How else does the shell store information?

The other kind of variable is called a shell variable, which is like a local variable in a programming language.

To create a shell variable, you simply assign a value to a name:

training=seasonal/summer.csv

without any spaces before or after the = sign. Once you have done this, you can check the variable's value with:

echo $training

seasonal/summer.csv

1
Define a variable called testing with the value seasonal/winter.csv.
2
Use head -n 1 SOMETHING to get the first line from seasonal/winter.csv using the value of the variable testing instead of the name of the file.