Python script to replace ${PLACEHOLDER} in config files with content from environment variables

update_config.py

#!/usr/bin/env python
#
# Replaces ${placeholder} in a config file by it's corresponding environment variable.
# Fails if a variable is missing.
#
# The latest version of this script can be found at https://gist.github.com/micw/d7c0e34aee751e81c5aa952b29b8631b
#

import argparse,re
from os import environ
from sys import exit

missing_vars=[]
verbose = False

def replace_var(match):
	var=match.group(1)
	if verbose:
		print " * Found placeholder ${%s}" % (var)
	if var in environ:
		return environ[var]

	missing_vars.append(var)
	return ""

def main():
	parser = argparse.ArgumentParser(description='Replace placeholders in config.')
	parser.add_argument('configfile', metavar='CONFIGFILE', help='Configfile to parse')
	parser.add_argument('outputfile', metavar='OUTPUTFILE', nargs='?', help='Optional output file. If ommitted, the configfile is overwritten')
	parser.add_argument('-v', action='store_true', help='Verbose')
	args = parser.parse_args()
	infile=args.configfile
	outfile=args.outputfile if args.outputfile else infile
	global verbose
	verbose=args.v

	if verbose:
		print "Processing placeholders in %s" % infile

	with open(infile, 'r') as instream:
		content = instream.read()

	content=re.sub("\${(.*?)}",replace_var,content)

	if len(missing_vars)>0:
		print
		print "Error processing %s:" % infile
		for missing_var in missing_vars:
			print " * Missing environment variable: %s" % missing_var
		print
		exit(1)

	if verbose:
		print " * Writing changes to %s" % outfile

	with open(outfile, 'w') as outstream:
		outstream.write(content)

if __name__ == "__main__":
	main()