Shell commands or script to unzip, add text file and rezip
Solution 1:
You don't even need to unzip the files, you can update an existing file:
zip -u existing.zip file.txt
from zip manual:
update (-u)
Update existing entries if newer on the file system and add new files.
If the archive does not exist issue warning then create a new archive.
If you want to add a complete folder, add -r.
To update a number of zip files, do something like this:
for z in *.zip; do
zip -u "$z" file.txt
done
See this related question on U&L.