Getting a list of files#
First, we need to get a list of all the files in the input folder so that we can process them one at a time. For this, we will use the function getFileList().
Important
The function getFileList is not that well named, since it actually returns...
...an array. 
There are no list objects in the ImageJ macro. We will use list and array as synonyms.
The syntax here should be familiar to you, but let's think about what we're doing here and refresh our memory. We're using a function, getFileList, and passing in the value of the folder in which we have our images stored (in this case, path/to/file, which we are storing in the variable folder). Under the hood, this function is going to look at this folder, and return an array of files that are contained within it. 
We can then store it in a variable, in order to use it later in our script. 
Variable Names
Remember that variable names should be clear and understandable. There was nothing stopping us from using the variable name dz334ldkx to store this file list, but we would have no idea later on in our code what that refers to. So, it's good practice to use names that are descriptive of what the variable is storing.
Let's take a look at what's stored in our file_list variable. Because it's an array, we need to use the special command to print it.
The output should look something like
normal_0.tif, normal_1.tif, normal_2.tif, normal_3.tif, normal_4.tif, normal_5.tif, normal_6.tif, normal_7.tif, normal_8.tif, normal_9.tif
Question
How can we loop through our list of files?
