What is the `-a 2` option?

The -a 2 option in the join command is used to include all lines from the second input file (file2), even if they do not have a matching line in the first input file (file1).

When you use -a 2, the command will output:

• All lines from file2, regardless of whether they have a corresponding match in file1.
• Lines from file2 that do not have a match will be displayed with empty fields for the corresponding fields from file1.

For example:

join -a 2 file1 file2

This command will join file1 and file2 based on their common fields, while also including all lines from file2, even those that do not match any lines in file1. If you want to include unpairable lines from both files, you can use -a 1 -a 2.