One can achieve this by, for example, using a for loop, which has the syntax:

for var in values; do things; done

This code puts the first of the list of values into var and performs things on it, then puts the next value into var and so on.

We often use this to do the same thing to a bunch of files, as you want to do here.

You can use some suitable expression to get Bash to create a list of your files (the list of values, as I've called it above). Here I suggest you use *File.txt. This would be expanded by the shell to a list of all the files in the current directory that end with File.txt. So the first bit of your code could be

for f in *File.txt; do

Instead of f you can use some other name for your variable, such as file. I suggest using a letter or word in lowercase (there are other variables already defined in the shell with uppercase words, and we don't want to over-ride them). When we call our variable in Bash, we precede its name with $. So you would refer to your variable with $f or $file, and the shell will expand this to the current value of that variable. To prevent further expansions on the result, we almost always use double quotes, so instead of writing $f, I would write "$f" when I call my variable.

To send some text into a file you can use simple tools like printf or echo with redirection. You have shown that you know how to use redirection to append content to a file. This: stuff > file replaces the contents of file with stuff and stuff >> file appends stuff to file.

We should be really careful with redirection as we can easily overwrite our important files - you won't even get a warning about it. So before you run such a powerful command, you might want to make a backup of your files in another directory, and/or do a dry run where you just check what files are going to be operated on, like this:

for f in *File.txt; do echo "$f"; done

The above simply returns the list of files that will be acted on.

So let's go back to how you are going to add text to the file. Since we already have the name of the file in our variable, we can use that to generate the text.

Here is how your code could look with echo:

for f in *File.txt; do echo "$f" >> "$f"; done

For firstFile.txt, that would run echo firstFile.txt >> firstFile.txt, and so on for all the other files matched by *File.txt.

But you mentioned that your files end with .txt and you don't want that to be in the appended text.

To avoid that you can perform some string manipulation

for f in *File.txt; do echo "${f%.*}" >> "$f"; done

This ${f%.*} trims characters from the last . character onwards from the string in $f, in this case the .txt.

You could also use printf. It's worth learning as it's more flexible than echo and, so I'm told, can be more reliable.

for f in *File.txt; do printf '%s

' "${f%.*}" >> "$f"; done

'%s
' is for formatting - %s means the text is a string (not a number, for example) and
appends a newline after it (echo automatically appends a newline).