespace or any other text.  The
# value of NAME should never include newlines.

source $1
shift

export D=\$

while (( "$#" )); do
  envsubst < "$1" \
    | sed '/^__IFTHEN__$/,/^__ENDIF__$/d' \
    | sed '/^__IF.*THEN__$/d' \
    | sed '/^__ENDIF__$/d' \
    > "$2"
  shift
  shift
done