How to use join -a option?

The -a option in the join command is used to include unpairable lines from one or both input files. Here's how to use it:

Syntax

join -a [file_number] file1 file2

Parameters

• file_number: This specifies which file's unpairable lines to include. Use 1 for the first file and 2 for the second file.

Examples

1. Include unpairable lines from the first file:

   join -a 1 file1.txt file2.txt

2. Include unpairable lines from the second file:

   join -a 2 file1.txt file2.txt

3. Include unpairable lines from both files:
   You can use both -a 1 and -a 2 together to include unpairable lines from both files:

   join -a 1 -a 2 file1.txt file2.txt

Example Files

file1.txt:

1 Alice
2 Bob
3 Charlie

file2.txt:

1 Electronics
2 Books
4 Furniture

Example Command

To include unpairable lines from the second file:

join -a 2 file1.txt file2.txt

Output

1 Alice Electronics
2 Bob Books
4 Furniture

In this output, you can see that the line with key 4 Furniture from file2.txt is included even though it has no match in file1.txt.