A shell script that checks if the filename variable is present on YAML, and it adds it if not.

add-filenames-to-posts.sh

#!/bin/bash
cd _posts
for file in $(ls *.md)
do
	if grep -Fq "filename: " $file
	then
       # code if found
	   echo "File: $file already processed"
	else
	   	# code if not found
    	echo "Adding the line on file: $file"
        awk -v n=2 -v s="filename: $file" 'NR == n {print s} {print}' $file > tmp.txt
		mv tmp.txt $file
	fi

done