In post #3 in this thread, you said that the command:

Code:

combining.executable infile1 infile2 > outfile1

puts the entire contents of lines from infile1 (the 1st input file operand) followed by the the last three fields of matching lines from infile2 (the 2nd input file operand) into outfile1.

Then you said that the command:

Code:

combining.executable infile3 outfile1 > outfile2

changes its behavior completely and puts the entire contents of lines from outfile1 (the 2nd input file operand) followed by the last three fields of matching lines from infile3 (the 1st input file operand) into outfile2.

Are you absolutely positive that combining.executable magically knows that the behavior should be different for the 1st pair of files being combined than it is for every other pair of files it is asked to combine?
I understand the concept of running a program repeatedly with the output of subsequent invocations being one of the inputs from the previous invocation. And that isn't hard to do; it just can't be done with the nested loops you showed us. If you answer the questions I posed above and the remaining questions I asked in my previous post, I think we will easily be able to suggest something that will work for you.

Therefore, I repeat: What is the format of the real names of your input files? (Please show us the actual names of your 1st input file and your last input file.) If all files don't have the same text with just a number that changes from file to file, we need to have a filename matching pattern that will match all of the input filenames you want to process and a way to determine the order in which those files should be processed.

And, is the number of input files a constant?

And, what name do you really want for your output file?