If you use -a 1 2 with the join command, it will include all lines from both file1 and file2, regardless of whether they have matching lines in the other file.
Here's how you would use it:
join -a 1 -a 2 file1.txt file2.txtIn this command:
-a 1includes all lines fromfile1(even those without matches infile2).-a 2includes all lines fromfile2(even those without matches infile1).
This results in a complete output that shows all lines from both files, with matches where available. Lines without matches will still be included, but they will show up with empty fields for the missing data from the other file.
