What is the purpose of using the '-a' option in the 'join' command with the value '1'?

The -a option in the join command is used to include unpairable lines from one or both input files in the output. When you specify -a 1, it means that you want to include all lines from the first file (file1), even if they do not have a matching line in the second file (file2).

Here's how it works:

• join -a 1 file1 file2: This command will join file1 and file2 based on their common fields, but it will also include all lines from file1, regardless of whether they have a corresponding match in file2. Lines from file1 that do not have a match will be displayed with empty fields for the corresponding fields from file2.

If you want to include unpairable lines from both files, you can use -a 1 -a 2.