I wanted to edit 123 text files by inserting a new line into each one. After some Googling I managed to create a shell script to do this:

LC_CTYPE=C find . -type f -exec sed -i '' '5i\
lang: en\
' {} \;

Explanations:

• LC_CTYPE=C resolves character encoding errors, but ignores Non-ASCII characters, so a different solution is needed to edit non-English text
• find . -type f -exec [command] {} \; finds all files and executes the command on them with {} as the input for each
• sed is a unix command which stands for stream editor. It can edit text within files with regex, for example replace, insert or delete text.
• -i '' is required for sed to work on MacOS, it stores an intermediary/backup file, but I skipped that since I could use git to roll back any unwanted changes (with git reset --hard)
• '5i\[newline] lang:en\[newline]' does the following: create a new line at line number 5 with the text lang:en

I did this to edit the front-matter of books I have read, which contains metadata for each one. I want to add a language filter for my book list, so I needed to add language information to every book.

As most of the books are in English, I added this line to all of them and then edited front matter for the the Non-English books manually.