How can I pass filenames to scripts?

A script that processes specific files is useful as a record of what you did, but one that allows you to process any files you want is more useful. To support this, you can use the special expression $@ (dollar sign immediately followed by at-sign) to mean "all of the command-line parameters given to the script".

For example, if unique-lines.sh contains sort $@ | uniq, when you run:

bash unique-lines.sh seasonal/summer.csv

the shell replaces $@ with seasonal/summer.csv and processes one file. If you run this:

bash unique-lines.sh seasonal/summer.csv seasonal/autumn.csv

it processes two data files, and so on.

As a reminder, to save what you have written in Nano, type Ctrl + O to write the file out, then Enter to confirm the filename, then Ctrl + X to exit the editor.