How to specify fields for joining in join command?

Specifying Fields for Joining in the join Command

The join command in Linux is used to merge two files based on a common field. By default, join uses the first field (column) of each file as the basis for the join operation. However, you can specify which fields to use for the join by using the appropriate options.

Specifying the Join Field

To specify the fields to use for the join, you can use the following options:

• -1 <field>: Specifies the field number (column number) from the first file to use for the join.
• -2 <field>: Specifies the field number (column number) from the second file to use for the join.

For example, let's say you have two files, file1.txt and file2.txt, with the following contents:

file1.txt:
1 apple
2 banana
3 cherry

file2.txt:
1 red
2 yellow
3 black

If you want to join the files based on the first field (the numbers), you can use the following command:

join file1.txt file2.txt

This will join the files based on the first field (column) of each file, resulting in the following output:

1 apple red
2 banana yellow
3 cherry black

If you want to join the files based on the second field (the fruit names) instead, you can use the following command:

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

This will join the files based on the second field (column 2) of file1.txt and the first field (column 1) of file2.txt, resulting in the following output:

apple 1 red
banana 2 yellow
cherry 3 black

Mermaid Diagram

Here's a Mermaid diagram to visualize the concept of specifying fields for joining in the join command:

In this diagram, the join command takes two input files and joins them based on a specified field. The -1 <field> and -2 <field> options allow you to specify the field numbers (column numbers) from the first and second files, respectively, to use for the join operation.

By using these options, you can control which fields are used for the join, allowing you to customize the join process to your specific needs.