Awk function that can in any text or configuration file substitute a key with its own value, if its value is assigned earlier in the file

substitute_key_in_file.sh

# In any text or configuration file substitute a key with its own value, if its value is assigned earlier in the file and the key is referenced with {{key}}
# This could be used on any text file, i.e an .ini file or a docker-compose env file
# The mechanism works like in shell script variable syntax in some ways : assignation, declaration, resolution order and comment symbol (#)
#   Usage : substitute_key_in_file "<file_path>"
#   Input file content:
#			N=10
#			The number is {{N}}
#			# FOO={{N}}
#			A=1
#			B={{A}}
#			C={{B}}
#			X={{Y}}
#			Y=4
#			X={{Y}}
#   Result file content:
#			N=10
#			# The number is 10
#			# FOO=10
#			A=1
#			B=1
#			C=1
#			X={{Y}}
#			Y=4
#			X=4
substitute_key_in_file() {

	local _file="$1"

	local _temp=$(mktmp)

	awk -F= '

 		function parsekey(str) {
			# if there is a value assignation to a key into this string
			# update val array
			if (match(str,/^([^=#]*)=(.*)/,tmp)) {
				val[tmp[1]]=tmp[2];
			}
		}

		# fill the key array which contains all existing key
		/^[^=#]*=/ {
			# FNR=NR only when reading first file
			if (FNR==NR) {
				key[$1]=1;
				next;
			}
		}
		/.*/ {
			# FNR=NR only when reading first file
			if (FNR==NR) {
				next;
			}
		}
	
		# this block is triggered at each line only if not bypassed by next
		# so this block is really triggered only when reading second file
		{
			for (k in key) {
				# transform any reference to the keyy in current line into its value, if it has a known value
				# key[] list all existing keys in file
				# val[] list only key which have a known value
				if (k in val) {
					# we replace any reference to the key with its value with gsub in current line
					gsub("{{"k"}}", val[k]);			
				}
			}
			# re-parse the current line to find any value assignation to a key
			parsekey($0);
			print $0;
		} 

		' "${_file}" "${_file}" > "${_temp}"
		cat "${_temp}" > "${_file}"
		rm -f "${_temp}"
}