You are in a directory with a large number of FIX logs that are over 1GB in size. You have a text file containing the names of 10 of those files. How can you search for all messages containing “50=MSFT”, only in those ten files?
If those filenames do not contain newlines and are one per line in the text file you can do:
cat file_with_filenames | xargs grep -F '50=MSFT'
(You can include the double quotes if they are part of the search string).