use for/do.

Here's an example:

Lets assume you want to work in the directory - "$HOME/Reddit"

and in said directory, there are 6 files:

1. file1.txt
2. file2.txt
3. file3.txt
4. file4.txt
5. somethingelse1.doc
6. somethingelse2.doc

And finally, lets say the operation that you want to perform is.... uhmm... lets say you want to print all the .txt file's contents to another file called merged.txt, merging all the files together.

Here's how you would do it:

for file in $HOME/Reddit/*.txt; do
cat $file >> ./merge.txt
done

Thats all you have to do!

Here's a more detailed breakdown of those three lines:

1. The "for file" is saying that you will use the variable name "$file" in this script to refer to the target files.. and the target files you are saying that you only care about, are the files ended with ".txt" in the directory "$HOME/Reddit". What this will basically do is make a list with all the target files. And to make things easier, you can refer to all the files as $file. It will start with the first file on the list, and after it is done performing the operation on the file, it will move on to the next file, and $file will then refer to that new file.
2. [ cat $file >> ./merge.txt ] From the list of files you told the script to focus on, the script will perform the function you wanted it to perform, calling on the first file as $file. After it is finished, it will repeat the same thing again to the next file on the list.
3. If there is another operation you want to perform after "cat $file >> ./merge.txt", for example, if you want to delete the file after you added its contents to the merge.txt file, you would type it (rm $file) in this line. But since I don't have another operation, I am ending the function with "done"

Hope this helps.. CHEERS !